Frequent shadow acne and/or peter panning

[tomas7770]

Description

When using lights with shadow casters, it's very easy to come across shadow acne and/or peter panning. This was apparent when adding shadows to some of our demos.

While the existing bias field of shadow casters can help mitigate this, it often requires manual adjustment for each light to get decent results, and it can be difficult to fix acne without causing peter panning, or vice versa. In addition, in some cases, both artifacts may be present.

Expected behaviour

Shadows should draw without acne or peter panning, preferably with minimum user intervention.

Screenshots

Some shadow acne is visible in this screenshot of Scraps vs Zombies, particularly the further away the light is from its origin.

Possible solutions

Initially, the possibility of automatically adjusting the shadow bias based on the light angle was explored. This helps reduce user intervention, but doesn't solve the problem of balancing acne with peter panning, and in some scenarios doesn't help at all (e.g. the spot lights in the Cars demo).

A better solution would be to use a shadow normal offset bias, as described here. Some quick testing on the demos reveals this to actually be a lot more effective than what we currently have.