Moving/rotating shapes in the vertex shader

Question

I'm writing a program that draws a number of moving/rotating polygons using OpenGL. Each polygon has a location in world coordinates while its vertices are expressed in local coordinates (relative to polygon location). Each polygon also has a rotation.

The only way I can think of doing this is calculate vertex positions by translation/rotation in each frame and push them to the GPU be drawn, but I was wondering if this could be performed in the vertex shader.

I thought I might express vertex locations in local coordinates and then add location and rotation attributes to each vertex, but then it occurred to me that this won't be any better than pushing new vertex positions on each frame.

Should I do this kind of calculation on the CPU, or is there a way to do it efficiently in the vertex shader?

Bartvbl Bartvbl · Accepted Answer · 2014-05-28T11:16:10

The vertex shader is indeed responsible for transforming your geometry. However, the vertex shader is run for every single vertex of your scene. If you do transformations inside the vertex shader, you'll do the same calculation over and over again which yields the same result every time (as opposed to simply multiplying the model view projection matrix with the vertex coordinate). So in terms of efficiency you're best off doing that on the CPU side.

If the models are small, like in your case, I don't expect there to be too much of a difference, because you still have to set the coordinates where the polygons are supposed to be drawn somehow. In this case doing the calculations once on the CPU side is still the best, given that it does the calculation once independent of the vertex count of your polygons, as well as that it will probably result in clearer code since it's easier to see what you're doing.

Moving/rotating shapes in the vertex shader

2 Answers