What does "//VTK::Light::Impl" mean?

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1

Hey all,

Sorry if this is a really simple question, I am working with Paraview (Paraview uses the VTK toolkit) and attempting to do a replacement shader, currently my code looks like this

{
  "type": "fragment",
  "original": "//VTK::Light::Impl",
  "replacement": "gl_FragData[0] = vec4(1.0,0.0,0.0,1.0);"
}

I understand that what I am doing is replacing (I think the main function?) of VTK::Light but I am not really too sure what VTK::Light is. Is there any source code for that and how would I find it?

2

Hi @shawn,

As you’ve noticed VTK’s mappers use a dynamic shader generation approach where we start with an empty shader file with some pre-defined tags like //VTK::Light::Impl.

A quick way to dump out the shader code at run time is to introduce an error, like a syntax error.

So, something like

{
  "type": "vertex",
  "original": "//VTK::Picking::Impl",
  "replacement": "My syntax error;
  //VTK::Picking::Impl"
}

would yield

ERROR: In vtkShaderProgram.cxx, line 427
vtkShaderProgram (000001807CFD0A90): 1: #version 150
2: #ifndef GL_ES
3: #define highp
4: #define mediump
5: #define lowp
6: #define texelFetchBuffer texelFetch
7: #endif // GL_ES
8: #define attribute in
9: #define varying out
10: 
11: 
12: // SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
13: // SPDX-License-Identifier: BSD-3-Clause
14: 
15: in vec4 vertexMC;
16: 
17: 
18: 
19: // frag position in VC
20: out vec4 vertexVCVSOutput;
21: 
22: // optional normal declaration
23: //VTK::Normal::Dec
24: in vec3 normalMC;
25: uniform mat3 normalMatrix;
26: out vec3 normalVCVSOutput;
27: 
28: // extra lighting parameters
29: //VTK::Light::Dec
30: 
31: // Texture coordinates
32: //VTK::TCoord::Dec
33: 
34: // material property values
35: //VTK::Color::Dec
36: 
37: // clipping plane vars
38: //VTK::Clip::Dec
39: 
40: // camera and actor matrix values
41: uniform mat4 MCDCMatrix;
42: uniform mat4 MCVCMatrix;
43: 
44: // Apple Bug
45: //VTK::PrimID::Dec
46: 
47: // Value raster
48: //VTK::ValuePass::Dec
49: 
50: // picking support
51: //VTK::Picking::Dec
52: 
53: // Surface with edges on GLES 3.0
54: //VTK::EdgesGLES30::Dec
55: 
56: // PointSize on GLES 3.0
57: //VTK::PointSizeGLES30::Dec
58: 
59: // LineWidth on GLES 3.0
60: //VTK::LineWidthGLES30::Dec
61: 
62: void main()
63: {
64:   //VTK::CustomBegin::Impl
65: 
66:   //VTK::PointSizeGLES30::Impl
67: 
68:   //VTK::Color::Impl
69: 
70:   normalVCVSOutput = normalMatrix * normalMC;
71: //VTK::Normal::Impl
72: 
73:   //VTK::TCoord::Impl
74: 
75:   //VTK::Clip::Impl
76: 
77:   //VTK::PrimID::Impl
78: 
79:   vertexVCVSOutput = MCVCMatrix * vertexMC;
80:   gl_Position = MCDCMatrix * vertexMC;
81: 
82: 
83:   //VTK::LineWidthGLES30::Impl
84: 
85:   //VTK::ValuePass::Impl
86: 
87:   //VTK::Light::Impl
88: 
89:   My syntax error;
90:   //VTK::Picking::Impl
91: 
92:   //VTK::CustomEnd::Impl
93: 
94:   //VTK::EdgesGLES30::Impl
95: }


ERROR: In vtkShaderProgram.cxx, line 428
vtkShaderProgram (000001807CFD0A90): 0(89) : error C0000: syntax error, unexpected ';', expecting "::" at token ";"

That should give you an idea of what the final generated shader code looks like. If you need to drill down to what the in-built tag replacement for a particular tag is, you’d have to browse through the mapper code. For example: https://gitlab.kitware.com/vtk/vtk/-/blob/66096615a1b90be9f401bfefea77ceb4e2189fae/Rendering/OpenGL2/vtkOpenGLPolyDataMapper.cxx#L620-627 is the default replacement for //VTK::Edges::Dec.

Shaders in VTK is an old wiki page that describes the logic behind these tags.

3

This is amazing information, thank you so much. I will be going through this and asking any questions I have after.

4

Let me see if I did this correctly, would the replacement tag for //VTK::Light::Dec be and this would be considered all the code in VTK::Light::Impl?

https://gitlab.kitware.com/vtk/vtk/-/blob/66096615a1b90be9f401bfefea77ceb4e2189fae/Rendering/OpenGL2/vtkOpenGLPolyDataMapper.cxx#L1104-1465 ?