# Strange Shader Behavior With VTK and WebAssembly
**URL:** https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183
**Category:** Web
**Created:** [September 10, 2020, 12:48am UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183 "2020-09-10T00:48:48Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 12:48am UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/1 "2020-09-10T00:48:48Z")
I have some code that reads a .vtp file with vtkXMLPolyDataReader that contains lines. I can run the code and see the lines with my desktop app, but when I compile it to WebAssembly I get the following error when running in the browser:
> ERROR: In /work/src/Rendering/OpenGL2/vtkShaderProgram.cxx, line 498
>
> put\_char @ SwisRadar.js:2270
>
> SwisRadar.js:2270 vtkShaderProgram (0x705458): Links failed: FRAGMENT varying clipDistancesGSOutput does not match any VERTEX varying
>
> put\_char @ SwisRadar.js:2270
I have attached the .vtp file I am using as a zipped file.[koax\_colines.zip](https://discourse.vtk.org/uploads/short-url/8EcUt7tjMw6n2UFLRcyieEaDEm9.zip) (3.5 MB)
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 1:06am UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/2 "2020-09-10T01:06:18Z")
I have discovered that if I don’t add my clipping planes collection (2 clipping planes) to the vtkPolyDataMapper the lines render correctly. Looks like a problem with using clipping planes in WebAssembly VTK.
Firefox is giving a little more descriptive error:
> ERROR: In /work/src/Rendering/OpenGL2/vtkShaderProgram.cxx, line 498 SwisRadar.js:2270:16
> WebGL warning: linkProgram: Varying `clipDistancesGSOutput` has static-use in the frag shader, but is undeclared in the vert shader. SwisRadar.js:11104:13
> vtkShaderProgram (0x70f4e0): Links failed: Varying `clipDistancesGSOutput` has static-use in the frag shader, but is undeclared in the vert shader.
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 3:35am UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/3 "2020-09-10T03:35:44Z")
In the source code for vtkOpenGLPolyDataMapper.cxx starting at line 1913:
> // geometry shader impl
> if (GSSource.length())
> {
> vtkShaderProgram::Substitute(VSSource, “//VTK::Clip::Dec”,
> “out vec4 clipVertexMC;”);
> vtkShaderProgram::Substitute(VSSource, “//VTK::Clip::Impl”,
> " clipVertexMC = vertexMC;\n");
> vtkShaderProgram::Substitute(GSSource,
> “//VTK::Clip::Dec”,
> “uniform int numClipPlanes;\n”
> “uniform vec4 clipPlanes[6];\n”
> “in vec4 clipVertexMC;\n”
> “out float clipDistancesGSOutput[6];”);
> vtkShaderProgram::Substitute(GSSource,
> “//VTK::Clip::Impl”,
> “for (int planeNum = 0; planeNum \< numClipPlanes; planeNum++)\n”
> " {\n"
> " clipDistancesGSOutput[planeNum] = dot(clipPlanes[planeNum], clipVertexMC[i]);\n"
> " }\n");
> }
> else // vertex shader impl
> {
> vtkShaderProgram::Substitute(VSSource, “//VTK::Clip::Dec”,
> “uniform int numClipPlanes;\n”
> “uniform vec4 clipPlanes[6];\n”
> “out float clipDistancesVSOutput[6];”);
> vtkShaderProgram::Substitute(VSSource, “//VTK::Clip::Impl”,
> “for (int planeNum = 0; planeNum \< numClipPlanes; planeNum++)\n”
> " {\n"
> " clipDistancesVSOutput[planeNum] = dot(clipPlanes[planeNum], vertexMC);\n"
> " }\n");
> }
>
> ```
> vtkShaderProgram::Substitute(FSSource, "//VTK::Clip::Dec",
> "uniform int numClipPlanes;\n"
> "in float clipDistancesVSOutput[6];");
> vtkShaderProgram::Substitute(FSSource, "//VTK::Clip::Impl",
> "for (int planeNum = 0; planeNum < numClipPlanes; planeNum++)\n"
> " {\n"
> " if (clipDistancesVSOutput[planeNum] < 0.0) discard;\n"
> " }\n");
>
> ```
>
> }
It looks like the fragment shader is also expecting clipDistancesVSOutput as an input, but if a geometry shader is used it is outputting clipDistancesGSOutput. I am going to try to modify the code so the geometry shader also outputs clipDistancesVSOutput instead of clipDistancesGSOutput.
