ADD - Adds texture color values to the underlying color values, using the alpha channel; light areas in the texture lighten the background while dark areas are more transparent
ALPHAKEY - One-bit transparency; pixels with alpha values greater than ~0.8 are treated as fully opaque and all other pixels are treated as fully transparent
BLEND - Normal color blending, using any alpha channel in the texture image
DISABLE - Ignores any alpha channel, displaying the texture as fully opaque
MOD - Ignores any alpha channel in the texture and multiplies texture color values by background color values; dark areas in the texture darken the background while light areas are more transparent
