> I'm looking for game using lightmap techniques. It seems :
> - quake2 and quake3 uses radiosity to produce lightmap texture
> - unreal uses blured shadows to produce lightmap texture
> What is the techniques really used . Is it :
> - first precalculate global illumination (with radiosity or blurring
> shadows). In this case, does BSP is interessant to accelerate computing
> ? This pass allows to produce lightmap textures for all faces of the
> scenes. It seems to me that takes a lot of memory space, no ?
Yes this is the method. It is computationally expensive, so is other
stuff like the visibility lists created for the convex leaves in the BSP
tree. I think id Software used a rather large multiprocessor SGI system
for the calculations. The kind of visibility structure Carmack created
for Quake would definitely help in radiosity calculations by limiting
the scope of surfaces you'd have to consider for transmitting light to
any surfaces within a particular leaf if you made use of that. It's
still a lot of computation.
Once computed the only thing you need to remember is the light map
information. This is a lot of texture but it is relatively low
resolution (or blurry), the downside is it can't be tiled like the other
textures, one image is unique to a particular location.
Quote:> - second, during real-time visualisation, the scenes is projecting twice
> : one for material texture and one fore lightmap textures ?
Basically yes, this is called multipass. The idea is you multiply the
surface color stored in the framebuffer by the lightmap value. There is
another way however, you could use two textures simultaneously and have
one modulate the other, this means you only have to draw the geometry
once and might get double the performance depending on the hardware.
Using common OpenGL extensions lot's of hardware supports multitexture
in OpenGL. Voodoo2 cards were the first but it's now a very common
feature on the latest PC Cards, from nVidia, ATI, 3Dfx, S3, Matrox &
3DLabs. Most of these have full speed multitexture in their cards,
certainly the most popular cards do, that's why they get such good
Quake2 performance numbers, the game uses multitexture instead of
multipass when it finds the extension.