RE: [Algorithms] Lightmap Terrain

[Martin F. <mf...@ac...>]

The trouble with outdoor area's is the sky. If you have a clear day you will
get very hard edged shadows as the sun light comes down more or less
parrallel. However if you have lots of cloud then the sunlight that is not
reflected back into space by the cloud layer tends to get diffused alot on
the ground. Thus there are no really hard shadows. 

Radiosity deals well with reflected light in shadowy areas it does not deal
very well with direct light. The best renders I have seen yet in computer
graphics are hybrid raytracing radiosity engines utilising raytracing only
for the specular light component and radiosity for the diffuse. (I did see a
paper on view depedant radiosity once but that thread of though seems to
have died)

A landscape is for all intensive purposes flat, the average variation in
altitude is pretty insignifcant compared to the size of the planet. For most
of the day except extremes of sunrise and sunset most things will be in
direct sunlight without shadow. Even during sunrise and sunset because the
world is nominally flat there is going to be very little reflection where
energy would bounce more than once of the world. The first radiosity bounce
is more likely than not to send energy straight back into space or cloud. If
you consider how much our atmosphere protects us from the direct sun (in the
first hit) any energy bounced back to the cloud layer and then back to earth
will have diminished to insignificant levels. Radiosity only noticably works
well in areas of shadow with flat surfaces. Texture (such as grass) tends to
scatter / average the light.

The trouble with the classic radiosity algorithm is that to work it requires
an enclosed environment from which energy cannot escape. Trying to deal with
this condition on a kind of universal scale provides all sorts of headaches.
A lamp in an entirely enclosed room with good flat surfaces is much more
trivial case. How Quake2 delt with energy loss from sky boxes I don't know
but I expect this was fudged and the sky box was allowed to reflect energy.
Certainly oudoor areas in Quake have always looked wrong while the enclosed
environments have been very convincing.

I would expect for outdoor situations raytracing to be the most successful
technique. For cloudy simply dropping the raycast and performing an
averaging fudge (don't like that word but..) on the light calculation to
simulate radiosity bleed might be sufficient to give convincing results. A
full blown radiosity solution is likely to be expensive in terms to
development time and processing and is IMO not going to give you good
results.

Cheers,
Martin
3D Programmer
Shadowman2



-----Original Message-----
From: Ivan-Assen Ivanov [mailto:as...@ha...]
Sent: 25 August 2000 02:02
To: gda...@li...
Subject: Re: [Algorithms] Lightmap Terrain


Neither embossing the heightmap nor dot-products work well. 
The resulting lightmap doesn't give you any clue of where the depths
and heights are (e.g. inverting the heightmap and moving the sun at the
opposite azimute produces the same image).
I'm currently thinking about a simple raytrace, but I feel it will be
effective
only with low elevations of the sun over the horizon. We don't have
any sharp features in the heightmap.

Has anybody tried "full" radiosity (whatever that means - e.g. different
reflectance/
absorption for textures/ecosystems) on heightfields? Do you think it would
be worth the trouble?

_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list