I've wanted to make a thread on this for a long time. I know I used to demand a shot of this in a few threads before but I procrastinated on making a whole thread about it. I know it's not a professional image quality/mipmap test but it's the one I know with consistent results in OpenGL.

This should start with maximum texture detail (important - affects the range), lightmaps, 640x480 and the option to enable coloring of mipmaps, with texture compression disabled. Color depth shouldn't matter

1quake3 +set r_picmip 0 +set r_vertexlight 0 +set r_colorMipLevels 1 +r_ext_compressed_textures 0 +devmap q3dm1

then hit F12 there (important because overbrights, don't 'snap' or prtscr it). png the tga or so. You should start behind the major statue to the right.

Different cards should exhibit varying behavior of the color transitions here. Things I know and to expect

Geforce2: a curve with 2x2 precision
PowerVR 1 and 2: a horizontal line (no curve)
PowerVR Kyro: an octagonal shape, with some variable stepping of precision across edges (up to 32 pixels)
Rage 128: an octagonal shape that's a bit spikier (and more banded in trilinear)
Voodoo2 and 3: a smooth curve that's a little farther away (cue the "3dFx VooDoo Superior OnLy The BeSt 3d GrAfIk" piss contest here)

Smooth geometry and models may appear jagged by this because Quake3 doesn't expose any normals to the driver as all the lighting is software calculated and the normals are kept to themselves in system memory. 😀