These are notes for myself about shadow mapping bias.
Good summary about all aspects of shadow mapping: http://mynameismjp.wordpress.com/2013/09/10/shadow-maps/
I’m not sure what’s wrong about Receiver Plane depth bias. What is interesting, it does work OK when there is no interpolation between samples.
In this presentation, there’s a comparison, but it also uses samples without interpolation: http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2012/10/Isidoro-ShadowMapping.pdf (page 39).
Here they also get strange artifact with it similar to one I have on sphere: http://www.digitalrune.com/Support/Blog/tabid/719/EntryId/218/Shadow-Acne.aspx
MJP also says that “When it works, it’s fantastic. However it will still run into degenerate cases where it can produce unpredictable results”.
So, maybe I implemented it wrong, or maybe I was unlucky enough to quickly get degenerate cases, but I’m not really willing to try this technique anymore.
Also this may better explain why it works: http://c0de517e.blogspot.ru/2011/05/shadowmap-bias-notes.html
There are 2 ways how to implement Normal Offset bias. One way is to inset geometry by normal when rendering the shadow map. The insetting amount is also scaled by slope aka dot(N,L) and also can be scaled by distance factor with FOV included for using with perspective projection.
Second way is to render shadow map normally, but add (instead of subtract) same scaled vertex normal to fragment position just before multiplying it by shadow map matrix and comparing.
The second method has less impact on shadow silhouette distortion and gives better results. It is, however, not easy to do with deferred rendering, because you need vertex normal, not normal mapped one!
Unity 5 seems to use 1st version exactly because it can’t hold vertex normal in G-Buffer.
Funnily enough, Infamous Second Son is OK with storing it there:
And they use it exactly for normal offset (and other stuff too): https://twitter.com/adrianb3000/status/464584971483893762
You can also try to calculate face normal from depth, BUT you’ll get unpredictable results on edges.
Even Intel guy couldn’t solve that: https://twitter.com/AndrewLauritzen/status/539669636912914432
2nd variant doesn’t suffer from this (you can still see a tiny hole there though… but it also exists with just constant bias, so it’s not a normal bias problem).
Real-time demo with Normal Offset 2nd version: http://geom.io/pc25d/demoShaders5.html
No acne, no peter-panning, yay!
Use RMB + WASD to fly around. Feel free to look into source.
You can tweak both normal/constant bias in browser console using