glsl

Source

readme.txt

VTK uses shaders to perform its OpenGL rendering.  VTK supports many different
options when it comes to rendering, resulting in potentially thousands of
possible combinations. While we could make one giant shader that uses defines or
uniforms to switch between all these possibilities it would be limiting. Instead
we build up the shader using string replacements on the fly, and then cache the
results for performance.

When writing your own shaders you can use any approach you want. In the end they
are just strings of code. For vtkOpenGLPolyDataMapper we make use of heavy
string replacements.  In other classes we do very little processing as the shader
has far fewer options. Regardless there are a few conventions you should be
aware of.

For shader replacements we tend to use a form of

//VTK::SomeConcept::SomeAction

For example

//VTK::Normal::Dec  - declaration any uniforms/varying needed for normals
//VTK::Normal::Impl - Implementation of shader code for handling normals

All shaders should start with the following line

//VTK::System::Dec

Which vtkOpenGLShaderCache will replace with a #version and some other values to
match the system and OpenGL Context it has. The other line you need (only in
your fragment shader) is

//VTK::Output::Dec

which VTK uses to map shader outputs to the framebuffer.

All vertex shaders should name their outputs with a postfix of VSOutput.
All geometry shaders should name their outputs with a postfix of GSOutput.
All fragment shaders should name their inputs with a postfix of VSOutput. Put
another way fragment shaders should assuming their input is coming from the
vertex shader. If a geometry shader is present VTK will rename the fragment
shader inputs from VSOutput to GSOuput automatically.

All variables that represent positions or directions usually have a suffix
indicating the coordinate system they are in. The possible values are

MC  - Model Coordinates
WC  - WC World Coordinates
VC  - View Coordinates
DC  - Display Coordinates
NVC - NormalizedViewCoordinates

vtkImageResliceMapperFS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkImageResliceMapperFS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
// Template for the gpu image mapper fragment shader

// VC position of this fragment
//VTK::PositionVC::Dec

// Texture coordinates
//VTK::TCoord::Dec

// picking support
//VTK::Picking::Dec

// handle coincident offsets
//VTK::Coincident::Dec

//VTK::ZBuffer::Dec

// the output of this shader
//VTK::Output::Dec

void main()
{
  // VC position of this fragment. This should not branch/return/discard.
  //VTK::PositionVC::Impl

  // Place any calls that require uniform flow (e.g. dFdx) here.
  //VTK::UniformFlow::Impl

  // Set gl_FragDepth here (gl_FragCoord.z by default)
  //VTK::Depth::Impl

  // Early depth peeling abort:
  //VTK::DepthPeeling::PreColor

  //VTK::TCoord::Impl

  if (gl_FragData[0].a <= 0.0)
    {
    discard;
    }

  //VTK::DepthPeeling::Impl

  //VTK::Picking::Impl

  // handle coincident offsets
  //VTK::Coincident::Impl

  //VTK::ZBuffer::Impl

  //VTK::RenderPassFragmentShader::Impl
}

vtkImageResliceMapperVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkImageResliceMapperVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/

// all variables that represent positions or directions have a suffix
// indicating the coordinate system they are in. The possible values are
// MC - Model coordinates
// WC - World coordinates
// VC - View coordinates
// DC - Display coordinates
// TC - Texture coordinates

// frag position in VC
//VTK::PositionVC::Dec

// Texture coordinates
//VTK::TCoord::Dec

// picking support
//VTK::Picking::Dec

// camera and actor matrix values
//VTK::Camera::Dec

void main()
{
  //VTK::PositionVC::Impl

  //VTK::TCoord::Impl

  //VTK::Picking::Impl
}

vtkPolyData2DFS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkPolyData2DFS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/

uniform int PrimitiveIDOffset;

// Texture coordinates
//VTK::TCoord::Dec

// Scalar coloring
//VTK::Color::Dec

// Depth Peeling
//VTK::DepthPeeling::Dec

// picking support
//VTK::Picking::Dec

// the output of this shader
//VTK::Output::Dec

// Apple Bug
//VTK::PrimID::Dec

void main()
{
  // Apple Bug
  //VTK::PrimID::Impl

  //VTK::Color::Impl
  //VTK::TCoord::Impl

  //VTK::DepthPeeling::Impl
  //VTK::Picking::Impl

  if (gl_FragData[0].a <= 0.0)
    {
    discard;
    }
}

vtkPolyData2DVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkPolyData2DVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/

// all variables that represent positions or directions have a suffix
// indicating the coordinate system they are in. The possible values are
// MC - Model Coordinates
// WC - WC world coordinates
// VC - View Coordinates
// DC - Display Coordinates

in vec4 vertexWC;

// frag position in VC
//VTK::PositionVC::Dec

// material property values
//VTK::Color::Dec

// Texture coordinates
//VTK::TCoord::Dec

// Apple Bug
//VTK::PrimID::Dec

uniform mat4 WCVCMatrix;  // World to view matrix

void main()
{
  // Apple Bug
  //VTK::PrimID::Impl

  gl_Position = WCVCMatrix*vertexWC;

  //VTK::TCoord::Impl

  //VTK::Color::Impl

  //VTK::PositionVC::Impl
}

vtkPolyDataFS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkPolyDataFS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
// Template for the polydata mappers fragment shader

uniform int PrimitiveIDOffset;

// VC position of this fragment
//VTK::PositionVC::Dec

// optional color passed in from the vertex shader, vertexColor
//VTK::Color::Dec

// optional surface normal declaration
//VTK::Normal::Dec

// extra lighting parameters
//VTK::Light::Dec

// define vtkImageLabelOutlineOn
//VTK::ImageLabelOutlineOn

// Texture coordinates
//VTK::TCoord::Dec

// picking support
//VTK::Picking::Dec

// Depth Peeling Support
//VTK::DepthPeeling::Dec

// clipping plane vars
//VTK::Clip::Dec

// label outline 
//VTK::LabelOutline::Dec

// the output of this shader
//VTK::Output::Dec

// Apple Bug
//VTK::PrimID::Dec

// handle coincident offsets
//VTK::Coincident::Dec

//VTK::ZBuffer::Dec

//VTK::LabelOutlineHelperFunction

void main()
{
  // VC position of this fragment. This should not branch/return/discard.
  //VTK::PositionVC::Impl

  // Place any calls that require uniform flow (e.g. dFdx) here.
  //VTK::UniformFlow::Impl

  // Set gl_FragDepth here (gl_FragCoord.z by default)
  //VTK::Depth::Impl

  // Early depth peeling abort:
  //VTK::DepthPeeling::PreColor

  // Apple Bug
  //VTK::PrimID::Impl

  //VTK::Clip::Impl

  //VTK::Color::Impl

  // Generate the normal if we are not passed in one
  //VTK::Normal::Impl

  //VTK::TCoord::Impl

  //VTK::Light::Impl

  if (gl_FragData[0].a <= 0.0)
    {
    discard;
    }

  //VTK::DepthPeeling::Impl

  //VTK::Picking::Impl

  // handle coincident offsets
  //VTK::Coincident::Impl

  //VTK::ZBuffer::Impl

  //VTK::RenderPassFragmentShader::Impl
}

vtkPolyDataVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkPolyDataVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/

attribute vec4 vertexMC;

// frag position in VC
//VTK::PositionVC::Dec

// optional normal declaration
//VTK::Normal::Dec

// extra lighting parameters
//VTK::Light::Dec

// Texture coordinates
//VTK::TCoord::Dec

// material property values
//VTK::Color::Dec

// clipping plane vars
//VTK::Clip::Dec

// camera and actor matrix values
//VTK::Camera::Dec

// Apple Bug
//VTK::PrimID::Dec

// picking support
//VTK::Picking::Dec

void main()
{
  //VTK::Color::Impl

  //VTK::Normal::Impl

  //VTK::TCoord::Impl

  //VTK::Clip::Impl

  //VTK::PrimID::Impl

  //VTK::PositionVC::Impl

  //VTK::Light::Impl

  //VTK::Picking::Impl
}

vtkSphereMapperVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkSphereMapperVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
// this shader implements imposters in OpenGL for Spheres

attribute vec4 vertexMC;
attribute vec2 offsetMC;

// optional normal declaration
//VTK::Normal::Dec

//VTK::Picking::Dec

// Texture coordinates
//VTK::TCoord::Dec

uniform mat3 normalMatrix; // transform model coordinate directions to view coordinates

// material property values
//VTK::Color::Dec

// clipping plane vars
//VTK::Clip::Dec

// camera and actor matrix values
//VTK::Camera::Dec

varying vec4 vertexVCVSOutput;
varying float radiusVCVSOutput;
varying vec3 centerVCVSOutput;

uniform int cameraParallel;
uniform float scaleFactor;

void main()
{
  //VTK::Picking::Impl

  //VTK::Color::Impl

  //VTK::Normal::Impl

  //VTK::TCoord::Impl

  //VTK::Clip::Impl

  // compute the projected vertex position
  vec2 scaledOffsetMC = scaleFactor * offsetMC;
  vertexVCVSOutput = MCVCMatrix * vertexMC;
  centerVCVSOutput = vertexVCVSOutput.xyz;
  radiusVCVSOutput = length(scaledOffsetMC)*0.5;

