Ray Tracing:  Ray t racing is the technique to determine which objects in the scene are visible.  In this to identify the visible surface for each pixel we continue to bounce the ray around the scene and collecting intensity contributions. This provides a simple and powerful rendering technique for obtaining global reflection and transmission effects. The basic ray tracing algorithm also provides for visible surface detection shadow effects transparency and multiple light source illumination.  For each scan line the intensity at the interaction of the scan line with a polygon edge is linearly interpolated from the intensities at the edge end points. For example the polygon edge with end point vertices at position 1 & 2 is intersected by the scan line at point 4. A fast method for obtaining the intensity at point 4 is to interpolate lower intensities I₁ & I₂ using only the vertical displacement of the scan line.