Allow using between 1 and 8 cascades for SDFGI

[Calinou]

Related to #39965 (can be merged independently).

This provides more flexibility between performance and quality adjustments, especially when using SDFGI for small-scale levels (which can be useful for real-time procedurally generated scenes).

Testing project: test_sdfgi_num_cascades.zip

Preview

Look at the FPS counter in the top-right corner.

6 cascades

5 cascades

4 cascades

3 cascades

2 cascades

1 cascade

Since GI would end really up close with a cell size of 0.4, I manually increased the cell size to 0.6.

SDFGI disabled

1 cascade + half-resolution GI + 8 SDFGI rays

The lowest-end possible setting. Only takes 0.5 ms of GPU time on a GTX 1080. Should be usable on recent or high-end integrated graphics.

[mrjustaguy]

This is really great, 4 cascades Stuttered on Low end GPUs during movement, while 1-2 don't. Thanks for this change!

Also, Just setting to 1 Cascade and going to a Large cell size, like 8,16 or such (to have SDF for the entire View) adds a significant improvement to overall quality without much of a performance hit as opposed to the previous minimum, albeit adding a bit Light Leaking (however even disabling most lights from GI totally would fix this while still improving quality)

[clayjohn]

Looks good to me too! I'm surprised how good the quality is with just one cascade. How does it look in motion?

[mrjustaguy]

Just fine, same as static really... now with pair this with 1 frame for converging and it could be a fun effect.. I mean it isn't a very strong "GI" effect that way, but it is an interesting and good looking effect that way.

It would be interesting to see how this would work if you could increase the size of a Cascade without affecting cell size..

[Calinou]

How does it look in motion?

In motion, SDFGI fade-in is quite noticeable indoors by default (but it's OK outdoors):

sdfgi-1-cascade-fade_.mp4

However, you can improve the appearance of indoor SDFGI fade-in (at the cost of outdoor appearance) by exploiting the fact that Environment ambient light settings can be adjusted independently of SDFGI ambient light settings.

Here's an example with Sky Contribution decreased to 0.25. Indoor fade-in becomes much less noticeable, but outdoor appearance is worse:

sdfgi-1-cascade-fade-low-sky-contribution_.mp4

In outdoor scenes, fog can be used to make SDFGI fade-in less noticeable. If you are OK with having coarser GI, you can also increase Min Cell Size to increase the area covered by SDFGI.

[clayjohn]

Looks much better than having no GI to me! Even in motion!

[akien-mga]

Thanks!