  // make the triangle face the camera
  if (cameraParallel == 0)
    {
    vec3 dir = normalize(-vertexVCVSOutput.xyz);
    vec3 base2 = normalize(cross(dir,vec3(1.0,0.0,0.0)));
    vec3 base1 = cross(base2,dir);
    vertexVCVSOutput.xyz = vertexVCVSOutput.xyz + scaledOffsetMC.x*base1 + scaledOffsetMC.y*base2;
    }
  else
    {
    // add in the offset
    vertexVCVSOutput.xy = vertexVCVSOutput.xy + scaledOffsetMC;
    }

  gl_Position = VCPCMatrix * vertexVCVSOutput;
}

vtkStickMapperVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkStickMapperVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
// this shader implements imposters in OpenGL for Sticks

attribute vec4 vertexMC;
attribute vec3 orientMC;
attribute vec4 offsetMC;
attribute float radiusMC;

// optional normal declaration
//VTK::Normal::Dec

//VTK::Picking::Dec

// Texture coordinates
//VTK::TCoord::Dec

uniform mat3 normalMatrix; // transform model coordinate directions to view coordinates

// material property values
//VTK::Color::Dec

// clipping plane vars
//VTK::Clip::Dec

// camera and actor matrix values
//VTK::Camera::Dec

varying vec4 vertexVCVSOutput;
varying float radiusVCVSOutput;
varying float lengthVCVSOutput;
varying vec3 centerVCVSOutput;
varying vec3 orientVCVSOutput;

uniform int cameraParallel;

void main()
{
  //VTK::Picking::Impl

  //VTK::Color::Impl

  //VTK::Normal::Impl

  //VTK::TCoord::Impl

  //VTK::Clip::Impl

  vertexVCVSOutput = MCVCMatrix * vertexMC;
  centerVCVSOutput = vertexVCVSOutput.xyz;
  radiusVCVSOutput = radiusMC;
  lengthVCVSOutput = length(orientMC);
  orientVCVSOutput = normalMatrix * normalize(orientMC);

  // make sure it is pointing out of the screen
  if (orientVCVSOutput.z < 0.0)
    {
    orientVCVSOutput = -orientVCVSOutput;
    }

  // make the basis
  vec3 xbase;
  vec3 ybase;
  vec3 dir = vec3(0.0,0.0,1.0);
  if (cameraParallel == 0)
    {
    dir = normalize(-vertexVCVSOutput.xyz);
    }
  if (abs(dot(dir,orientVCVSOutput)) == 1.0)
    {
    xbase = normalize(cross(vec3(0.0,1.0,0.0),orientVCVSOutput));
    ybase = cross(xbase,orientVCVSOutput);
    }
  else
    {
    xbase = normalize(cross(orientVCVSOutput,dir));
    ybase = cross(orientVCVSOutput,xbase);
    }

  vec3 offsets = offsetMC.xyz*2.0-1.0;
  vertexVCVSOutput.xyz = vertexVCVSOutput.xyz +
    radiusVCVSOutput*offsets.x*xbase +
    radiusVCVSOutput*offsets.y*ybase +
    0.5*lengthVCVSOutput*offsets.z*orientVCVSOutput;

  gl_Position = VCPCMatrix * vertexVCVSOutput;
}

vtkVolumeFS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkVolumeFS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
// Template for the volume mappers fragment shader

// the output of this shader
//VTK::Output::Dec

varying vec3 vertexVCVSOutput;

// first declare the settings from the mapper
// that impact the code paths in here

// always set vtkNumComponents 1,2,3,4
//VTK::NumComponents

// possibly define vtkTrilinearOn
//VTK::TrilinearOn

// possibly define UseIndependentComponents
//VTK::IndependentComponentsOn

// possibly define vtkCustomComponentsColorMix
//VTK::CustomComponentsColorMixOn

// possibly define any "proportional" components
//VTK::vtkProportionalComponents

// possibly define any components that are forced to nearest interpolation
//VTK::vtkForceNearestComponents

// Define the blend mode to use
#define vtkBlendMode //VTK::BlendMode

// Possibly define vtkImageLabelOutlineOn
//VTK::ImageLabelOutlineOn

#ifdef vtkImageLabelOutlineOn

uniform float outlineOpacity;
uniform float vpWidth;
uniform float vpHeight;
uniform float vpOffsetX;
uniform float vpOffsetY;
uniform mat4 PCWCMatrix;
uniform mat4 vWCtoIDX;
#endif

// define vtkLightComplexity
//VTK::LightComplexity
#if vtkLightComplexity > 0
uniform float vSpecularPower;
uniform float vAmbient;
uniform float vDiffuse;
uniform float vSpecular;
//VTK::Light::Dec
#endif

//VTK::VolumeShadowOn
//VTK::SurfaceShadowOn
//VTK::localAmbientOcclusionOn
//VTK::LAO::Dec
//VTK::VolumeShadow::Dec

// define vtkComputeNormalFromOpacity
//VTK::vtkComputeNormalFromOpacity

// possibly define vtkGradientOpacityOn
//VTK::GradientOpacityOn
#ifdef vtkGradientOpacityOn
uniform float goscale0;
uniform float goshift0;
uniform float gomin0;
uniform float gomax0;
#ifdef UseIndependentComponents
#if vtkNumComponents > 1
uniform float goscale1;
uniform float goshift1;
uniform float gomin1;
uniform float gomax1;
#if vtkNumComponents > 2
uniform float goscale2;
uniform float goshift2;
uniform float gomin2;
uniform float gomax2;
#if vtkNumComponents > 3
uniform float goscale3;
uniform float goshift3;
uniform float gomin3;
uniform float gomax3;
#endif
#endif
#endif
#endif
#endif

// if you want to see the raw tiled
// data in webgl1 uncomment the following line
// #define debugtile

// camera values
uniform float camThick;
uniform float camNear;
uniform float camFar;
uniform int cameraParallel;

// values describing the volume geometry
uniform vec3 vOriginVC;
uniform vec3 vSpacing;
uniform ivec3 volumeDimensions; // 3d texture dimensions
uniform vec3 vPlaneNormal0;
uniform float vPlaneDistance0;
uniform vec3 vPlaneNormal1;
uniform float vPlaneDistance1;
uniform vec3 vPlaneNormal2;
uniform float vPlaneDistance2;
uniform vec3 vPlaneNormal3;
uniform float vPlaneDistance3;
uniform vec3 vPlaneNormal4;
uniform float vPlaneDistance4;
uniform vec3 vPlaneNormal5;
uniform float vPlaneDistance5;

//VTK::ClipPlane::Dec

// opacity and color textures
uniform sampler2D otexture;
uniform float oshift0;
uniform float oscale0;
uniform sampler2D ctexture;
uniform float cshift0;
uniform float cscale0;

#if vtkNumComponents >= 2
uniform float oshift1;
uniform float oscale1;
uniform float cshift1;
uniform float cscale1;
#endif
#if vtkNumComponents >= 3
uniform float oshift2;
uniform float oscale2;
uniform float cshift2;
uniform float cscale2;
#endif
#if vtkNumComponents >= 4
uniform float oshift3;
uniform float oscale3;
uniform float cshift3;
uniform float cscale3;
#endif

// jitter texture
uniform sampler2D jtexture;
uniform sampler2D ttexture;


// some 3D texture values
uniform float sampleDistance;
uniform vec3 vVCToIJK;

// the heights defined below are the locations
// for the up to four components of the tfuns
// the tfuns have a height of 2XnumComps pixels so the
// values are computed to hit the middle of the two rows
// for that component
#ifdef UseIndependentComponents
#if vtkNumComponents == 1
uniform float mix0;
#define height0 0.5
#endif
#if vtkNumComponents == 2
uniform float mix0;
uniform float mix1;
#define height0 0.25
#define height1 0.75
#endif
#if vtkNumComponents == 3
uniform float mix0;
uniform float mix1;
uniform float mix2;
#define height0 0.17
#define height1 0.5
#define height2 0.83
#endif
#if vtkNumComponents == 4
uniform float mix0;
uniform float mix1;
uniform float mix2;
uniform float mix3;
#define height0 0.125
#define height1 0.375
#define height2 0.625
#define height3 0.875
#endif
#endif

uniform vec4 ipScalarRangeMin;
uniform vec4 ipScalarRangeMax;

// declaration for intermixed geometry
//VTK::ZBuffer::Dec

//=======================================================================
// global and custom variables (a temporary section before photorealistics rendering module is complete)
vec3 rayDirVC;
float sampleDistanceISVS;
float sampleDistanceIS;

#define SQRT3    1.7321
#define INV4PI   0.0796
#define EPSILON  0.001
#define PI       3.1415
#define PI2      9.8696

//=======================================================================
// Webgl2 specific version of functions
#if __VERSION__ == 300

uniform highp sampler3D texture1;

vec4 getTextureValue(vec3 pos)
{
  vec4 tmp = texture(texture1, pos);

