Painting Windows Front-to-Back

This problem (and my solution) were for a tiny embedded system many years ago. Modern hardware and windowing systems don’t need this kind of optimization any more.

This project will probably end up being just a description of the algorithm without any code, or perhaps a reference implementation. Any real implementation should probably be in assembly language.

Entering the Wayback Machine

Most computer windowing systems paint windows back-to-front, because it is simple and correct. However, that technique also results in writing multiple times to the same pixels, wherever there is overlap. Writing to video memory should be avoided when possible because of the overhead involved in moving data between RAM, the CPU, and video RAM.

It is possible to efficiently paint windows front-to-back, so that every pixel is only written once. This technique works best when window content is stored in off-screen video RAM, so that fast bit blit operations can be performed entirely using video hardware. The algorithm is very amenable to expression in assembly language, because that allows you to test the sign and carry bits directly without performing a second operation.