Tuesday, November 25, 2008

Ad-Hoc Stenciled Shadow Maps

Previously I blogged a design for combining G-Buffering with shadow mapping using the stencil buffer. I doubt that this is an original idea; the GPU Gems 2 chapter on G-Buffering (a la CRYSIS) mentions that G-Buffering and shadow mapping work well together.

I would describe the G-Buffering + stencil + shadow mapping approach as "ad-hoc" shadow mapping because the approach lets you compute any number of arbitrary shadow volumes and apply them to screen-space areas.  Because we are using the stencil buffer, it doesn't matter if the shadow volumes overlap.  We can simply pick out the most important parts of the scene (closest to camera, biggest, important to user, flagged by artist) and shadow those.  We can shadow fewer models or all models, determined by runtime settings.

Wait, what do I mean by "shadow volume"?  Well, a shadow map is a 2-d texture, but the value of a pixel in that 2-d texture is the "nearest occluder" from the sun.  Since the minimum and maximum gray-scale value correspond to distances from the sun, we can think of the shadow map effectively specifying occluder information within a cube that is aligned such that the sun sees exactly one square face of the cube.

Cascading Shadow Maps (CSM) uses a similar approach to fix some of the weaknesses of shadow mapping, namely:
  1. That shadow volume is very much resolution limited - both by texture dimension (X and Y axes) and texture precision (Z axis).  For very large scenes, you run out of res long before you get a nice looking shadow.
  2. Often the alignment of the sun and user's viewpoint are such that your pixels are being spent where the user can't see them, which is wasteful.
CSM solves both of these problems by using multiple shadow volumes in multiple maps, using a smaller volume for the near part of the view frustum (which is naturally smaller since the view frustum gets larger as it goes away from your eyeball).

Typical CSM designs will simply use 2, 3, or 4 shadow maps simultaneously, looking up the shadow per pixel in each one.  But this design can be adapted to ad-hoc shadow mapping -- with ad-hoc shadow mapping, we simply build each shadow map (near, far, very far away) in series, and apply each one to the stencil buffer.

Since there is no penalty for overlapping shadow volumes, we can even combine ad-hoc and CSM approaches - we can run several shadow volumes along the view frustum (a CSM-like approach), excluding "high value" content - and then separately shadow that "high value" content, each with its own shadow map.  The result is generally good shadow precision, and arbitrarily precise shadows for models that require extra res.

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