RE: [Algorithms] Doom lightmaps

[Alex C. <al...@ar...>]

We don't use shadow maps (== nasty horrible artefacts).
Adding full on volumetric shadows to an engine is not a
light weight thing to do.

Depending on the method used, they tend to:

* Burn fill rate
* Stencil methods in particular have very variable fill rate cost
* Require skinning to be done CPU side (so you can calculate the silhouette)
* Require fancy silhouette extraction algorithms (brute force extraction =
expensive)
* Require extremely accurate raycasts (to determine if the camera is shadow)
* Require a very near clipping plane (so you _have_ to use a z-buffer)
* Mess up loads of lighting optimisations (coalescing multiple lights into
an average light doesn't work well due to the shadows affecting the
blending)
* They only really work for dynamic lights and typically you can only
have a few on screen before your frame rate suffers very badly. 
* Force the artists to cap objects
* Make it essential for all meshes to be manifold
* May make lit semi-transparent materials a total nightmare
* Require the artists to be very careful when modelling to avoid
unfortunate self shadowing.
* Require that the artists and designers don't try to use too many
polys. You can forget about having 100k triangle / frame scenes with
full on volumetric shadows

However,

They are quite pretty.

Alex Clarke, Programmer
Argonaut Games PLC

> > It is funny you mention the stencil+shadow map method 
> because that is
> > exactly how I do dynamic shadows in my engine.  For any 
> object that might
> 
> I think that's also what they do in Malice (just my guesses 
> of course).
> 
> 
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