Can anybody else verify if this seems like a bug in VTK?
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 3:45pm UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/4 "2020-09-10T15:45:12Z")
I have tried modifying the code in the vktOpenGLPolyDataMapper without any changes in the result.
After further research, it looks like I need to try and use the new vtkShaderProperty class with the actor to try and find where this possible bug is.
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 8:21pm UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/5 "2020-09-10T20:21:10Z")
Below is the shader code that is causing the error. Vertex shader is first, followed by geometry shader, and then the fragment shader. Can anybody that knows glsl better than me figure this out?
> VS: //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](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._
>
> _=========================================================================_/
>
> in 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
> out vec4 clipVertexMC;
>
> // camera and actor matrix values
> uniform mat4 MCDCMatrix;
>
> // Apple Bug
> //VTK::PrimID::Dec
>
> // Value raster
> //VTK::ValuePass::Dec
>
> // picking support
> //VTK::Picking::Dec
>
> void main()
> {
> //VTK::Color::Impl
>
> //VTK::Normal::Impl
>
> //VTK::TCoord::Impl
>
> clipVertexMC = vertexMC;
>
> //VTK::PrimID::Impl
>
> gl\_Position = MCDCMatrix \* vertexMC;
>
> //VTK::ValuePass::Impl
>
> //VTK::Light::Impl
>
> //VTK::Picking::Impl
> }
>
> GS: //VTK::System::Dec
>
> /_=========================================================================_
>
> _Program: Visualization Toolkit
> Module: vtkPolyDataWideLineGS.glsl_
>
> _Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
> All rights reserved.
> See Copyright.txt or [http://www.kitware.com/Copyright.htm](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 geometry shader
>
> // VC position of this fragment
> //VTK::PositionVC::Dec
>
> // primitiveID
> //VTK::PrimID::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
>
> // Texture coordinates
> //VTK::TCoord::Dec
>
> // picking support
> //VTK::Picking::Dec
>
> // Depth Peeling Support
> //VTK::DepthPeeling::Dec
>
> // clipping plane vars
> uniform int numClipPlanes;
> uniform vec4 clipPlanes[6];
> in vec4 clipVertexMC;
> out float clipDistancesGSOutput[6];
>
> // the output of this shader
> //VTK::Output::Dec
>
> uniform vec2 lineWidthNVC;
>
> layout(lines) in;
> layout(triangle\_strip, max\_vertices = 4) out;
>
> void main()
> {
> // compute the lines direction
> vec2 normal = normalize(
> gl\_in[1].gl\_Position.xy/gl\_in[1].gl\_Position.w -
> gl\_in[0].gl\_Position.xy/gl\_in[0].gl\_Position.w);
>
> // rotate 90 degrees
> normal = vec2(-1.0_normal.y,normal.x);_
>
> _//VTK::Normal::Start_
>
> _for (int j = 0; j \< 4; j++)
> {
> int i = j/2;_
>
> _gl\_PrimitiveID = gl\_PrimitiveIDIn;_
>
> _for (int planeNum = 0; planeNum \< numClipPlanes; planeNum++)
> {
> clipDistancesGSOutput[planeNum] = dot(clipPlanes[planeNum], clipVertexMC[i]);
> }_
>
> _//VTK::Color::Impl_
>
> _//VTK::Normal::Impl_
>
> _//VTK::Light::Impl_
>
> _//VTK::TCoord::Impl_
>
> _//VTK::DepthPeeling::Impl_
>
> _//VTK::Picking::Impl_
>
> _// VC position of this fragment
> //VTK::PositionVC::Impl_
>
> _gl\_Position = vec4(
> gl\_in[i].gl\_Position.xy + (lineWidthNVC_normal)\*((j+1)%2 - 0.5)\*gl\_in[i].gl\_Position.w,
> gl\_in[i].gl\_Position.z,
> gl\_in[i].gl\_Position.w);
> EmitVertex();
> }
> EndPrimitive();
> }
>
> FS: //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](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
>
> // Camera prop
> uniform int cameraParallel;
>
> // optional color passed in from the vertex shader, vertexColor
> uniform float ambientIntensity; // the material ambient
