如何在顶点着色器外部将顶点与模型矩阵相乘

Question

I am using OpenGL ES2 to render a fairly large number of mostly 2d items and so far I have gotten away by sending a premultiplied model/view/projection matrix to the vertex shader as a uniform and then multiplying my vertices with the resulting MVP in there.

All items are batched using texture atlases and I use one MVP per batch. So all my vertices are relative to the translation of that MVP.

Now I want to have rotation and scaling for each of the separate items, which means I need a different model for each of them. So I modified my vertex to include the model (16 floats!) and added a mat4 attribute in my shader and it all works well. But I'm kinda dissapointed with this solution since it dramatically increased the vertex size.

So as I was staring at my screen trying to think of a different solution I thought about transforming my vertices to world space before I send them over to the shader. Or even to screen space if its possible. The vertices I use are unnormalized coordinates in pixels.

So the question is, is such a thing possible? And if yes how do you do it? I can't think why it shouldn't be since its just maths but after a fairly long search on google, it doesn't look like a lot of people are actually doing this...

Strange cause if it is indeed possible, it would be quite a major optimization in cases like this one.

Answer 1

If the number of matrices per batch are limited then you can pass all those matrices as uniforms (preferably in a UBO) and expand the vertex data with an index which specifies which matrix you need to use.

This is similar to GPU skinning used for skeletal animation .