如何在OpenGL中有效处理大量的每个顶点属性？

Question

The number of per vertex attributes that I need to calculate my vertex shader output is bigger than GL_MAX_VERTEX_ATTRIBS. Is there an efficient way to eg point to a number of buffers using a uniform array of indices and to access the per vertex data this way?

Answer 1

This is a hardware limitation so the short answer is no. If you consider workarounds for other ways, like using uniforms that also got limitations so that is a no way to go either.

One possible way I can think of which is rather hackish is to get the extra data from a texture. Since you can access textures from the vertex shader, but texture filtering is not supported ( you wont need it so it doesn't matter for you ). With the newer OpenGLs its possible to store rather large amount of data in textures and access them without limitation even in the vertex shader, it seems to be one way to go.

Altho with this approach there is a problem you need to face, how do you know the current index, ie which vertex it is? You can check out gl_VertexID built-in for that.

Answer 2

You could bypass the input assembler and bind the extra attributes in an SSBO or texture. Then you can use gl_VertexID in the vertex shader to get the value of index buffer entry you are currently rendering (eg: the index in the vertex data you need to read from)

So for example in a VS the following code is essentially identical (it may however have different performance characteristics depending on your hardware)

in vec3 myAttr;

void main() {
 vec3 vertexValue = myAttr;
 //etc
}

vs.

buffer myAttrBuffer {
 vec3 myAttr[];
};

void main() {
 vec3 vertexValue = myAttr[gl_VertexID];
 //etc
} 

The CPU-side binding code is different, but generally that's the concept. myAttr counts towards GL_MAX_VERTEX_ATTRIBS, but myAttrBuffer does not since it is loaded explicitly by the shader.

You could even use the same buffer object in both cases by binding with a different target.

Answer 3

If you can not absolutely limit yourself to GL_MAX_VERTEX_ATTRIBS attributes, I would advise using multi pass shaders. Redesign your code to work with data with half set of attributes in first pass, and the remaining in second pass.