[3.x] Fix SSAO stairstepping artifacts when using a high Camera Far property

[Calinou]

This matches master behavior where high Camera Far property values can be used just fine with SSAO.

This closes #42390.

Testing project: test_ssao_2_3.x.zip

Preview

Camera Far 100 (project default in 3.x)

Before	After

Camera Far 500 (editor default in 3.x)

Before	After

Camera Far 4,000 (default for both project and editor in master)

Before	After

[clayjohn]

I don't think this is the right approach. What will happen with this change is that the depth buffer will be treated as if it is only 200 units deep (even if it is representing a scene that is 4000 units deep) so to the SSAO objects that are 1000 units apart will look like they are only 50 units apart. More concerningly, objects that are 20 units apart will cast SSAO as if they are one unit apart.

If you want to clamp distant SSAO, you should copy the approach used in ASSAO which is to fade the SSAO out as it approaches the maximum distance (which is configurable with a project setting)

godot/servers/rendering/renderer_rd/shaders/ssao.glsl

Line 431 in d956904

float fade_out = clamp(pix_center_pos.z * params.fade_out_mul + params.fade_out_add, 0.0, 1.0);

[Calinou]

If you want to clamp distant SSAO, you should copy the approach used in ASSAO which is to fade the SSAO out as it approaches the maximum distance (which is configurable with a project setting)

I don't think this will resolve the stairstepping artifacts that appear close to the camera though (especially when approaching a surface that's parallel to the camera). It's likely caused by a lack of depth buffer precision.

Can we do both approaches here (fade distant SSAO and limit camera Z far)?

[clayjohn]

I don't think this will resolve the stairstepping artifacts that appear close to the camera though (especially when approaching a surface that's parallel to the camera). It's likely caused by a lack of depth buffer precision.
Can we do both approaches here (fade distant SSAO and limit camera Z far)?

Perhaps. We can't control the depth buffer precision, because that is set elsewhere. But maybe doing a combined approach can work to reduce the depth precision artifacts while also reducing the artifacts that come from arbitrarily scaling the depth values.

What is the highest Z-far we can use without introducing the stair stepping artifacts? I am thinking it you set the max Z-far to something like 2000 then the scaling artifacts should be minimal.

[Calinou]

What is the highest Z-far we can use without introducing the stair stepping artifacts? I am thinking it you set the max Z-far to something like 2000 then the scaling artifacts should be minimal.

I already get significant artifacts when up close at Z-far = 1000. I'd say 500 is the maximum value where it's acceptable, but you can still notice artifacts if you look for them (compared to 200).