Add SSShadowPass (screen space shadow)

[gonnavis]

Related:
https://twitter.com/mrdoob/status/1400345686537916418
https://twitter.com/BrianKaris/status/1283513723852689409

Description:

Made a quick prototype of SSShadowPass (screen space shadow), I'll keep improving.

---EDIT---
Demo: https://raw.githack.com/gonnavis/three.js/SSShadowPassPr/examples/webgl_postprocessing_ssshadow.html

[gonnavis]

Another branch that trying to use MeshBasicMaterial and pure screen-space-lighting-and-shadowing.

Demo: https://raw.githack.com/gonnavis/three.js/SSLightPass/examples/webgl_postprocessing_ssshadow.html

[lyonsno]

Hey @gonnavis, this is a cool technique, are you still working on it? Is there any way I could contribute?

[gonnavis]

Hello @lyonsno, currently it lacks a lot of feature, such as:

1. Selective: Like which already implemented in SSRPass and SSRrPass.
2. Blur shadow: Especially for hide screen-space flaws at shadow's edge.
3. More light types: Such as DirectionalLight.
4. Compatible with lights parameters which already in THREE.js, prevent create new lights.
5. PBR, MRT, and so on.

But recently I may have not enough time and ability to do all these.
If you interested in, feel free to keep developing and make new PRs, very welcome!

[lyonsno]

Why is this declared using var?

[gonnavis]

It's copy-pasted from old SSRShader.js, should be ok with let const.

[andreasrosdal]

SSShadowPass looks amazing, I would like to use this in Fciv.net to implement shadows. I therefore hope this could be merged and included in the next Three.js release please.

https://github.com/fciv-net/fciv-net
https://www.fciv.net

[gonnavis]

@andreasrosdal Thanks! I'll try to ship a basic version first.