uniform mat4  CameraRotation; // From camera space to world space.
uniform float Fovy;
uniform float Aspect;

layout (location = 0) in vec2 Position;

out vec3 Ray;

// We would typically normalize the vector, but there is no need to do so when
// sampling a cube map.
vec3 sky_ray(vec2 FilmPosition)
{
  //float d = 0.5 / tan(Fovy/2.0);
  // return vec3((FilmPosition.x - 0.5) * Aspect,
  //              FilmPosition.y - 0.5,
  //              -d);
  float d = 0.5 / tan(Fovy/2.0);
  return vec3(FilmPosition, -d);
}

void main()
{
  vec2 FilmPosition = Position * 0.5; // map [-1,1] -> [-1/2, +1/2]
  Ray = mat3(CameraRotation) * sky_ray(FilmPosition);
  // Set z to the background.
  gl_Position = vec4(Position, 0.99999, 1.0); // z=1 -> send to background
}