  #if defined(vtkComponent0ForceNearest) || \
      defined(vtkComponent1ForceNearest) || \
      defined(vtkComponent2ForceNearest) || \
      defined(vtkComponent3ForceNearest)
    vec3 nearestPos = (floor(pos * vec3(volumeDimensions)) + 0.5) / vec3(volumeDimensions);
    vec4 nearestValue = texture(texture1, nearestPos);
    #ifdef vtkComponent0ForceNearest
      tmp[0] = nearestValue[0];
    #endif
    #ifdef vtkComponent1ForceNearest
      tmp[1] = nearestValue[1];
    #endif
    #ifdef vtkComponent2ForceNearest
      tmp[2] = nearestValue[2];
    #endif
    #ifdef vtkComponent3ForceNearest
      tmp[3] = nearestValue[3];
    #endif
  #endif

  #ifndef UseIndependentComponents
    #if vtkNumComponents == 1
      tmp.a = tmp.r;
    #endif
    #if vtkNumComponents == 2
      tmp.a = tmp.g;
    #endif
    #if vtkNumComponents == 3
      tmp.a = length(tmp.rgb);
    #endif
  #endif

  return tmp;
}

//=======================================================================
// WebGL1 specific version of functions
#else

uniform sampler2D texture1;

uniform float texWidth;
uniform float texHeight;
uniform int xreps;
uniform int xstride;
uniform int ystride;

// if computing trilinear values from multiple z slices
#ifdef vtkTrilinearOn
vec4 getTextureValue(vec3 ijk)
{
  float zoff = 1.0/float(volumeDimensions.z);
  vec4 val1 = getOneTextureValue(ijk);
  vec4 val2 = getOneTextureValue(vec3(ijk.xy, ijk.z + zoff));

  float indexZ = float(volumeDimensions)*ijk.z;
  float zmix =  indexZ - floor(indexZ);

  return mix(val1, val2, zmix);
}

vec4 getOneTextureValue(vec3 ijk)
#else // nearest or fast linear
vec4 getTextureValue(vec3 ijk)
#endif
{
  vec3 tdims = vec3(volumeDimensions);

#ifdef debugtile
  vec2 tpos = vec2(ijk.x, ijk.y);
  vec4 tmp = texture2D(texture1, tpos);
  tmp.a = 1.0;

#else
  int z = int(ijk.z * tdims.z);
  int yz = z / xreps;
  int xz = z - yz*xreps;

  int tileWidth = volumeDimensions.x/xstride;
  int tileHeight = volumeDimensions.y/ystride;

  xz *= tileWidth;
  yz *= tileHeight;

  float ni = float(xz) + (ijk.x*float(tileWidth));
  float nj = float(yz) + (ijk.y*float(tileHeight));

  vec2 tpos = vec2(ni/texWidth, nj/texHeight);

  vec4 tmp = texture2D(texture1, tpos);

#if vtkNumComponents == 1
  tmp.a = tmp.r;
#endif
#if vtkNumComponents == 2
  tmp.g = tmp.a;
#endif
#if vtkNumComponents == 3
  tmp.a = length(tmp.rgb);
#endif
#endif

  return tmp;
}

// End of Webgl1 specific code
//=======================================================================
#endif

//=======================================================================
// transformation between VC and IS space

// convert vector position from idx to vc
#if (vtkLightComplexity > 0) || (defined vtkClippingPlanesOn)
vec3 IStoVC(vec3 posIS){
  vec3 posVC = posIS / vVCToIJK;
  return posVC.x * vPlaneNormal0 +
         posVC.y * vPlaneNormal2 +
         posVC.z * vPlaneNormal4 +
         vOriginVC;
}

// convert vector position from vc to idx
vec3 VCtoIS(vec3 posVC){
  posVC = posVC - vOriginVC;
  posVC = vec3(
    dot(posVC, vPlaneNormal0),
    dot(posVC, vPlaneNormal2),
    dot(posVC, vPlaneNormal4));
  return posVC * vVCToIJK;
}
#endif

//Rotate vector to view coordinate
#if (vtkLightComplexity > 0) || (defined vtkGradientOpacityOn)
void rotateToViewCoord(inout vec3 dirIS){
  dirIS.xyz =
    dirIS.x * vPlaneNormal0 +
    dirIS.y * vPlaneNormal2 +
    dirIS.z * vPlaneNormal4;
}

//Rotate vector to idx coordinate
vec3 rotateToIDX(vec3 dirVC){
  vec3 dirIS;
  dirIS.xyz = vec3(
    dot(dirVC, vPlaneNormal0),
    dot(dirVC, vPlaneNormal2),
    dot(dirVC, vPlaneNormal4));
  return dirIS;
}
#endif

//=======================================================================
// Given a normal compute the gradient opacity factors
float computeGradientOpacityFactor(
  float normalMag, float goscale, float goshift, float gomin, float gomax)
{
  return clamp(normalMag * goscale + goshift, gomin, gomax);
}

//=======================================================================
// compute the normal and gradient magnitude for a position, uses forward difference
#if (vtkLightComplexity > 0) || (defined vtkGradientOpacityOn)
  #ifdef vtkClippingPlanesOn
    void adjustClippedVoxelValues(vec3 pos, vec3 texPos[3], inout vec3 g1)
    {
      vec3 g1VC[3];
      for (int i = 0; i < 3; ++i)
      {
        g1VC[i] = IStoVC(texPos[i]);
      }
      vec3 posVC = IStoVC(pos);
      for (int i = 0; i < clip_numPlanes; ++i)
      {
        for (int j = 0; j < 3; ++j)
        {
          if(dot(vec3(vClipPlaneOrigins[i] - g1VC[j].xyz), vClipPlaneNormals[i]) > 0.0)
          {
            g1[j] = 0.0;
          }
        }
      }
    }
  #endif

  #ifdef vtkComputeNormalFromOpacity
    vec4 computeDensityNormal(vec3 opacityUCoords[2], float opactityTextureHeight, float gradientOpacity) {
      vec3 opacityG1, opacityG2;
      opacityG1.x = texture2D(otexture, vec2(opacityUCoords[0].x, opactityTextureHeight)).r;
      opacityG1.y = texture2D(otexture, vec2(opacityUCoords[0].y, opactityTextureHeight)).r;
      opacityG1.z = texture2D(otexture, vec2(opacityUCoords[0].z, opactityTextureHeight)).r;
      opacityG2.x = texture2D(otexture, vec2(opacityUCoords[1].x, opactityTextureHeight)).r;
      opacityG2.y = texture2D(otexture, vec2(opacityUCoords[1].y, opactityTextureHeight)).r;
      opacityG2.z = texture2D(otexture, vec2(opacityUCoords[1].z, opactityTextureHeight)).r;
      opacityG1.xyz *= gradientOpacity;
      opacityG2.xyz *= gradientOpacity;

      vec4 opacityG = vec4(opacityG1 - opacityG2, 1.0f);
      // divide by spacing
      opacityG.xyz /= vSpacing;
      opacityG.w = length(opacityG.xyz);
      // rotate to View Coords
      rotateToViewCoord(opacityG.xyz);
      if (!all(equal(opacityG.xyz, vec3(0.0)))) {
        return vec4(normalize(opacityG.xyz),opacityG.w);
      } else {
        return vec4(0.0);
      }
    }

    vec4 computeNormalForDensity(vec3 pos, vec3 tstep, out vec3 scalarInterp[2], const int opacityComponent)
    {
      vec3 xvec = vec3(tstep.x, 0.0, 0.0);
      vec3 yvec = vec3(0.0, tstep.y, 0.0);
      vec3 zvec = vec3(0.0, 0.0, tstep.z);
      vec3 texPosPVec[3];
      texPosPVec[0] = pos + xvec;
      texPosPVec[1] = pos + yvec;
      texPosPVec[2] = pos + zvec;
      vec3 texPosNVec[3];
      texPosNVec[0] = pos - xvec;
      texPosNVec[1] = pos - yvec;
      texPosNVec[2] = pos - zvec;
      vec3 g1, g2;

      scalarInterp[0].x = getTextureValue(texPosPVec[0])[opacityComponent];
      scalarInterp[0].y = getTextureValue(texPosPVec[1])[opacityComponent];
      scalarInterp[0].z = getTextureValue(texPosPVec[2])[opacityComponent];
      scalarInterp[1].x = getTextureValue(texPosNVec[0])[opacityComponent];
      scalarInterp[1].y = getTextureValue(texPosNVec[1])[opacityComponent];
      scalarInterp[1].z = getTextureValue(texPosNVec[2])[opacityComponent];

      #ifdef vtkClippingPlanesOn
        adjustClippedVoxelValues(pos, texPosPVec, scalarInterp[0]);
        adjustClippedVoxelValues(pos, texPosNVec, scalarInterp[1]);
      #endif
      vec4 result;
      result.x = scalarInterp[0].x - scalarInterp[1].x;
      result.y = scalarInterp[0].y - scalarInterp[1].y;
      result.z = scalarInterp[0].z - scalarInterp[1].z;
      // divide by spacing
      result.xyz /= vSpacing;
      result.w = length(result.xyz);
      // rotate to View Coords
      rotateToViewCoord(result.xyz);
      if (length(result.xyz) > 0.0) {
        return vec4(normalize(result.xyz),result.w);
      } else {
        return vec4(0.0);
      }
    }
  #endif

