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

The traditional method is to pass a matrix to the shader, whether vertex or fragment. If you don't know how to fill in a rotation matrix, Google and Wikipedia can help.

The main thing is that you're going to run into is the simple fact that a 2D rotation is not enough. gl_PointCoord goes from [0, 1]. A pure rotation matrix rotates around the origin, which is the bottom-left in point-coord space. So you need more than a pure rotation matrix.

You need a 3x3 matrix, which has part rotation and part translation. This matrix should be generated as follows (using GLM for math stuff):

glm::mat4 currMat(1.0f);
currMat = glm::translate(currMat, glm::vec3(0.5f, 0.5f, 0.0f));
currMat = glm::rotate(currMat, angle, glm::vec3(0.0f, 0.0f, 1.0f));
currMat = glm::translate(currMat, glm::vec3(-0.5f, -0.5f, 0.0f));

You then pass currMat to the shader as a 4x4 matrix. Your shader does this:

vec2 texCoord = (rotMatrix * vec4(gl_PointCoord, 0, 1)).xy
gl_FragColor = texture2D(texture, texCoord) * f_color;

I'll leave it as an exercise for you as to how to move the translation from the fourth column into the third, and how to pass it as a 3x3 matrix. Of course, in that case, you'll do vec3(gl_PointCoord, 1) for the matrix multiply.