Soft shadows banding

[tomas7770]

Description

Soft shadows currently exhibit banding artifacts, instead of appearing smooth.

Screenshots

Notice how there is visible banding around the edges of shadows:

Possible solutions

The current implementation of soft shadows uses PCF (Percentage Closer Filtering), which involves sampling multiple pixels of the shadow map and averaging their shadowed/unshadowed results. The most obvious way to reduce banding is to increase the number of samples, but this appears to cause a noticeable performance impact while still not looking smooth enough.

Modern GPUs support hardware-accelerated shadow smoothing (linear interpolation), exposed in OpenGL through sampler2DShadow, which samples an area of 2x2 pixels. This article describes a possible way to leverage this capability, while still being able to sample a larger area, to allow a configurable blurRadius.