S2DEX microcode was developed to simplify development of 2D games. It provides macro commands to create sprites and backgrounds. S2DEX2 modification of S2DEX can self-load with F3DEX2 microcode and thus can be used to create 2D sprites and backgrounds in 3D games. Many N64 games use S2DEX, so all modern HLE graphics plugins implement it.
S2DEX implementation in GLideN64 was too high level. Actual microcode commands are very complex. The microcode is documented, but the documentation does not cover some tiny details in internal mechanics. Thus, S2DEX implementation not always worked right. olivieryuyu decided to not rely on documentation and decode the microcode itself. The goal of that new decoding project was to implement commands in HLE as close as possible to LLE and obtain the result, which is as good as with LLE or better.
It was very long and hard project, which took circa six months. We started form relatively simple sprite commands: OBJ_RECTANGLE, OBJ_RECTANGLE_R and OBJ_SPRITE. We fixed calculation of screen and texture coordinates of sprites. There are several revisions of the microcode, and each revision does the calculations in slightly different way. The fix eliminated several minor glitches and a major one, in Parlor! Pro 64: