I wrote a chapter for GPU Zen 2 focused on emulating dappled soft shadows.

Inspired by depth of field splatting techniques, this chapter focuses on an approximation that identifies points of high variance in a shadow map and reuses such points to project a slice of the light source shape. This reduces the cost of searching, filtering and sampling the depth map, common in other shadow mapping techniques. It also gives artistic control of the light shape to the user, making certain effects like rendering crescent shadows possible.

It can be used to render non-photoreal effects as well:

Because it is designed for environments with high frequency shadow maps, it is ideal for scenes with many trees or any geometry that can generate small pinholes. The proposed technique is subtractive and can be implemented on top of any other conventional shadow mapping algorithm, while considering mitigations to potential spatio temporal artifacts.

To my knowledge, it is the first technique that emulates and animates shadows under a solar eclipse, called crescent shadows, in realtime.

You can find the reference source code at GitHub!