Saturday, November 29, 2008


If you use CGAL with GMP for numerics, you might want to compile a debug version of GMP. The problem is: GMP will abort if it gets bogus input (like a NaN floating point initializer).  But GMP is normally coded with stack back-links disabled for speed.  This means that if you have a NaN in your code, as soon as CGAL passes it to GMP you get a crash with no back-trace to see how it happened.

The fix is to recompile GMP in some kind of debug mode for development purposes.

Wednesday, November 26, 2008

CSM vs. Ad Hoc Shadows - Quality

Comparing the quality of ad-hoc shadows vs. CSM...there are some cases where ad-hoc does a lot better.  In particular, given a character or vehicle near the camera, ad-hoc usually looks better, because a lot of shadow map res is dedicated to a relatively small area.

Where CSM excels is in very large models or very large terrains where there is no good scene-graph based decomposition.  For example, ad-hoc shadows on terrain by decomposing sub-parts of the mesh works rather poorly - decompositions really need to be based around view frustums (which is exactly what CSM does), not based on world coordinates.

One thing I haven't been able to quantify yet is the cost in triangle count to CSM.  If you look carefully at the CSM scheme, you'll see that at certain camera vs. sun angles, there can be significant overlap between the shadow volumes, and that means multiple iterations over the scene graph content that is in the "shared" location.  If the model in that location is expensive, this can be a potential performance problem.  

(For example, if you work on, oh I don't know, a flight simulator, there is a chance that the user's airplane is significantly more expensive than anything else in the universe...if it spans several CSM volumes, you're going to feel the pain.)

Finally, from what I can tell, while it is more efficient (fill-rate wise) to apply all CSM volumes at once, it is not strictly necessary from a quality standpoint - I'm not seeing a ton of artifacting from applying CSM volumes separately via stenciling.  

(The artifacts would be from the overlap of low and higher res shadow maps..what I have found is that if the CSM scheme uses enough splits to really look good, the overlap regions are small and don't differ that much in quality between the lower and higher res shadow map that overlap.)

nVidia's CSM demo uses four shadow maps -- my tests required six at first, but upon examination, it looks like the closest and farthest map are too close and too far to be useful, and could be dropped.

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.