Re: [Algorithms] Combining shadow maps and preprocessed lighting

[Alen L. <ale...@cr...>]

>So if a person walks beneath the shadow of a building, he is not shadowed.
This only occurs
>because I don't render the static objects in to the shadow map.

Have you considered a middle-ground solution? Have a software-based shadow
map of static geometry for the static light(s) that is precalculated and
stored in the main memory. At runtime just check center of the dynamic
object's box (or several points on the box) against that map (using the CPU)
and see whether and how much is that object in the shadow, then use that
info to darken it accordingly. It doesn't actually lay shadows over dynamic
objects, only darkens them by average shadowing, but at least they are not
standing up full-bright in the shadow. We use that trick with success for
sun in our current project.

Alen


----- Original Message -----
From: igrok
To: gda...@li...
Sent: Friday, September 30, 2005 4:33 PM
Subject: [Algorithms] Combining shadow maps and preprocessed lighting


I've been implementing a technique that seems to be quite popular in games
right now.
In an outdoor environment, using lightmaps to store light occlusion from the
sun, and then
combining the results with a shadow map (updated each frame) in a pixel
shader.

That way, you get nice smooth pre-processed shadows on static objects like
buildings,
but dynamic or fine-detail objects like people, vehicles or trees can cast
shadows using
the shadow map. The pixel shader just takes the darker of the lightmap or
shadow map.
So when a character walks in to a shadowed area, there is none of this
'double-shadows'
rubbish, it all blends in correctly.

Anyway. all is well and good apart from one thing. I can't see a nice way to
use the shadow
map to also occlude the dynamic objects.
So if a person walks beneath the shadow of a building, he is not shadowed.
This only occurs
because I don't render the static objects in to the shadow map.
If I DO render the static objects in to the shadow map, it is obviously
fixed instantly, but then
you have the problem that the ground is receiving both the smooth lightmap
shadow from a
building AND the sharp outline from the shadow map of the same building.

This results in the smooth lightmaps being lost as towards the edges where
it fades out, the
shadow map is being overlaid and is the darker of the two.
hmmm. I hope that is a good explanation. :/

The only way I can see to fix it is to have TWO shadow maps. One renders
only the non-lightmap
objects on to itself, and is used when combining lightmap meshes with
dynamic shadows.
The other renders every object on to itself, and is used by dynamic objects.

But that will be no small hit on the GPU as I'm already rendering several
shadow maps to cover the
entire scene.

It seems such a waste, I feel sure there must be a better way.
Can anyone help?