OpenGL point sprites rotation in fragment shader

Question

I\'m following this tutorial to learn something more about OpenGL and in particular point sprites. But I\'m stuck on one of the exercises at the end of the page:

Answer 1

You're right - a 2x2 rotation matrix will do what you want.

This page: http://www.cg.info.hiroshima-cu.ac.jp/~miyazaki/knowledge/teche31.html shows how to compute the elements. Note that you will be rotating the texture coordinates, not the vertex positions - the result will probably not be what you're expecting - it will rotate around the 0,0 texture coordinate, for example.

You may alse need to multiply the point_size by 2 and shrink the gl_PointCoord by 2 to ensure the whole texture fits into the point sprite when it's rotated. But do that as a second change. Note that a straight scale of texture coordinates move them towards the texture coordinate origin, not the middle of the sprite.

If you use a higher dimension matrix (3x3) then you will be able to combine the offset, scale and rotation into one operation.