LOD shader to reduce popping doesn't fully works (Documentation bug)

Unity Engine
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1

Heyo

I’ve created a subgraph to manage my LOD’s like so

7937929--1014739--upload_2022-3-3_5-27-54.png
7937929--1014739--upload_2022-3-3_5-27-54.png1238×512 45.2 KB

Plugged it in to Alpha and Clip threshold respectively which with combination with the LOD group component setup with a “Cross Fade” fade mode and fade transition of 0.2 for each LOD like so
7937929--1014745--upload_2022-3-3_5-29-20.png
7937929--1014745--upload_2022-3-3_5-29-20.png701×231 16.9 KB

It works… well half works…
The issue is that while the LOD objects fade out nicely, they still pops in without fade.
Like in this demonstration:
7937929--1014763--AnimationCin.gif
As you can see, as soon as LOD0 starts to fade out, LOD1 immediately pops in without fading.
Which doesn’t reflect what’s the manual says
7937929--1014766--upload_2022-3-3_6-14-31.png
7937929--1014766--upload_2022-3-3_6-14-31.png769×193 13.7 KB

If there is something I did wrong, it’s probably in the shader… someone knows and can help?
Thanks

2

Bug case 1408617

3

Maybe try to test it in play mode. The editor view doesn’t really reflect trutly the LOD transition from my experience. But I’m not entirely sure about it.

4

Thanks!
Actually the GIF above was made in play mode.
I have also created a fresh new URP project, same results… it is also how it behave in builds.
Also build that project in 2021.2.8, same results.

5

I think I had similar issues before. It was a long time ago tho. My solution was to use cross-fade with animated cross-fade mode on. That will make the cross fade stick at a 2s interval. The interval can be changed I believe.

6

I think the issue is beyond that.
In animated cross-fade mode the “next” LOD still pops…

7

Try using the CopySign function, this handles the issue you’re seeing and is also used in the SpeedTree shaders. Oddly enough, not needed in the Built-in RP.

This is a snippet from the function I’m using for LOD crossfading:

#if LOD_FADE_CROSSFADE
    float hash = GenerateHashedRandomFloat(screenPos.xy * _ScreenParams.xy);
    float sign = CopySign(hash, unity_LODFade.x);
   
    clip(unity_LODFade.x - sign);
    #endif
8

Thanks looks like a nice workaround but I got it!
The documentation for LOD’s is not intuitive at all. By reading this portion it was clear to me that the x component can only be 0-1. (with the assumption that it may corelates to alpha)
7948093--1017418--upload_2022-3-8_6-58-54.png
But after lot more digging I found this post
7948093--1017421--upload_2022-3-8_7-7-35.png
(and also your suggestion of using the sign of x) and got it to work as intended!

So looks like it is a documentation bug.
And I did done things wrong but only because I followed it.

I moved the code to a file and attached it.

7948093–1017424–ThisIsHowToProperlyUseLODFade.hlsl.txt (149 Bytes)

9

Documentation seems wrong, maybe that changed in the SRPs? The range being -1…1 instead of 0…1 is also obvious from the UnityCG.cginc:

    float sgn = unity_LODFade.x > 0 ? 1.0f : -1.0f;
    clip(unity_LODFade.x - mask * sgn);