  // only works with dependent components
  vec4 computeNormal(vec3 pos, vec3 tstep)
  {
    vec3 xvec = vec3(tstep.x, 0.0, 0.0);
    vec3 yvec = vec3(0.0, tstep.y, 0.0);
    vec3 zvec = vec3(0.0, 0.0, tstep.z);
    vec3 texPosPVec[3];
    texPosPVec[0] = pos + xvec;
    texPosPVec[1] = pos + yvec;
    texPosPVec[2] = pos + zvec;
    vec3 texPosNVec[3];
    texPosNVec[0] = pos - xvec;
    texPosNVec[1] = pos - yvec;
    texPosNVec[2] = pos - zvec;
    vec3 g1, g2;
    g1.x = getTextureValue(texPosPVec[0]).a;
    g1.y = getTextureValue(texPosPVec[1]).a;
    g1.z = getTextureValue(texPosPVec[2]).a;
    g2.x = getTextureValue(texPosNVec[0]).a;
    g2.y = getTextureValue(texPosNVec[1]).a;
    g2.z = getTextureValue(texPosNVec[2]).a;
    #ifdef vtkClippingPlanesOn
      adjustClippedVoxelValues(pos, texPosPVec, g1);
      adjustClippedVoxelValues(pos, texPosNVec, g2);
    #endif
    vec4 result;
    result = vec4(g1 - g2, -1.0);
    // divide by spacing
    result.xyz /= vSpacing;
    result.w = length(result.xyz);
    if (result.w > 0.0){
      // rotate to View Coords
      rotateToViewCoord(result.xyz);
      return vec4(normalize(result.xyz),result.w);
    } else {
      return vec4(0.0);
    }
  }
#endif

#ifdef vtkImageLabelOutlineOn
vec3 fragCoordToIndexSpace(vec4 fragCoord) {
  vec4 pcPos = vec4(
    (fragCoord.x / vpWidth - vpOffsetX - 0.5) * 2.0,
    (fragCoord.y / vpHeight - vpOffsetY - 0.5) * 2.0,
    (fragCoord.z - 0.5) * 2.0,
    1.0);

  vec4 worldCoord = PCWCMatrix * pcPos;
  vec4 vertex = (worldCoord/worldCoord.w);

  vec3 index = (vWCtoIDX * vertex).xyz;

  // half voxel fix for labelmapOutline
  return (index + vec3(0.5)) / vec3(volumeDimensions);
}
#endif

//=======================================================================
// compute the normals and gradient magnitudes for a position
// for independent components
mat4 computeMat4Normal(vec3 pos, vec4 tValue, vec3 tstep)
{
  mat4 result;
  vec4 distX = getTextureValue(pos + vec3(tstep.x, 0.0, 0.0)) - tValue;
  vec4 distY = getTextureValue(pos + vec3(0.0, tstep.y, 0.0)) - tValue;
  vec4 distZ = getTextureValue(pos + vec3(0.0, 0.0, tstep.z)) - tValue;

  // divide by spacing
  distX /= vSpacing.x;
  distY /= vSpacing.y;
  distZ /= vSpacing.z;

  mat3 rot;
  rot[0] = vPlaneNormal0;
  rot[1] = vPlaneNormal2;
  rot[2] = vPlaneNormal4;

#if !defined(vtkComponent0Proportional)
  result[0].xyz = vec3(distX.r, distY.r, distZ.r);
  result[0].a = length(result[0].xyz);
  result[0].xyz *= rot;
  if (result[0].w > 0.0)
  {
    result[0].xyz /= result[0].w;
  }
#endif

// optionally compute the 2nd component
#if vtkNumComponents >= 2 && !defined(vtkComponent1Proportional)
  result[1].xyz = vec3(distX.g, distY.g, distZ.g);
  result[1].a = length(result[1].xyz);
  result[1].xyz *= rot;
  if (result[1].w > 0.0)
  {
    result[1].xyz /= result[1].w;
  }
#endif

// optionally compute the 3rd component
#if vtkNumComponents >= 3 && !defined(vtkComponent2Proportional)
  result[2].xyz = vec3(distX.b, distY.b, distZ.b);
  result[2].a = length(result[2].xyz);
  result[2].xyz *= rot;
  if (result[2].w > 0.0)
  {
    result[2].xyz /= result[2].w;
  }
#endif

// optionally compute the 4th component
#if vtkNumComponents >= 4 && !defined(vtkComponent3Proportional)
  result[3].xyz = vec3(distX.a, distY.a, distZ.a);
  result[3].a = length(result[3].xyz);
  result[3].xyz *= rot;
  if (result[3].w > 0.0)
  {
    result[3].xyz /= result[3].w;
  }
#endif

  return result;
}

//=======================================================================
// global shadow - secondary ray
#if defined(VolumeShadowOn) || defined(localAmbientOcclusionOn)
float random()
{
  float rand = fract(sin(dot(gl_FragCoord.xy,vec2(12.9898,78.233)))*43758.5453123);
  float jitter=texture2D(jtexture,gl_FragCoord.xy/32.).r;
  uint pcg_state = floatBitsToUint(jitter);
  uint state = pcg_state;
  pcg_state = pcg_state * uint(747796405) + uint(2891336453);
  uint word = ((state >> ((state >> uint(28)) + uint(4))) ^ state) * uint(277803737);
  return (float((((word >> uint(22)) ^ word) >> 1 ))/float(2147483647) + rand)/2.0;
}
#endif

#ifdef VolumeShadowOn
// henyey greenstein phase function
float phase_function(float cos_angle)
{
  // divide by 2.0 instead of 4pi to increase intensity
  return ((1.0-anisotropy2)/pow(1.0+anisotropy2-2.0*anisotropy*cos_angle, 1.5))/2.0;
}

// Computes the intersection between a ray and a box
struct Hit
{
  float tmin;
  float tmax;
};

struct Ray
{
  vec3 origin;
  vec3 dir;
  vec3 invDir;
};

bool BBoxIntersect(vec3 boundMin, vec3 boundMax, const Ray r, out Hit hit)
{
  vec3 tbot = r.invDir * (boundMin - r.origin);
  vec3 ttop = r.invDir * (boundMax - r.origin);
  vec3 tmin = min(ttop, tbot);
  vec3 tmax = max(ttop, tbot);
  vec2 t = max(tmin.xx, tmin.yz);
  float t0 = max(t.x, t.y);
  t = min(tmax.xx, tmax.yz);
  float t1 = min(t.x, t.y);
  hit.tmin = t0;
  hit.tmax = t1;
  return t1 > max(t0,0.0);
}

// As BBoxIntersect requires the inverse of the ray coords,
// this function is used to avoid numerical issues
void safe_0_vector(inout Ray ray)
{
  if(abs(ray.dir.x) < EPSILON) ray.dir.x = sign(ray.dir.x) * EPSILON;
  if(abs(ray.dir.y) < EPSILON) ray.dir.y = sign(ray.dir.y) * EPSILON;
  if(abs(ray.dir.z) < EPSILON) ray.dir.z = sign(ray.dir.z) * EPSILON;
}

float volume_shadow(vec3 posIS, vec3 lightDirNormIS)
{
  float shadow = 1.0;
  float opacity = 0.0;

  // modify sample distance with a random number between 1.5 and 3.0
  float sampleDistanceISVS_jitter = sampleDistanceISVS * mix(1.5, 3.0, random());
  float opacityPrev = texture2D(otexture, vec2(getTextureValue(posIS).r * oscale0 + oshift0, 0.5)).r;

  // in case the first sample near surface has a very tiled light ray, we need to offset start position
  posIS += sampleDistanceISVS_jitter * lightDirNormIS;

  // compute the start and end points for the ray
  Ray ray;
  Hit hit;
  ray.origin = posIS;
  ray.dir = lightDirNormIS;
  safe_0_vector(ray);
  ray.invDir = 1.0/ray.dir;

  if(!BBoxIntersect(vec3(0.0),vec3(1.0), ray, hit))
  {
    return 1.0;
  }
  float maxdist = hit.tmax;

  // interpolate shadow ray length between: 1 unit of sample distance in IS to SQRT3, based on globalIlluminationReach
  float maxgi = mix(sampleDistanceISVS_jitter,SQRT3,giReach);
  maxdist = min(maxdist,maxgi);
  if(maxdist < EPSILON) {
    return 1.0;
  }

  float current_dist = 0.0;
  float current_step = length(sampleDistanceISVS_jitter * lightDirNormIS);
  float clamped_step = 0.0;

  vec4 scalar = vec4(0.0);
  while(current_dist < maxdist)
  {
#ifdef vtkClippingPlanesOn
    vec3 posVC = IStoVC(posIS);
    for (int i = 0; i < clip_numPlanes; ++i)
    {
      if (dot(vec3(vClipPlaneOrigins[i] - posVC), vClipPlaneNormals[i]) > 0.0)
      {
        current_dist = maxdist;
      }
    }
#endif
    scalar = getTextureValue(posIS);
    opacity = texture2D(otexture, vec2(scalar.r * oscale0 + oshift0, 0.5)).r;
    #if defined(vtkGradientOpacityOn) && !defined(UseIndependentComponents)
      vec4 normal = computeNormal(posIS, vec3(1.0/vec3(volumeDimensions)));
      opacity *= computeGradientOpacityFactor(normal.w, goscale0, goshift0, gomin0, gomax0);
    #endif
    shadow *= 1.0 - opacity;

