uniform vec2 size;
uniform vec3 center;

vec3 billboard(vec3 v, mat4 view) {
    vec3 up = vec3(view[0][1], view[1][1], view[2][1]);
    vec3 right = vec3(view[0][0], view[1][0], view[2][0]);
    vec3 pos = center + right * v.x * size.x + up * v.y * size.y;
    return pos;
}

void main() {
   vec3 worldPos = billboard(position, viewMatrix);
   gl_Position = projectionMatrix * modelViewMatrix * vec4(worldPos, 1.0);
}