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

I was stuck in the same problem too, but I found a tutorial that explain how to perform a 2d texture rotation in the same fragment shader with only with passing the rotate value (vRotation).

#version 130

uniform sampler2D tex;
varying float vRotation;
void main(void)
{

    float mid = 0.5;
    vec2 rotated = vec2(cos(vRotation) * (gl_PointCoord.x - mid) + sin(vRotation) * (gl_PointCoord.y - mid) + mid,
                        cos(vRotation) * (gl_PointCoord.y - mid) - sin(vRotation) * (gl_PointCoord.x - mid) + mid);

    vec4 rotatedTexture=texture2D(tex, rotated);
    gl_FragColor =  gl_Color * rotatedTexture;
}

Maybe this method is slow but is only to prove/show that you have an alternative to perform a texture 2D rotation inside fragment shader instead of passing a Matrix.

Note: vRotation should be in Radians.

Cheers,