// Compute the ray direction for a given pixel in the image plane.
// Arguments:
// i, j - pixel coordinates in the image plane (int)
// llc - lower-left corner of the image plane (vec3)
// px, py - plane vectors in the x-direction and y-direction (vec3)
// res - resolution (x,y) of the image plane (vec2)
// cop - center of projection (vec3)
// Returns:
// ray direction vector(vec3)
raydirFunc = function(i, j, llc, px, py, res, cop){
return vec3.fromValues(0,0,0);
};