> uniform float diffuseIntensity; // the material diffuse
> uniform float opacityUniform; // the fragment opacity
> uniform vec3 ambientColorUniform; // ambient color
> uniform vec3 diffuseColorUniform; // diffuse color
>
> // optional surface normal declaration
> //VTK::Normal::Dec
>
> // extra lighting parameters
> uniform vec3 lightColor0;
>
> // Texture maps
> //VTK::TMap::Dec
>
> // Texture coordinates
> //VTK::TCoord::Dec
>
> // picking support
> //VTK::Picking::Dec
>
> // Depth Peeling Support
> //VTK::DepthPeeling::Dec
>
> // clipping plane vars
> uniform int numClipPlanes;
> in float clipDistancesVSOutput[6];
>
> // the output of this shader
> //VTK::Output::Dec
>
> // Apple Bug
> //VTK::PrimID::Dec
>
> // handle coincident offsets
> //VTK::Coincident::Dec
>
> // Value raster
> //VTK::ValuePass::Dec
>
> // surface with edges
> //VTK::Edges::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
>
> // Apple Bug
> //VTK::PrimID::Impl
>
> for (int planeNum = 0; planeNum \< numClipPlanes; planeNum++)
> {
> if (clipDistancesVSOutput[planeNum] \< 0.0) discard;
> }
>
> //VTK::ValuePass::Impl
>
> vec3 ambientColor = ambientIntensity \* ambientColorUniform;
> vec3 diffuseColor = diffuseIntensity \* diffuseColorUniform;
> float opacity = opacityUniform;
>
> //VTK::Edges::Impl
>
> // Generate the normal if we are not passed in one
> //VTK::Normal::Impl
>
> gl\_FragData[0] = vec4(ambientColor + diffuseColor, opacity);
> //VTK::Light::Impl
>
> //VTK::TCoord::Impl
>
> if (gl\_FragData[0].a \<= 0.0)
> {
> discard;
> }
>
> gl\_FragData[0] = vec4(gl\_FragData[0].rgb\*gl\_FragData[0].a, gl\_FragData[0].a);
> gl\_FragData[1].r = gl\_FragData[0].a;
>
> //VTK::Picking::Impl
>
> // handle coincident offsets
> //VTK::Coincident::Impl
> }
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 9:04pm UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/6 "2020-09-10T21:04:58Z")
Ahhhhh. Sorry for the repeated posts, but I just discovered that WebGL doesn’t support geometry shaders! Is there an easy way to tell the vtkOpenGLPolyDataMapper to not use geometry shaders?
---
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [September 10, 2020, 10:04pm UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/7 "2020-09-10T22:04:19Z")
Ok, I finally got it to work. I put an #ifdef check for Emscripten and write an empty string to the geometry shader if it is defined.
I then ran into a problem with the vertex and fragment shader code not having the uniform numClipPlanes at the same precision. After some research I discovered that in GLES, if the uniform in the fragment shader is defined as an int, then it must be prefixed with highp.
Not sure if my changes should be included in the main VTK source or not?
---
### Author:  [@ken-martin](https://discourse.vtk.org/u/ken-martin)
#### Post date: [September 11, 2020, 1:26pm UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/8 "2020-09-11T13:26:45Z")
I’ll try catching up on this next week. We do want to incorporate fixes for shader code to make it work with WebGL (usually webgl is a bit more picky but the fixes tend to be easy)
---
### Author:  [@sunlin0oo](https://discourse.vtk.org/u/sunlin0oo)
#### Post date: [October 9, 2022, 2:49am UTC](https://discourse.vtk.org/t/strange-shader-behavior-with-vtk-and-webassembly/4183/9 "2022-10-09T02:49:09Z")
Hi, I would like to ask a few questions about shaders.
First,Did you define the parameters in the code of the shader you presented yourself?
E.g in FS ` // extra lighting parameters uniform vec3 lightColor0; // clipping plane vars uniform int numClipPlanes; in float clipDistancesVSOutput[6];`
Second,Do you know how to get vtk’s own shader to get external data? I can only replace my own shader with Polydatamapper now？
Looking forward to your reply!