shared attribute vec4 VERTEX_POSITION;
                
void main(void) {
  /*
    we do our own projection to form the paraboloid, so we
    can ignore the projection matrix entirely.
   */
  vec4 pos = mvMatrix * VERTEX_POSITION;

  pos = vec4(pos.xyz / pos.w, pos.w);

  pos.z *= DP_DIRECTION;

  float L = length(pos.xyz);
  pos /= L;
  vClip = pos.z;

  pos.z += 1.0;
  pos.x /= pos.z;
  pos.y /= pos.z;
  pos.z = (L - DP_SHADOW_NEAR) / (DP_SHADOW_FAR - DP_SHADOW_NEAR);
  pos.w = 1.0;

  vPos = pos;
  export(vec4, Position, pos);
  gl_Position = pos;
}