The High Quality mode is less demanding. Although it too uses uncompressed normal maps and textures, specular and diffuse maps are compressed with DXT1, 3 or 5 algorithms. The load on the graphics memory subsystem is thus reduced – only 256MB of memory is recommended for that mode.
All high-performance graphics cards today have that amount of memory on board (GeForce 6800 Ultra/GT, GeForce FX 5950 Ultra, RADEON X800 XT/PRO, RADEON 9800 XT). Some mainstream models, down to RADEON 9600/9200 and GeForce FX 5700, can have that much memory, too, as it only depends on the desire and ability of the manufacturer of the particular graphics card. If you choose an Enable Anisotropic Filtering by Application Request option in the driver’s settings, the game, like in the Ultra High Quality mode, automatically enables 8x AF to improve the image quality.
As a rule, you can also enable the High Quality mode on a graphics card with 128MB of memory, but you are discouraged to turn on full-screen antialiasing in this case since it results in a terrible performance hit, especially in high resolutions. For example, the GeForce 6800, being an excellent card to play Doom 3 on in the ordinary mode, issued less than one frame per second after our attempt to force 4x FSAA in 1600x1200 resolution – the graphics memory subsystem just cannot digest the increased amounts of data.
The Medium Quality mode is for the owners of graphics cards with 128MB of memory. To pack all the graphics data into this size, the game compresses normal, diffuse and specular maps as well as textures, but retains their regular resolution. Thus, image quality shouldn’t deteriorate too much, visually.
The last Low Quality mode is recommended for graphics cards with 64MB of memory on board. Everything is compressed here, and all the textures bigger than 512x512 pixels are scaled down to this resolution, while all the specular maps are diminished to 64x64 pixels. Of course, this negatively affects the image quality, but still allows playing Doom 3 on such graphics cards.