RE: [Algorithms] Doom lightmaps

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

> >* They only really work for dynamic lights and typically you can only
> >have a few on screen before your frame rate suffers very badly.
> 
> Eh?  You can do static lights quite well too, e.g. shadow volume BSPs.
Not really what I meant.  I was thinking of shadows and pre-calculated light
maps.
It would be quite challenging to get dynamic shadows from static lighting
since apart from anything the static lighting could be for hundreds of
lights,
or even worse a radiosity solution.

> >* Force the artists to cap objects
> 
> I have not found this to be the case.
> 
> >* Make it essential for all meshes to be manifold
> 
> Nor this.

Err, you must have the worlds most robust silhouette extractor!
I've never come across one that deals gracefully with non-manifold
meshes.

We can flag polys with open edges not to cast shadows, which
helps avoid a lot of capping for world geometry, however if we 
have an edge shared by 3 triangles then we are hosed.  We provide
the artists with tools to find bad geometry, and they are quite
proficient at correcting them.

I would point out that for animated objects, capping is essential,
otherwise you run the risk of ugly shadow artefacts. 

> >* May make lit semi-transparent materials a total nightmare
> 
> Can be approximated with projected lights.

Ok a stained glass window could be approximated this way, however
this does not even come close to solving the problems with
semi-transparent objects.

Stencil methods only calculate if the front most surface is
on shadow or not.  This means that they can't light semi-transparent
objects in the general case (you have to use shadow maps). 

I suppose we could use render to texture and projective methods
to deal with dynamic objects, but I prefer the look of things 
to be internally consistent (artefacts for projective methods 
are different to stencil methods).

> >* Require the artists to be very careful when modelling to avoid
> >unfortunate self shadowing.

Some of the character meshes our artists modelled had the creases in
clothes modelled.  Without the shadows it looked great.  With the
shadows it was ugly.  The reason being that stencil shadows are
either on or off, you would get ugly dark areas from the ridges
in the clothing, often with a dotted line of bright pixels
(due to z-buffer inaccuracies).  The artist had to remodel
to get rid of the ridges in the clothing (they went into
the bumpmap instead).

> At this point I strongly believe that high polygon counts can 
> be safely 
> eschewed for better and more accurate lighting.  We're at the 
> point of 
> significantly diminished returns for cranking up polygon 
> counts (especially 
> with bump mapping available now), so exchanging triangle 
> density for more 
> complex lighting is definitely worth while.

I tend to agree, however due to marketing nonsense,
artists and designers often initially believe that they
have a much higher poly budget than they really do.

Alex Clarke, Programmer
Argonaut Games PLC