Editing Ray casting (section)

== Anti-aliasing ==

The jagged edges caused by aliasing is an undesirable effect of point sampling techniques and is a classic problem with raster display algorithms. Linear or smoothly curved edges will appear jagged and are particularly objectionable in animations because movement of the image makes the edges appear fuzzy or look like little moving escalators. Also, details in the scene smaller than the spacing between rays may be lost. The jagged edges in a line drawing can be smoothed by edge following. The purpose of such an algorithm is to minimize the number of lines needed to draw the picture within one pixel accuracy. Smooth edges result. The line drawings above were drawn this way.

To smooth the jagged edges in a shaded picture with subpixel accuracy, additional rays should be cast for information about the edges. (See [[Supersampling]] for a general approach.) Edges are formed by the intersection of surfaces or by the profile of a curved surface. Applying "Coherence" as described above via binary search, if the visible surface at pixel (X,Y) is different than the visible surface at pixel (X+1,Y), then a ray could be generated midway between them at (X+½,Y) and the visible surface there identified. The distance between sample points could be further subdivided, but the search need not be deep. The primary search depth to smooth jagged edges is a function of the intensity gradient across the edge. The cost for smoothing jagged edges is affordable, since:

* the area of the image that contains edges is usually a small percentage of the total area; and
* the extra rays cast in binary searches can be bounded in depth (that of the visible primitives forming the edges).