you could define a sun angle and look from each vertex of the model where the trace (direction is the angle) it hits the ground or trace from each pixel with the angle so just like this
Code:
vec_set(temp.x,vector(10000,0,0));
vec_rotate(temp.x,sunangle);
vec_add(temp.x,pixelpos);
trace(pixelpos,temp.x,....);
if result==0 or you==sky
no shadow on this pixel
