Why don't high-res shadow maps produce aliasing artifacts due to minification?

Question

Shadow quality generally improves with increased shadow map resolution (i.e. shadow edges are less "jagged"). I noticed this is also true for shadow maps with resolution far greater than the viewport (screen) resolution.

High-res maps like this are sampled at the lower frequency of the viewport resolution, and as such are akin to texture minification. (Assume point-sampling for simplicity.) Furthermore, minification exacerbates as the camera moves further away from the shadow, resulting in pixels "covering" multiple shadow texels.

Texture minification normally produces aliasing/shimmering artifacts, so I fail to understand why this is not the case with high-res shadow maps as well.

Answer 1

Turns out minification aliasing is a real problem, just as with regular (color) textures. Following are a couple of papers demonstrating the artifacts and presenting solutions (basically low-pass filtering techniques to downsample the textures):

1. Summed Area Variance Shadow Maps
2. Convolution Shadow Maps

A shadow map that produces a lot of "high-frequency" shadow/light transitions, e.g. shadows cast by a wire fence, will likely produce aliasing artifacts at distance. The determining factor here is the distance from the eye to the pixel in shadow. If it is large enough that the pixel "covers" multiple shadow map texels (AKA minification), aliasing is likely.