Ray Tracing

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For ray tracing, we represent the window on the projection plane as a
rectangular area of pseudo-pixels (PP). Each pseudo-pixel is then a small
rectangle and is identified with the location of its center.  Ray tracing will
determine the correct color to display for each pseudo-pixel using the
algorithm:

For each PP in window {
    Find RAY from COP to center of PP
    Dnearest = INFINITY; /* Distance from COP to nearest intersection */
    Onearest = NULL;     /* OBJECT on which Dnearest is achieved */
    For each OBJECT in SCENE {
        if (OBJECT intersects RAY) {
	    P = closest intersection of RAY with OBJECT
            D = dist(COP,P)
            if (D < Dnearest) {
                Pnearest = P
		Dnearest = D
                Onearest = OBJECT
            }
         }
    }
    WRITE_PSEUDO_PIXEL(color(Onearest,Pnearest), PP)
}

If the pseudo pixels map directly to pixels on the screen, then
WRITE_PSEUDO_PIXEL just becomes write_pixel(). However more generally,
WRITE_PSEUDO_PIXEL draws the rectangle PP with color color(Onearest,Pearest))