During work on Marathon Recompiled, developers dug into Sonic 06's core systems, including rendering. The game's visuals vary widely. Crisis City features reflections, particle effects and dramatic lighting, while Castle Town looks flat and drab. Beta footage hints at what was lost: Kingdom Valley had full bump maps, Castle Town had more complex geometry and proper lighting and shadows, and New City's collision suggests a very different building layout earlier in development.
Much of the cut content is still in the game but disabled for performance reasons. Even with the scaled-back visuals, the game struggles to hit its target frame rate on real hardware. Given the crunch Sonic Team faced, these features were likely among the first to go.
The answer lies in the shader variants. Most objects use either the Terrain or Billboard shaders, each with feature levels from 00 to 07, where higher numbers offer better feature sets. The most used shader, Billboard02, supports only per-vertex lighting, no cascaded shadow maps and no point lights. That explains why much of the game is rendered with only basic albedo textures and ambient lighting. Swapping shaders is difficult because of differing I/O layouts, and some models may not reference their normal and bump maps at all.
Shadows are also limited. The engine supports four cascades, but several areas, including twn_a, drop to three or two, significantly shortening the distance at which shadows render.
Source: Hacker News · Summarized by HeadlinesBriefing