Real-time soft shadows in dynamic scenes using spherical harmonic exponentiation

Abstract

Previous methods for soft shadows numerically integrate over many light directions at each receiver point, testing blocker visibility in each direction. We introduce a method for real-time soft shadows in dynamic scenes illuminated by large, low-frequency light sources where such integration is impractical. Our method operates on vectors representing low-frequency visibility of blockers in the spherical harmonic basis. Blocking geometry is modeled as a set of spheres; relatively few spheres capture the low-frequency blocking effect of complicated geometry. At each receiver point, we compute the product of visibility vectors for these blocker spheres as seen from the point. Instead of computing an expensive SH product per blocker as in previous work, we perform inexpensive vector sums to accumulate the log of blocker visibility. SH exponentiation then yields the product visibility vector over all blockers. We show how the SH exponentiation required can be approximated accurately and efficiently for low-order SH, accelerating previous CPUbased methods by a factor of 10 or more, depending on blocker complexity, and allowing real-time GPU implementation.

Hardware accelerated ambient occlusion techniques on GPUs

“…We use this notion of a negligible ambient occlusion value to limit the ambient occlusion influence due to an occluder within a certain distance. This is the significant difference between other methods such as [Kontkanen and Laine 2005], [Ren et al 2006], [Kontkanen and Aila 2006], [Bunnell 2005] and ours. We avoid the problem of iterating through every possible occluder for all pixels in the ND − bu f f er by providing a limit to such occlusion.…”

“…[ Ren et al 2006] uses spherical occluders and spherical harmonics to calculate approximate ambient occlusion (or low-frequency soft shadows). This solution attacks the problem of over-occlusion and provides a simple pre-computation step while supporting deformable objects.…”

“…The effect of ambient occlusion can roughly be described as the effect seen in a cloudy day; here we consider the sky to be a uniform area light source resulting in soft, low-frequency non-directional shadows [Ren et al 2006]. Throughout this paper, we use the notion of approximated ambient occlusion (A ψ ) due to a sphere S with center C and radius r (written as < C, r > ) at a receiver point P having a normaln ( fig.…”

“…We can use the methods described in [Kontkanen and Laine 2005], [Ren et al 2006], [Kontkanen and Aila 2006] or [Bunnell 2005] for this purpose. However we solve this problem using a different and a faster approach.…”

“…One major advantage of the distant-occluder approach as compared to [Ren et al 2006;Bunnell 2005;Hegeman et al 2006;Kontkanen and Laine 2005] is that we use the various GPU units to their advantage. We achieve frame-rates in excess of 80 fps on a nVidia GeForce 8800GTX for over 6,000 spheres (using the distantoccluder approach alone); this is because, regardless of the number of spheres present in the scene, the rasterizer clips the billboards to the view-frustum and generates the targeted fragments.…”