Perspective Distortion - Polygon Rendering and Ray Tracing Methods

Anomalies are introduced since interpolation is performed after perspective transformation in the 3-D screen-coordinate system, quite than in the WC system. For illustration, linear interpolation causes the shading information to be incremented through a constant amount from one scan line to the other along each edge. Identify what happens when vertex 1 is more distant than vertex 2. Perspective foreshortening implies that the dissimilarity from one scan line to the other in the untransformed z value along an edge rises in the direction of the farther coordinate. Hence, if y_s  = (y₁ + y₂ ) , then I_s = (I1 + I₂ ) / 2 , although z_s will not equal  (z₁ + z₂)/ 2 . This issue can also be solved by utilizing a superior number of smaller polygons. Reducing the size of the polygons raises the number of points at that the information to be interpolated is sampled and thus increases the accuracy of the shading.            

Figure: (a)

Figure: (b)

In figure 17 (a) and (b) as in above demonstrates Interpolated values derived for point P upon similar polygon at diverse orientations that are different from (a) to (b). P interpolates A, B, D in (a) and A, B, C in (b).