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Husbj

Standard approach to shadow mapping multiple light sources?

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So I've been contemplating this lately, is there any standard approach to how to (efficiently) handle dynamic shadow mapping of multiple light sources?

As I've understood it the common advice is to just render separate depth maps for each visible light in the scene and then let the scene shader(s) iterate over all of those. However this just sounds like it would get extremely wasteful with relatively few lights.

Assume for example that I have a moderately complex scene lit by three point lights; this translates into having to render the scene 18 times just to generate the depth maps and then those maps have to be stored in memory as well (assuming 2048x2048x16 maps that alone will use 144Mb VRAM, I suppose that isn't overly much, but it still adds up with further lights).

Another big issue is that this approach would quickly eat up texture slots for the actual scene shader (I suppose you could put multiple shadow maps into a texture atlas but that has its problems as well).

I'd just imagine there should be a way to somehow combine shadow calculations, or is it really all about the art of cheating (ie. only make the x most significant lights in the current frame actually cast shadows)?

 

If anybody would like to share some information, thoughts or links to papers or similar on this subject it would be greatly appreciated smile.png

Edited by Husbjörn
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Use a cube map and a geometry shader and you don't need to render the scene 6 times per light; it goes back down to once per light.

 

Also consider using smaller shadow textures for lower intensity lights.

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Use a cube map and a geometry shader and you don't need to render the scene 6 times per light; it goes back down to once per light.

 

 

This is true, but it comes with a significant GPU cost.

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Use a cube map

Admittedly I'm not all too familiar with cube maps but won't they essentially render as 6 separate maps anyway?

 

 

@MJP: Thanks for the insight.

My planned game probably won't "require" that many lights, I'm basically just planning ahead and trying to come up with the most flexible solution to start with so as to not have to rewrite too large portions should it turn out I need an extra light or four somewhere along the way. Frustum culling the lights makes sense and I've heard of faking point light shadows by actually generating shadows for an imaginary spot light pointing at the most lit area of said point light (such as straight down on the ground).

As for pre-generating static lighting on static meshes, won't that by definition fail and you'll have to regenerate the whole thing the minute any dynamic objects enter the shadowed zone however? I suppose it would still work for distant / unreachable areas though (although I'm planning a mostly outdoors game that would require dynamic directional sun / moon lighting anyway so I guess this isn't too applicable at my particular situation unfortunately. Good to know for later endeavours though.).

Also thanks for the links, will read through them during the upcoming rainy vacation days! That dual paraboloid shadow mapping seems like it could be quite useable at a first glance smile.png

 

[b]Edit:[/b] forgot to mention it but I was under the impression that you could at most have up to 16 textures bound to the graphics pipeline for any single draw call. It would however seem I was dead wrong as MSDN suggests the number to be 128 for shader model 4 (however no such number is given for SM5). That at least gives some more headroom to work with, even though it will of course be limited by other factors instead in the end then.

Edited by Husbjörn
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Use a cube map and a geometry shader and you don't need to render the scene 6 times per light; it goes back down to once per light.

That reduces the number of draw calls, but still requires rasterizing of the scene 6 times (once for each 2D surface) so the primary cost is still present.

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I use subsivision of a square atlas for multiple depth maps to use the same texture slot. By removing all allocations in the atlas each render of the world, there will be no more than 3 unused squares per size after allocating a new square and a fixed size array can be used to store them.

 

https://code.google.com/p/david-piuvas-graphics-engine/source/browse/trunk/Engine/QuadAllocator.cpp

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