How to add attributes in Shader？

Web
1

For example in vtkPolyDataVS.glsl
attribute vec4 vertexMC;
I want to add another attribute like vertexMC;
These operations need to be placed in the shader.
I now call back the code data based on the PloydataMapper-CallBack. I think this processing can add uniform and attribute to the shader in Mapper; how to add attribute I have no clue, I just found these codes,

cellBO
.getVAO()
// VertexArrayObject
.addAttributeArray(
cellBO.getProgram(),
cellBO.getCABO(),
‘vertexMC’,
cellBO.getCABO().getVertexOffset(),
cellBO.getCABO().getStride(),
model.context.FLOAT,
3,
model.context.FALSE
);

but I can’t find where I add the data.
I hope you can give me some suggestions and methods, I look forward to your reply!

2

All the shader replacements happen before any attribute values are passed by the mapper. You should be able add a replacement for one of the Dec tags like:

 mapperViewProp.addShaderReplacements(
        'Vertex',
        '//VTK::PositionVC::Dec',
        true,
        '//VTK::PositionVC::Dec\n  attribute vec4 vertexMCN;\n',
        false
      );

As far as passing data for the attribute, you’d have to access the VAO and call addAttributeArray

3

Thank for your reply.
My idea is similar to what you think. I use mapperViewProp.addShaderReplacements to replace code in shaders.And,Get VAO by using PloydataMapper CallBack, and then call addAttributeArray.
Like this:

const mapperSpecificProp = mapper.getViewSpecificProperties();
mapperSpecificProp.ShaderCallbacks = [];
mapperSpecificProp.ShaderCallbacks.push({
	userData: ...,
	callback: function(userData, cellBO, ren, actor, model) {
		cellBO
        .getVAO()
        // VertexArrayObject
        .addAttributeArray(
        cellBO.getProgram(),
        cellBO.getCABO(),
        ‘vertexMC’,
        cellBO.getCABO().getVertexOffset(),
        cellBO.getCABO().getStride(),
        model.context.FLOAT,
        3,
        model.context.FALSE
        );
	}
});

But for passing data for the attribute, I did not find the corresponding parameter in addAttributeArray.
Maybe I don’t understand the code well enough. I hope you can give me some clear suggestions.
Looking forward to your reply!

4

I looked up the definition of VAO (details on how the data is stored and how to use it), if you want to import data you need to use a VBO (the vertex buffer object is responsible for the actual data storage).
The specific details are being advanced. If you have any better suggestions, I hope to share and discuss together!
Looking forward to your reply!

5

Right, they might be advanced topics but there are some good tutorials that explain the inner workings like Tutorial 32 - Vertex Array Objects

6

Thank you for your reply, I would like to ask if you have added attributes to Shader in VTK before?

7

Yes, there are attributes in both the polydata and volume mappers. If you’re asking about adding attributes via replacement shaders, I don’t think there is an existing example of it.

8

Thanks for your help this time, I can already customize the shader!