    // optimization: early termination
    if (shadow < EPSILON){
      return 0.0;
    }

    clamped_step = min(maxdist - current_dist, current_step);
    posIS += clamped_step * lightDirNormIS;
    current_dist += current_step;
  }

  return shadow;
}

vec3 applyShadowRay(vec3 tColor, vec3 posIS, vec3 viewDirectionVC)
{
  vec3 vertLight = vec3(0.0);
  vec3 secondary_contrib = vec3(0.0);
  // here we assume only positional light, no effect of cones
  for (int i = 0; i < lightNum; i++)
  {
    #if(vtkLightComplexity==3)
      if (lightPositional[i] == 1){
        vertLight = lightPositionVC[i] - IStoVC(posIS);
      }else{
        vertLight = - lightDirectionVC[i];
      }
    #else
      vertLight = - lightDirectionVC[i];
    #endif
    // here we assume achromatic light, only intensity
    float dDotL = dot(viewDirectionVC, normalize(vertLight));
    // isotropic scatter returns 0.5 instead of 1/4pi to increase intensity
    float phase_attenuation = 0.5;
    if (abs(anisotropy) > EPSILON){
      phase_attenuation = phase_function(dDotL);
    }
    float vol_shadow = volume_shadow(posIS, normalize(rotateToIDX(vertLight)));
    secondary_contrib += tColor * vDiffuse * lightColor[i] * vol_shadow * phase_attenuation;
    secondary_contrib += tColor * vAmbient;
  }
  return secondary_contrib;
}
#endif

//=======================================================================
// local ambient occlusion
#ifdef localAmbientOcclusionOn
vec3 sample_direction_uniform(int i)
{
  float rand = random() * 0.5;
  float theta = PI2 * (kernelSample[i][0] + rand);
  float phi = acos(2.0 * (kernelSample[i][1] + rand) -1.0) / 2.5;
  return normalize(vec3(cos(theta)*sin(phi), sin(theta)*sin(phi), cos(phi)));
}

// return a matrix that transform startDir into z axis; startDir should be normalized
mat3 zBaseRotationalMatrix(vec3 startDir){
  vec3 axis = cross(startDir, vec3(0.0,0.0,1.0));
  float cosA = startDir.z;
  float k = 1.0 / (1.0 + cosA);
  mat3 matrix = mat3((axis.x * axis.x * k) + cosA, (axis.y * axis.x * k) - axis.z, (axis.z * axis.x * k) + axis.y,
              (axis.x * axis.y * k) + axis.z, (axis.y * axis.y * k) + cosA, (axis.z * axis.y * k) - axis.x,
              (axis.x * axis.z * k) - axis.y, (axis.y * axis.z * k) + axis.x, (axis.z * axis.z * k) + cosA);
  return matrix;
}

float computeLAO(vec3 posIS, float op, vec3 lightDir, vec4 normal){
  // apply LAO only at selected locations, otherwise return full brightness
  if (normal.w > 0.0 && op > 0.05){
    float total_transmittance = 0.0;
    mat3 inverseRotateBasis = inverse(zBaseRotationalMatrix(normalize(-normal.xyz)));
    vec3 currPos, randomDirStep;
    float weight, transmittance, opacity;
    for (int i = 0; i < kernelSize; i++)
    {
      randomDirStep = inverseRotateBasis * sample_direction_uniform(i) * sampleDistanceIS;
      weight = 1.0 - dot(normalize(lightDir), normalize(randomDirStep));
      currPos = posIS;
      transmittance = 1.0;
      for (int j = 0; j < kernelRadius ; j++){
        currPos += randomDirStep;
        // check if it's at clipping plane, if so return full brightness
        if (all(greaterThan(currPos, vec3(EPSILON))) && all(lessThan(currPos,vec3(1.0-EPSILON)))){
          opacity = texture2D(otexture, vec2(getTextureValue(currPos).r * oscale0 + oshift0, 0.5)).r;
          #ifdef vtkGradientOpacityOn
             opacity *= computeGradientOpacityFactor(normal.w, goscale0, goshift0, gomin0, gomax0);
          #endif
          transmittance *= 1.0 - opacity;
        }
        else{
          break;
        }
      }
      total_transmittance += transmittance / float(kernelRadius) * weight;

      // early termination if fully translucent
      if (total_transmittance > 1.0 - EPSILON){
        return 1.0;
      }
    }
    // average transmittance and reduce variance
    return clamp(total_transmittance / float(kernelSize), 0.3, 1.0);
  } else {
    return 1.0;
  }
}
#endif

//=======================================================================
// surface light contribution
#if vtkLightComplexity > 0
  void applyLighting(inout vec3 tColor, vec4 normal)
  {
    vec3 diffuse = vec3(0.0, 0.0, 0.0);
    vec3 specular = vec3(0.0, 0.0, 0.0);
    float df, sf = 0.0;
    for (int i = 0; i < lightNum; i++){
        df = abs(dot(normal.rgb, -lightDirectionVC[i]));
        diffuse += df * lightColor[i];
        sf = pow( abs(dot(lightHalfAngleVC[i],normal.rgb)), vSpecularPower);
        specular += sf * lightColor[i];
    }
    tColor.rgb = tColor.rgb*(diffuse*vDiffuse + vAmbient) + specular*vSpecular;
  }
  #ifdef SurfaceShadowOn
  #if vtkLightComplexity < 3
    vec3 applyLightingDirectional(vec3 posIS, vec4 tColor, vec4 normal)
    {
      // everything in VC
      vec3 diffuse = vec3(0.0);
      vec3 specular = vec3(0.0);
      #ifdef localAmbientOcclusionOn
        vec3 ambient = vec3(0.0);
      #endif
      vec3 vertLightDirection;
      for (int i = 0; i < lightNum; i++){
        float ndotL,vdotR;
        vertLightDirection = lightDirectionVC[i];
        ndotL = dot(normal.xyz, vertLightDirection);
        if (ndotL < 0.0 && twoSidedLighting)
        {
          ndotL = -ndotL;
        }
        if (ndotL > 0.0)
        {
          diffuse += ndotL * lightColor[i];
          //specular
          vdotR = dot(-rayDirVC, normalize(2.0 * ndotL * -normal.xyz + vertLightDirection));
          if (vdotR > 0.0)
          {
            specular += pow(vdotR, vSpecularPower) * lightColor[i];
          }
        }
        #ifdef localAmbientOcclusionOn
            ambient += computeLAO(posIS, tColor.a, vertLightDirection, normal);
        #endif
      }
      #ifdef localAmbientOcclusionOn
        return tColor.rgb * (diffuse * vDiffuse + vAmbient * ambient) + specular*vSpecular;
      #else
        return tColor.rgb * (diffuse * vDiffuse + vAmbient) + specular*vSpecular;
      #endif
    }
  #else
    vec3 applyLightingPositional(vec3 posIS, vec4 tColor, vec4 normal, vec3 posVC)
    {
      // everything in VC
      vec3 diffuse = vec3(0.0);
      vec3 specular = vec3(0.0);
      #ifdef localAmbientOcclusionOn
        vec3 ambient = vec3(0.0);
      #endif
      vec3 vertLightDirection;
      for (int i = 0; i < lightNum; i++){
        float distance,attenuation,ndotL,vdotR;
        vec3 lightDir;
        if (lightPositional[i] == 1){
          lightDir = lightDirectionVC[i];
          vertLightDirection = posVC - lightPositionVC[i];
          distance = length(vertLightDirection);
          vertLightDirection = normalize(vertLightDirection);
          attenuation = 1.0 / (lightAttenuation[i].x
                              + lightAttenuation[i].y * distance
                              + lightAttenuation[i].z * distance * distance);
          // per OpenGL standard cone angle is 90 or less for a spot light
          if (lightConeAngle[i] <= 90.0){
            float coneDot = dot(vertLightDirection, lightDir);
            if (coneDot >= cos(radians(lightConeAngle[i]))){  // if inside cone
              attenuation = attenuation * pow(coneDot, lightExponent[i]);
            }
            else {
              attenuation = 0.0;
            }
          }
          ndotL = dot(normal.xyz, vertLightDirection);
          if (ndotL < 0.0 && twoSidedLighting)
          {
            ndotL = -ndotL;
          }
          if (ndotL > 0.0)
          {
            diffuse += ndotL * attenuation * lightColor[i];
            //specular
            vdotR = dot(-rayDirVC, normalize(2.0 * ndotL * -normal.xyz + vertLightDirection));
            if (vdotR > 0.0)
            {
              specular += pow(vdotR, vSpecularPower) * attenuation * lightColor[i];
            }
          }
          #ifdef localAmbientOcclusionOn
            ambient += computeLAO(posIS, tColor.a, vertLightDirection, normal);
          #endif
        } else {
          vertLightDirection = lightDirectionVC[i];
          ndotL = dot(normal.xyz, vertLightDirection);
          if (ndotL < 0.0 && twoSidedLighting)
          {
            ndotL = -ndotL;
          }
          if (ndotL > 0.0)
          {
            diffuse += ndotL * lightColor[i];
            //specular
            vdotR = dot(-rayDirVC, normalize(2.0 * ndotL * -normal.xyz + vertLightDirection));
            if (vdotR > 0.0)
            {
              specular += pow(vdotR, vSpecularPower) * lightColor[i];
            }
          }
          #ifdef localAmbientOcclusionOn
            ambient += computeLAO(posIS, tColor.a, vertLightDirection, normal);
          #endif
        }
      }
      #ifdef localAmbientOcclusionOn
        return tColor.rgb * (diffuse * vDiffuse + vAmbient * ambient) + specular*vSpecular;
      #else
        return tColor.rgb * (diffuse * vDiffuse + vAmbient) + specular*vSpecular;
      #endif
    }
  #endif
  #endif
#endif

