- The center position: I think a good idea would be to put the center of the frustra at 1/3 of the current size of the frustum, in the direction of camera. Like this you waste a lot less SM space. What offset sizes do you think between the different frustum sizes? I tried times 2, but then you get a frustum with 4 divisions such as this:



Is this is a good way to subdivide ? If you start with a shadow map of 32m x 32m, you can cover 'only' 256m / 3 * 2 = 170m .. which perhaps might even not be enough.

The thing I don't like about the multiple shadow maps is that you have to shade your scene also with the subdivided frustums, and this doesn't really work well with deferred shading and feels counterintuitive. You could select the shadow map to render depending on the distance (as wolf suggested I think), but then you have to give all the shadow maps to the shader, have a lot of texture memory swapping (shadow maps are rather big uncompressed textures), and it's always better to avoid branching I think. I tried it before and it works fine. I even increased the PCF when sampling the shadow maps closer to the camera.

I presume (not sure yet, cause I'm writing here as I am thinking :)), that you could do the following:

- cull scene with all object which will be shaded (shadow receiver objects)
- cull all possible object which can contribute shadows into a single visible array (shadow caster objects)
- devide the visible array into different frustra, 4 here
- repeat for each frustra (starting with furthest)
* render shadow caster objects from frustum in a shadow map
* render shadow receiver objects within this frustum, using shadow map to another RGBA texture in screenspace, and only store black and white and perhaps perform some PCF here
- now we have the complete projected shadow map into a single texture for the scene -> blur it.
- render the shadow receiver objects again (all of them), using the RGBA shadow map.

This seams like the easiest method, because you don't really have to modify your shading system to work with multiple frustra.

Someone else posted another very nice link to Variance Shadow Map. But I would only use the Variance Shadow Maps for point and spotlights, cause I think it would work best there.

All comments welcome of course

to magnus:

that's what I though in the beginning as well, but have you already taken a good look at battlefield or battle for middle earth 2. The shadows are bad! With wrong directions you always manage to make them look really really ugly if you use a perspective correction. Perhaps if you would blur them somehow it could work. But I think with method I described above, you only have to render the objects once more, with a cheap shader, and it doesn't consume more texture memory than normal TSM, and it has far less special cases on quality level (at least I think :))

hmm .. I don't really understand how you subdivide your frustra then. Could you make a quick sketch or explain in a little bit more detail then pls. I have modified my preview a little bit. Each region has its own color, and overlaps the previous and next region.

I see, but like you mentioned. How do you take care of tall object casting shadows into the scene like this, but not visible in your frustum. That's what I had this big region behind me.

well I haven't been able to test it, but it seams possible to me to render all the lights into the same black and white texture. It basically just the scene shadowed, without shading, so using depth testing, and z-write this should work no? Or am I forgetting something important here again? ;)

I'm probably missing something but, wouldn't storing intermediate results in a texture more or less invalidate any MSAA scheme?
Maybe it's just one of those things that looks good enough, even though you don't get correct results.

Consider a multisampled pixel like this:


The simplified equation for the pixel then becomes something like this:
Output = (ShadowA * LightingA + ShadowB * LightingB + ShadowC * LightingC + ShadowD * LightingD) / 4

Now for the intermediate shadow texture, we can't have AA (right?), so we get:
IntermediateShadow = (ShadowA + ShadowB + ShadowC + ShadowD) / 4

And the output:
Output = (IntermediateShadow * LightingA + IntermediateShadow * LightingB + IntermediateShadow * LightingC + IntermediateShadow * LightingD) / 4

Which is different from the original.

Edit: Should have mentioned that I did the same thing but instead i build the shadow mask using stencil volumes and precomputed shadow maps and I did get artifacts (halos) when using MSAA.