Design Decisions

We decided at the outset that we didn't want to deal with a lot of PNG bit depths; we have plenty of that in the front-end code (at least for the X version...sigh). Being fond of alpha transparency and the nice effects it can produce, we did want to retain full transparency information, however. In both cases, we were willing to sacrifice a minimal memory footprint in favor of simplicity and, to some extent, speed. Thus, we chose to expand or reduce all PNG image types to 24-bit RGB, optionally with a full 8-bit alpha channel. In other words, the output would always be either three channels (RGB) or four channels (RGBA).

Handling all alpha blending on our own, in the front end, is not strictly necessary. In the case of a flat background color, which is all I've discussed so far, libpng can be instructed to blend the background color (either from the PNG file or as supplied by the user) with the foreground pixels, thereby eliminating the alpha channel; the relevant function is png_set_background(). The result would have been a single output format to deal with: three-channel, 24-bit RGB. But we had in mind from the outset the possibility of loading or generating a complete background image, not just a background color, and libpng currently has no provision for blending two images.