// LAO of surface shadows and volume shadows only work with dependent components
vec3 applyAllLightning(vec3 tColor, float alpha, vec3 posIS, vec4 normalLight) {
  #if vtkLightComplexity > 0
    // surface shadows if needed
    #ifdef SurfaceShadowOn
      #if vtkLightComplexity < 3
        vec3 tColorS = applyLightingDirectional(posIS, vec4(tColor, alpha), normalLight);
      #else
        vec3 tColorS = applyLightingPositional(posIS, vec4(tColor, alpha), normalLight, IStoVC(posIS));
      #endif
    #endif

    // volume shadows if needed
    #ifdef VolumeShadowOn
      vec3 tColorVS = applyShadowRay(tColor, posIS, rayDirVC);
    #endif

    // merge
    #ifdef VolumeShadowOn
      #ifdef SurfaceShadowOn
        // surface shadows + volumetric shadows
        float vol_coef = volumetricScatteringBlending * (1.0 - alpha / 2.0) * (1.0 - atan(normalLight.w) * INV4PI);
        tColor = (1.0-vol_coef) * tColorS + vol_coef * tColorVS;
      #else
        // volumetric shadows only
        tColor = tColorVS;
      #endif
    #else
      #ifdef SurfaceShadowOn
        // surface shadows only
        tColor = tColorS;
      #else
        // no shadows
        applyLighting(tColor, normal3);
      #endif
    #endif
  #endif
  return tColor;
}

//=======================================================================
// Given a texture value compute the color and opacity
//
vec4 getColorForValue(vec4 tValue, vec3 posIS, vec3 tstep)
{
#ifdef vtkImageLabelOutlineOn
  vec3 centerPosIS = fragCoordToIndexSpace(gl_FragCoord); // pos in texture space
  vec4 centerValue = getTextureValue(centerPosIS);
  bool pixelOnBorder = false;
  vec4 tColor = texture2D(ctexture, vec2(centerValue.r * cscale0 + cshift0, 0.5));

  // Get alpha of segment from opacity function.
  tColor.a = texture2D(otexture, vec2(centerValue.r * oscale0 + oshift0, 0.5)).r;

  int segmentIndex = int(centerValue.r * 255.0);
  
  // Use texture sampling for outlineThickness
  float textureCoordinate = float(segmentIndex - 1) / 1024.0;
  float textureValue = texture2D(ttexture, vec2(textureCoordinate, 0.5)).r;

  int actualThickness = int(textureValue * 255.0);


  // If it is the background (segment index 0), we should quickly bail out. 
  // Previously, this was determined by tColor.a, which was incorrect as it
  // prevented the outline from appearing when the fill is 0.
  if (segmentIndex == 0){
    return vec4(0, 0, 0, 0);
  }

  // Only perform outline check on fragments rendering voxels that aren't invisible.
  // Saves a bunch of needless checks on the background.
  // TODO define epsilon when building shader?
  for (int i = -actualThickness; i <= actualThickness; i++) {
    for (int j = -actualThickness; j <= actualThickness; j++) {
      if (i == 0 || j == 0) {
        continue;
      }

      vec4 neighborPixelCoord = vec4(gl_FragCoord.x + float(i),
        gl_FragCoord.y + float(j),
        gl_FragCoord.z, gl_FragCoord.w);

      vec3 neighborPosIS = fragCoordToIndexSpace(neighborPixelCoord);
      vec4 value = getTextureValue(neighborPosIS);

      // If any of my neighbours are not the same value as I
      // am, this means I am on the border of the segment.
      // We can break the loops
      if (any(notEqual(value, centerValue))) {
        pixelOnBorder = true;
        break;
      }
    }

    if (pixelOnBorder == true) {
      break;
    }
  }

  // If I am on the border, I am displayed at full opacity
  if (pixelOnBorder == true) {
    tColor.a = outlineOpacity;
  }

  return tColor;

#else
  // compute the normal and gradient magnitude if needed
  // We compute it as a vec4 if possible otherwise a mat4

  #ifdef UseIndependentComponents
    // sample textures
    vec3 tColor0 = texture2D(ctexture, vec2(tValue.r * cscale0 + cshift0, height0)).rgb;
    float pwfValue0 = texture2D(otexture, vec2(tValue.r * oscale0 + oshift0, height0)).r;

    #if vtkNumComponents > 1
      vec3 tColor1 = texture2D(ctexture, vec2(tValue.g * cscale1 + cshift1, height1)).rgb;
      float pwfValue1 = texture2D(otexture, vec2(tValue.g * oscale1 + oshift1, height1)).r;

      #if vtkNumComponents > 2
        vec3 tColor2 = texture2D(ctexture, vec2(tValue.b * cscale2 + cshift2, height2)).rgb;
        float pwfValue2 = texture2D(otexture, vec2(tValue.b * oscale2 + oshift2, height2)).r;

        #if vtkNumComponents > 3
          vec3 tColor3 = texture2D(ctexture, vec2(tValue.a * cscale3 + cshift3, height3)).rgb;
          float pwfValue3 = texture2D(otexture, vec2(tValue.a * oscale3 + oshift3, height3)).r;
        #endif
      #endif
    #endif

    #if !defined(vtkCustomComponentsColorMix)
      // default path for component color mix

      // compute the normal vectors as needed
      #if (vtkLightComplexity > 0) || defined(vtkGradientOpacityOn)
        mat4 normalMat = computeMat4Normal(posIS, tValue, tstep);
      #endif

      // compute gradient opacity factors as needed
      vec4 goFactor = vec4(1.0, 1.0 ,1.0 ,1.0);
      #if defined(vtkGradientOpacityOn)
        #if !defined(vtkComponent0Proportional)
          goFactor.x =
            computeGradientOpacityFactor(normalMat[0].a, goscale0, goshift0, gomin0, gomax0);
        #endif
        #if vtkNumComponents > 1
          #if !defined(vtkComponent1Proportional)
            goFactor.y =
              computeGradientOpacityFactor(normalMat[1].a, goscale1, goshift1, gomin1, gomax1);
          #endif
          #if vtkNumComponents > 2
            #if !defined(vtkComponent2Proportional)
              goFactor.z =
                computeGradientOpacityFactor(normalMat[2].a, goscale2, goshift2, gomin2, gomax2);
            #endif
            #if vtkNumComponents > 3
              #if !defined(vtkComponent3Proportional)
                goFactor.w =
                  computeGradientOpacityFactor(normalMat[3].a, goscale3, goshift3, gomin3, gomax3);
              #endif
            #endif
          #endif
        #endif
      #endif

      // process color and opacity for each component
      #if !defined(vtkComponent0Proportional)
        float alpha = goFactor.x*mix0*pwfValue0;
        #if vtkLightComplexity > 0
          applyLighting(tColor0, normalMat[0]);
        #endif
      #else
        tColor0 *= pwfValue0;
        float alpha = mix(pwfValue0, 1.0, (1.0 - mix0));
      #endif

      #if vtkNumComponents > 1
        #if !defined(vtkComponent1Proportional)
          alpha += goFactor.y*mix1*pwfValue1;
          #if vtkLightComplexity > 0
            applyLighting(tColor1, normalMat[1]);
          #endif
        #else
          tColor1 *= pwfValue1;
          alpha *= mix(pwfValue1, 1.0, (1.0 - mix1));
        #endif

        #if vtkNumComponents > 2
          #if !defined(vtkComponent2Proportional)
            alpha += goFactor.z*mix2*pwfValue2;
            #if vtkLightComplexity > 0
              applyLighting(tColor2, normalMat[2]);
            #endif
          #else
            tColor2 *= pwfValue2;
            alpha *= mix(pwfValue2, 1.0, (1.0 - mix2));
          #endif
        #endif

        #if vtkNumComponents > 3
          #if !defined(vtkComponent3Proportional)
            alpha += goFactor.w*mix3*pwfValue3;
            #if vtkLightComplexity > 0
              applyLighting(tColor3, normalMat[3]);
            #endif
          #else
            tColor3 *= pwfValue3;
            alpha *= mix(pwfValue3, 1.0, (1.0 - mix3));
          #endif
        #endif
      #endif

