4.2.5 Exhaustive Subpixel Testing

After an uninformative pixel test, where

exhaustive subpixel testing may be performed. If

then we set to , since

Subpixel testing aims to prove a solution exists within a pixel; exhaustive subpixel testing aims to prove no solution exists within a pixel. Of course, if a solution is discovered during exhaustive subpixel testing, the test may abort prematurely and we may set to .

Consider rendering y;SPMlt;x-x; here are two renderings, produced using, and not using, exhaustive subpixel testing:

  denotes a rendering produced using exhaustive subpixel testing. The uncertainty is caused by the form chosen for S: the rendering restricts the uncertainty to the border. A rendering produced using subpixel testing, but not exhaustive subpixel testing, follows:

A rendering produced using both subpixel sample testing and exhaustive subpixel testing follows:

The two subpixel tests complement one another; after an uninformative pixel test both subpixel sample testing and exhaustive subpixel testing may be applied. If, after such testing, , further subpixel testing may be applied.

Jeff Tupper

March 1996