gen 3d direction

spherical.comp

//x is theta in range(0, pi), y is phi in range(0, 2pi), solid angle pi/8
layout(local_size_x = 8, local_size_y = 16, local_size_z = 1) in;

//using local group for theta and phi

#define M_PI 3.141592653f

vec3 get_direction()
{
  vec2 pos = vec2(gl_LocalInvocationID.xy) / vec2(gl_WorkGroupSize.xy);

  float theta = mix(0.0f, M_PI, pos.x);         //theta is 0 at north pole, pi at south pole
  float phi   = mix(0.0f, 2.0f * M_PI, pos.y);  //phi is 0 at +x direction, PI at -x direction.

  return vec3(sin(theta) * cos(phi),  //this is right hand y-up coordinate system.
              cos(theta),
              sin(theta) * sin(phi));
}