      // perform final independent blend
      vec3 tColor = mix0 * tColor0;
      #if vtkNumComponents > 1
        tColor += mix1 * tColor1;
        #if vtkNumComponents > 2
          tColor += mix2 * tColor2;
          #if vtkNumComponents > 3
            tColor += mix3 * tColor3;
          #endif
        #endif
      #endif

      return vec4(tColor, alpha);
    #else
      /*
       * Mix the color information from all the independent components to get a single rgba output
       * Gradient opactity factors and normals are not computed
       *
       * You can compute these using:
       * - computeMat4Normal: always available, compute normal only for non proportional components, used by default independent component mix
       * - computeDensityNormal & computeNormalForDensity: available if ((LightComplexity > 0) || GradientOpacityOn) && ComputeNormalFromOpacity),
       *                                                   used by dependent component color mix, see code for Additive preset in OpenGl/VolumeMapper
       * - computeGradientOpacityFactor: always available, used in a lot of places
       *
       * Using applyAllLightning() is advised for shading but some features don't work well with it (volume shadows, LAO)
       * mix0, mix1, ... are defined for each component that is used and correspond to the componentWeight
       */
      //VTK::CustomComponentsColorMix::Impl
    #endif
  #else
    // dependent components

    // compute normal if needed
    #if (vtkLightComplexity > 0) || defined(vtkGradientOpacityOn)
      // use component 3 of the opacity texture as getTextureValue() sets alpha to the opacity value
      #ifdef vtkComputeNormalFromOpacity
        vec3 scalarInterp[2];
        vec4 normal0 = computeNormalForDensity(posIS, tstep, scalarInterp, 3);
      #else
        vec4 normal0 = computeNormal(posIS, tstep);
      #endif
    #endif

    // compute gradient opacity factor enabled
    #if defined(vtkGradientOpacityOn)
      float gradientOpacity = computeGradientOpacityFactor(normal0.a, goscale0, goshift0, gomin0, gomax0);
    #else
      const float gradientOpacity = 1.0;
    #endif

    // get color and opacity
    #if vtkNumComponents == 1
      vec3 tColor = texture2D(ctexture, vec2(tValue.r * cscale0 + cshift0, 0.5)).rgb;
      float alpha = gradientOpacity*texture2D(otexture, vec2(tValue.r * oscale0 + oshift0, 0.5)).r;
      if (alpha < EPSILON){
        return vec4(0.0);
      }
    #endif
    #if vtkNumComponents == 2
      vec3 tColor = vec3(tValue.r * cscale0 + cshift0);
      float alpha = gradientOpacity*texture2D(otexture, vec2(tValue.a * oscale1 + oshift1, 0.5)).r;
    #endif
    #if vtkNumComponents == 3
      vec3 tColor;
      tColor.r = tValue.r * cscale0 + cshift0;
      tColor.g = tValue.g * cscale1 + cshift1;
      tColor.b = tValue.b * cscale2 + cshift2;
      float alpha = gradientOpacity*texture2D(otexture, vec2(tValue.a * oscale0 + oshift0, 0.5)).r;
    #endif
    #if vtkNumComponents == 4
      vec3 tColor;
      tColor.r = tValue.r * cscale0 + cshift0;
      tColor.g = tValue.g * cscale1 + cshift1;
      tColor.b = tValue.b * cscale2 + cshift2;
      float alpha = gradientOpacity*texture2D(otexture, vec2(tValue.a * oscale3 + oshift3, 0.5)).r;
    #endif

    // lighting
    #if (vtkLightComplexity > 0)
      #ifdef vtkComputeNormalFromOpacity
        vec4 normalLight;
        if (!all(equal(normal0, vec4(0.0)))) {
          scalarInterp[0] = scalarInterp[0] * oscale0 + oshift0;
          scalarInterp[1] = scalarInterp[1] * oscale0 + oshift0;
          normalLight = computeDensityNormal(scalarInterp, 0.5, gradientOpacity);
          if (all(equal(normalLight, vec4(0.0)))) {
            normalLight = normal0;
          }
        }
      #else
        vec4 normalLight = normal0;
      #endif
      tColor = applyAllLightning(tColor, alpha, posIS, normalLight);
    #endif

    return vec4(tColor, alpha);
  #endif // dependent
#endif
}

bool valueWithinScalarRange(vec4 val, vec4 min, vec4 max) {
  bool withinRange = false;
  #if vtkNumComponents == 1
    if (val.r >= min.r && val.r <= max.r) {
      withinRange = true;
    }
  #else
    #ifdef UseIndependentComponents
      #if vtkNumComponents == 2
        if (val.r >= min.r && val.r <= max.r &&
            val.g >= min.g && val.g <= max.g) {
          withinRange = true;
        }
      #else
        if (all(greaterThanEqual(val, ipScalarRangeMin)) &&
            all(lessThanEqual(val, ipScalarRangeMax))) {
          withinRange = true;
        }
      #endif
    #endif
  #endif
  return withinRange;
}

//=======================================================================
// Apply the specified blend mode operation along the ray's path.
//
void applyBlend(vec3 posIS, vec3 endIS, vec3 tdims)
{
  vec3 tstep = 1.0/tdims;

  // start slightly inside and apply some jitter
  vec3 delta = endIS - posIS;
  vec3 stepIS = normalize(delta)*sampleDistanceIS;
  float raySteps = length(delta)/sampleDistanceIS;

  // avoid 0.0 jitter
  float jitter = 0.01 + 0.99*texture2D(jtexture, gl_FragCoord.xy/32.0).r;
  float stepsTraveled = jitter;

  // local vars for the loop
  vec4 color = vec4(0.0, 0.0, 0.0, 0.0);
  vec4 tValue;
  vec4 tColor;

  // if we have less than one step then pick the middle point
  // as our value
  // if (raySteps <= 1.0)
  // {
  //   posIS = (posIS + endIS)*0.5;
  // }

  // Perform initial step at the volume boundary
  // compute the scalar
  tValue = getTextureValue(posIS);

  #if vtkBlendMode == 0 // COMPOSITE_BLEND
    // now map through opacity and color
    tColor = getColorForValue(tValue, posIS, tstep);

    // handle very thin volumes
    if (raySteps <= 1.0)
    {
      tColor.a = 1.0 - pow(1.0 - tColor.a, raySteps);
      gl_FragData[0] = tColor;
      return;
    }

    tColor.a = 1.0 - pow(1.0 - tColor.a, jitter);
    color = vec4(tColor.rgb*tColor.a, tColor.a);
    posIS += (jitter*stepIS);

    for (int i = 0; i < //VTK::MaximumSamplesValue ; ++i)
    {
      if (stepsTraveled + 1.0 >= raySteps) { break; }

      // compute the scalar
      tValue = getTextureValue(posIS);

      // now map through opacity and color
      tColor = getColorForValue(tValue, posIS, tstep);

      float mix = (1.0 - color.a);

      // this line should not be needed but nvidia seems to not handle
      // the break correctly on windows/chrome 58 angle
      //mix = mix * sign(max(raySteps - stepsTraveled - 1.0, 0.0));

      color = color + vec4(tColor.rgb*tColor.a, tColor.a)*mix;
      stepsTraveled++;
      posIS += stepIS;
      if (color.a > 0.99) { color.a = 1.0; break; }
    }

    if (color.a < 0.99 && (raySteps - stepsTraveled) > 0.0)
    {
      posIS = endIS;

      // compute the scalar
      tValue = getTextureValue(posIS);

      // now map through opacity and color
      tColor = getColorForValue(tValue, posIS, tstep);
      tColor.a = 1.0 - pow(1.0 - tColor.a, raySteps - stepsTraveled);

      float mix = (1.0 - color.a);
      color = color + vec4(tColor.rgb*tColor.a, tColor.a)*mix;
    }

    gl_FragData[0] = vec4(color.rgb/color.a, color.a);
  #endif
  #if vtkBlendMode == 1 || vtkBlendMode == 2
    // MAXIMUM_INTENSITY_BLEND || MINIMUM_INTENSITY_BLEND
    // Find maximum/minimum intensity along the ray.

    // Define the operation we will use (min or max)
    #if vtkBlendMode == 1
    #define OP max
    #else
    #define OP min
    #endif

    // If the clipping range is shorter than the sample distance
    // we can skip the sampling loop along the ray.
    if (raySteps <= 1.0)
    {
      gl_FragData[0] = getColorForValue(tValue, posIS, tstep);
      return;
    }

    vec4 value = tValue;
    posIS += (jitter*stepIS);

    // Sample along the ray until MaximumSamplesValue,
    // ending slightly inside the total distance
    for (int i = 0; i < //VTK::MaximumSamplesValue ; ++i)
    {
      // If we have reached the last step, break
      if (stepsTraveled + 1.0 >= raySteps) { break; }

      // compute the scalar
      tValue = getTextureValue(posIS);

      // Update the maximum value if necessary
      value = OP(tValue, value);

      // Otherwise, continue along the ray
      stepsTraveled++;
      posIS += stepIS;
    }

    // Perform the last step along the ray using the
    // residual distance
    posIS = endIS;
    tValue = getTextureValue(posIS);
    value = OP(tValue, value);

    // Now map through opacity and color
    gl_FragData[0] = getColorForValue(value, posIS, tstep);
  #endif
  #if vtkBlendMode == 3 || vtkBlendMode == 4 //AVERAGE_INTENSITY_BLEND || ADDITIVE_BLEND
    vec4 sum = vec4(0.);

    if (valueWithinScalarRange(tValue, ipScalarRangeMin, ipScalarRangeMax)) {
      sum += tValue;
    }

    if (raySteps <= 1.0) {
      gl_FragData[0] = getColorForValue(sum, posIS, tstep);
      return;
    }

    posIS += (jitter*stepIS);

