How to rotate parts of a repeated texture at the parts pivot?

Question

I'm currently working on a flying game, which handles perlin-based island rendering completely in a fragment shader, so I'm able to zoom out as far as I want. I now want to render little details like grass as objects in-shader too, as rendering them using quads would be too performance-heavy on larger zoom levels.

My only problem with that is, that I can't seem to figure out how to handle camera rotation, so that my objects are always facing the camera like billboards. How would I achieve that?

Screenshots:

Grass without rotation

Grass with rotation

Code:

pos -= vec2(0.5);
pos = rotate(pos, CameraRotation);
pos += vec2(0.5);

vec2 fraction = vec2(fract(pos.x), fract(pos.y));
vec4 texColor = texture2D(tex, fraction);

My rotate function:

vec2 rotate(vec2 position, float degrees)
{
    float rotation = degrees * Pi / 180.0f;

    float rx = position.x * cos(rotation) - position.y * sin(rotation);
    float ry = position.x * sin(rotation) + position.y * cos(rotation);

    return vec2(rx, ry);
}

Thanks in advance for any help on this!

Answer 1

If you want the matrix to not rotate the vertices, then you need to take the model-view matrix and set the top-left 3*3 part to a 3*3 identity matrix, because that's where rotations are handled.