Halo 4 is upon us! Cleaner bitmaps without super grunge details for forerunner IS possible!!!
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Unfortunately, cleaner bitmaps only really look good in a newer gfx engine. But there's always OS I guess.

Thats... disagreeable.

well, more modern games create textures with the shader effects in mind, ie bump map/specular/ambient occlusion, whereas earlier gfx engines pre-baked the details onto the texture. a fine example is halo 1 to halo 2; textures in halo 1 had pre-baked lighting and bump whereas halo 2 textures are flat because bump and specular lighting are calculated in-game. in terms of newer textures looking good in older games, that varies greatly on the game itself.

Yeah, you wont see anything like Halo 1's bitmaps now-a-days. They incorporate shadows and what-not within the single diffuse map. Not entirely sure about a bump map though. I don't think they integrated them.

If you look at a modern game's textures, the diffuse is usually pretty simple, where as the bump brings out most of the detail, and then the specular brings out all of the highlights.

This can be pulled off with OS, however combining Halo 1's bitmaps with these newer effects won't be very visually pleasing.

Bump mapping was available in the origional Halo, same with specular. Specular was just the alpha in the diffuse. Otherwise, on non-bsp objects, bump maps were put in on the diffuse, and then the multipurpose map controlled the specular and 3 other effects. You can still create very awesome looking affects in Halo 1, take DSalimanders maps for example.