    // Sample along the ray until MaximumSamplesValue,
    // ending slightly inside the total distance
    for (int i = 0; i < //VTK::MaximumSamplesValue ; ++i)
    {
      // If we have reached the last step, break
      if (stepsTraveled + 1.0 >= raySteps) { break; }

      // compute the scalar
      tValue = getTextureValue(posIS);

      // One can control the scalar range by setting the AverageIPScalarRange to disregard scalar values, not in the range of interest, from the average computation.
      // Notes:
      // - We are comparing all values in the texture to see if any of them
      //   are outside of the scalar range. In the future we might want to allow
      //   scalar ranges for each component.
      if (valueWithinScalarRange(tValue, ipScalarRangeMin, ipScalarRangeMax)) {
        // Sum the values across each step in the path
        sum += tValue;
      }
      stepsTraveled++;
      posIS += stepIS;
    }

    // Perform the last step along the ray using the
    // residual distance
    posIS = endIS;

    // compute the scalar
    tValue = getTextureValue(posIS);

    // One can control the scalar range by setting the IPScalarRange to disregard scalar values, not in the range of interest, from the average computation
    if (valueWithinScalarRange(tValue, ipScalarRangeMin, ipScalarRangeMax)) {
      sum += tValue;

      stepsTraveled++;
    }

    #if vtkBlendMode == 3 // Average
      sum /= vec4(stepsTraveled, stepsTraveled, stepsTraveled, 1.0);
    #endif

    gl_FragData[0] = getColorForValue(sum, posIS, tstep);
  #endif
  #if vtkBlendMode == 5 // RADON
    float normalizedRayIntensity = 1.0;

    // handle very thin volumes
    if (raySteps <= 1.0)
    {
      tValue = getTextureValue(posIS);
      normalizedRayIntensity = normalizedRayIntensity - sampleDistance*texture2D(otexture, vec2(tValue.r * oscale0 + oshift0, 0.5)).r;
      gl_FragData[0] = texture2D(ctexture, vec2(normalizedRayIntensity, 0.5));
      return;
    }

    posIS += (jitter*stepIS);

    for (int i = 0; i < //VTK::MaximumSamplesValue ; ++i)
    {
      if (stepsTraveled + 1.0 >= raySteps) { break; }

      // compute the scalar value
      tValue = getTextureValue(posIS);

      // Convert scalar value to normalizedRayIntensity coefficient and accumulate normalizedRayIntensity
      normalizedRayIntensity = normalizedRayIntensity - sampleDistance*texture2D(otexture, vec2(tValue.r * oscale0 + oshift0, 0.5)).r;

      posIS += stepIS;
      stepsTraveled++;
    }

    // map normalizedRayIntensity to color
    gl_FragData[0] = texture2D(ctexture, vec2(normalizedRayIntensity , 0.5));

  #endif
}

//=======================================================================
// Compute a new start and end point for a given ray based
// on the provided bounded clipping plane (aka a rectangle)
void getRayPointIntersectionBounds(
  vec3 rayPos, vec3 rayDir,
  vec3 planeDir, float planeDist,
  inout vec2 tbounds, vec3 vPlaneX, vec3 vPlaneY,
  float vSize1, float vSize2)
{
  float result = dot(rayDir, planeDir);
  if (abs(result) < 1e-6)
  {
    return;
  }
  result = -1.0 * (dot(rayPos, planeDir) + planeDist) / result;
  vec3 xposVC = rayPos + rayDir*result;
  vec3 vxpos = xposVC - vOriginVC;
  vec2 vpos = vec2(
    dot(vxpos, vPlaneX),
    dot(vxpos, vPlaneY));

  // on some apple nvidia systems this does not work
  // if (vpos.x < 0.0 || vpos.x > vSize1 ||
  //     vpos.y < 0.0 || vpos.y > vSize2)
  // even just
  // if (vpos.x < 0.0 || vpos.y < 0.0)
  // fails
  // so instead we compute a value that represents in and out
  //and then compute the return using this value
  float xcheck = max(0.0, vpos.x * (vpos.x - vSize1)); //  0 means in bounds
  float check = sign(max(xcheck, vpos.y * (vpos.y - vSize2))); //  0 means in bounds, 1 = out

  tbounds = mix(
   vec2(min(tbounds.x, result), max(tbounds.y, result)), // in value
   tbounds, // out value
   check);  // 0 in 1 out
}

//=======================================================================
// given a
// - ray direction (rayDir)
// - starting point (vertexVCVSOutput)
// - bounding planes of the volume
// - optionally depth buffer values
// - far clipping plane
// compute the start/end distances of the ray we need to cast
vec2 computeRayDistances(vec3 rayDir, vec3 tdims)
{
  vec2 dists = vec2(100.0*camFar, -1.0);

  vec3 vSize = vSpacing*tdims;

  // all this is in View Coordinates
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal0, vPlaneDistance0, dists, vPlaneNormal2, vPlaneNormal4,
    vSize.y, vSize.z);
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal1, vPlaneDistance1, dists, vPlaneNormal2, vPlaneNormal4,
    vSize.y, vSize.z);
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal2, vPlaneDistance2, dists, vPlaneNormal0, vPlaneNormal4,
    vSize.x, vSize.z);
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal3, vPlaneDistance3, dists, vPlaneNormal0, vPlaneNormal4,
    vSize.x, vSize.z);
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal4, vPlaneDistance4, dists, vPlaneNormal0, vPlaneNormal2,
    vSize.x, vSize.y);
  getRayPointIntersectionBounds(vertexVCVSOutput, rayDir,
    vPlaneNormal5, vPlaneDistance5, dists, vPlaneNormal0, vPlaneNormal2,
    vSize.x, vSize.y);

  //VTK::ClipPlane::Impl

  // do not go behind front clipping plane
  dists.x = max(0.0,dists.x);

  // do not go PAST far clipping plane
  float farDist = -camThick/rayDir.z;
  dists.y = min(farDist,dists.y);

  // Do not go past the zbuffer value if set
  // This is used for intermixing opaque geometry
  //VTK::ZBuffer::Impl

  return dists;
}

//=======================================================================
// Compute the index space starting position (pos) and end
// position
//
void computeIndexSpaceValues(out vec3 pos, out vec3 endPos, vec3 rayDir, vec2 dists)
{
  // compute starting and ending values in volume space
  pos = vertexVCVSOutput + dists.x*rayDir;
  pos = pos - vOriginVC;
  // convert to volume basis and origin
  pos = vec3(
    dot(pos, vPlaneNormal0),
    dot(pos, vPlaneNormal2),
    dot(pos, vPlaneNormal4));

  endPos = vertexVCVSOutput + dists.y*rayDir;
  endPos = endPos - vOriginVC;
  endPos = vec3(
    dot(endPos, vPlaneNormal0),
    dot(endPos, vPlaneNormal2),
    dot(endPos, vPlaneNormal4));

  float delta = length(endPos - pos);

  pos *= vVCToIJK;
  endPos *= vVCToIJK;

  float delta2 = length(endPos - pos);
  sampleDistanceIS = sampleDistance*delta2/delta;
  #ifdef VolumeShadowOn
    sampleDistanceISVS = sampleDistanceIS * volumeShadowSamplingDistFactor;
  #endif
}

void main()
{

  if (cameraParallel == 1)
  {
    // Camera is parallel, so the rayDir is just the direction of the camera.
    rayDirVC = vec3(0.0, 0.0, -1.0);
  } else {
    // camera is at 0,0,0 so rayDir for perspective is just the vc coord
    rayDirVC = normalize(vertexVCVSOutput);
  }

  vec3 tdims = vec3(volumeDimensions);

  // compute the start and end points for the ray
  vec2 rayStartEndDistancesVC = computeRayDistances(rayDirVC, tdims);

  // do we need to composite? aka does the ray have any length
  // If not, bail out early
  if (rayStartEndDistancesVC.y <= rayStartEndDistancesVC.x)
  {
    discard;
  }

  // IS = Index Space
  vec3 posIS;
  vec3 endIS;
  computeIndexSpaceValues(posIS, endIS, rayDirVC, rayStartEndDistancesVC);

  // Perform the blending operation along the ray
  applyBlend(posIS, endIS, tdims);
}

vtkVolumeVS.glsl

//VTK::System::Dec

/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkPolyDataVS.glsl

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/

attribute vec4 vertexDC;

varying vec3 vertexVCVSOutput;
uniform mat4 PCVCMatrix;

uniform float dcxmin;
uniform float dcxmax;
uniform float dcymin;
uniform float dcymax;

void main()
{
  // dcsmall is the device coords reduced to the
  // x y area covered by the volume
  vec4 dcsmall = vec4(
    dcxmin + 0.5 * (vertexDC.x + 1.0) * (dcxmax - dcxmin),
    dcymin + 0.5 * (vertexDC.y + 1.0) * (dcymax - dcymin),
    vertexDC.z,
    vertexDC.w);
  vec4 vcpos = PCVCMatrix * dcsmall;
  vertexVCVSOutput = vcpos.xyz/vcpos.w;
  gl_Position = dcsmall;
}