>    This might not be the best possible place to ask this, but at least it is
> related to computer graphics (and there are lots of programmers here), and
> I don't feel like subscribing to yet another random online forum.
>
>    This is something I really should know, but I don't. And, more
> surprisingly, I just can't find this info even though it should be
> something that's trivial to find. It just doesn't seem to be. I was
> wondering if someone with more experience on this subject could illuminate
> me a bit.
>
>    Light mapping is a common efficient method for lighting static scenes,
> especially in older 3D games. I had always assumed that light maps could
> both make the underlying texture darker *and* brighter (from complete black
> to completely overexposed white where the lighting is the strongest, and
> then of course an unmodified texture in between the two extremes).
>
>    However, after searching info online about how to use light maps with
> OpenGL, this possibility is actually not clear at all. There are tons and
> tons of tutorials online (most of them dealing with how to *create* light
> maps, which is not what I'm looking for rather than how to *use* them in
> OpenGL). Whether the same light map can both make the underlying texture
> darker and brighter (than the original texture) is inconclusive after my
> extensive search. Some example images seem to indicate that it is possible,
> but the details of *how* it's done are sketchy at best.
>
>    When I tried to search information on how exactly do this with OpenGL,
> I just can't find it. You can make the light map modulate (ie. multiply)
> the underlying texture, which can only make the texture darker, or you
> can make the light map add to the underlying texture, which can only make
> the texture brighter, but I can't find the info of how to make it do both,
> or whether it's actually possible at all.
>
>    I am almost certain that in many games light maps do both. However,
> trying to search for examples has also proven difficult. For example
> screenshots of Quake 2 (which is a perfect example of a game using
> light mapping) are inconclusive. It's difficult to say whether the
> light maps are brightening the textures besides darkening them, or
> whether they are only darkening them. I was pretty sure that they do
> both, but I can't find conclusive screenshots demonstrating this.
>
>    (One possibility could perhaps be layering *two* light maps on top
> of the textures, one for darkening and another for brightening. However,
> it seems that at least vanilla OpenGL does not easily support layering
> more than two textures at a time.)
>
>    Could someone more knowledgeable about the subject shed some light on it?
> (Pun very much intended.)
>

If you multiply with a map whose values go from 0 to 10, values under 1 
will darken, while values larger than 1 will brighten. It's just a 
question of applying some offset or stretching the range of your light map.

Same with addition, here, you need negative values to darken things.

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