[VFX] Fix soft particles

[PaulDemeulenaere]

Purpose of this PR

TLDR, it fixes two fogbugz :

• https://fogbugz.unity3d.com/f/cases/1309961/
• https://fogbugz.unity3d.com/f/cases/1330697/

More details :

• Fix soft particles orthographic projection : it was initially fixed by @ludovic-theobald in this PR.
• Fix missing Flip-Y : implicitly integrated in GetNormalizedScreenSpaceUV. The issue occurs when there isn't an HDR or Opaque texture as intermediate texture (⚠️This intermediate texture can be implicitly enabled by renderer feature like decal)
• Fix wrong behavior on OGL : We were loading instead of sampling, when the depth texture isn't available, the default texture can be either black or white depending on reverse_z
• Fix missing scale integration : See e73718c I'm not sure if the issue was already there, I didn't test it before, maybe relative to the change from Load to Sample

---

Testing status

1) Orthographic Depth
Before

After : Fix Orthographic Depth Sampling

Also an equivalent test from Ludovic :

2) Missing Y-Flip
Before

⚠️ Decal has the same effect than enabling hdr/opaque texture

After

3) Fallback when there isn't a depth texture
Before DX11 (no issue actually)

Before OGL (on the left Shuriken, on the right VFX)

After

4) Screen scale
Before

After

---

Comments to reviewers

This change will require a manual backport in 10.x.x & 7.x.x
See also this PR : https://github.cds.internal.unity3d.com/unity/vfx-graphics/pull/211

[ludovic-theobald]

Looks good to me!

[VitaVFX]

Thank you for shiny tests and detailed description, @PaulDemeulenaere!

Tested:

• All camera settings (ortho + perspective)
• URP asset settings
• Forward decal
• Output settings/inspector
• Standalone
• Graphics APIs (DX11, Vulkan, Metal)

Everything you fixed works like a charm, but observed several issues:

1. Soft particles do not work with camera stacking
   
2. Decal particles have 'sliding' problem when tweaking 'Render Scale' property in URP asset
   

Repro can be found here

Is this something that could be fixed in this PR?

[ludovic-theobald]

@VitaSkruibyte For the second issue, it's possible that it is fixed in the VFX HDRP Decals PR (at the same time as the sliding artifact was fixed).
I'll test and come back to you.

Edit : It's not fixed in the other PR.

[PaulDemeulenaere]

Edit : It's not fixed in the other PR.

I have an idea about this issue, forward decal use a simple sampling which require to consider the renderscale while sampling the depth. I will take a look, if it's a simple fix, we could append it to this PR ⏳

[PaulDemeulenaere]

Update for @VitaSkruibyte

1. About Soft particles do not work with camera stacking
   
   Can you confirm it doesn't work with shuriken as well ? If it's the case, it's probably a common issue within URP and should be reported in URP grab bag.

2. About Decal particles have 'sliding', the issue is actually here
   It should be
   clipPos.xy = (i.pos.xy * rcp(GetScaledScreenParams().xy)) * 2.0f - 1.0f;
   Instead of
   clipPos.xy = (i.pos.xy / _ScreenParams.xy) * 2.0f - 1.0f; (cc @ludovic-theobald)
   Can you open a fogbugz (and assign it to me) for this one ? I will create an independent PR for this issue, it will probably require an new interface in VFX (GetScaledScreenParams is specific to URP).
   I'm also wondering if there is a specific issue on SubpixelAA & ScreenSpaceSize with RenderScale & URP.

[PaulDemeulenaere]

I isolated the fix about forward decal here : 5a4cb98 (not included ⚠️)

_scale_screen_space_size.mp4

In the end, should I integrate this change in this PR @julienf-unity ?

[VitaVFX]

Heya @PaulDemeulenaere! Thank you for the update!

Here's the bug for URP team regarding soft particles
Here's the bug report regarding decals. Also, bumped into another issue where forward decal is not properly projected onto the surface with Overlay Camera, so regarding your comment could this be looked into in the independent PR you've mentioned? (of course, if you have bandwidth for this, otherwise I'll just open a new FB)

[VitaVFX]

All PR related issues were fixed, newly found bugs are not PR related and gonna be fixed in following PR, hence approving.

[PaulDemeulenaere]

VFX_HDRP on Win_DX11_editmode_mono_Linear is expected to fail 🔴

[PaulDemeulenaere]

TODO : backport on 21.1, 10.x.x & 7.x.x