Hello everyone,
I’m encountering an issue with path tracing in Unity HDRP related to the visibility of the sun disk through transparent materials.
As an exemple, I’ve made a scene with two identical windows positioned side-by-side. The left window has no glass pane, while the right window has a transparent material applied (using the HDRP/Lit shader).
Here’s what’s happening in different scenarios:
1.Without Path Tracing:
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When 'Affect Physically Based Sky" is enabled on the directional light, the sun disk appears through both windows as expected
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When ‘Affect Physically Based Sky’ is disabled, the sun disk disappears correctly behind both windows (as expected)
2. With Path Tracing Enabled:
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When ‘Affect Physically Based Sky’ is enabled, the sun disk appears through both windows, which is expected behavior
-
When ‘Affect Physically Based Sky’ is disabled, the sun disk disappears behind the window without a material (left window), but it remains visible through the window with the transparent material (right window), which is unexpected:
It seems like the path tracing renderer is not respecting the ‘Affect Physically Based Sky’ setting in the same way that the standard renderer does, at least when dealing with transparent materials. This discrepancy leads to the sun disk being visible through transparent materials even when it shouldn’t be.
Some context:
-Unity Version: 6000.0.24.f1
-HDRP Version: 17.0.3
-The scene is instantiated at runtime, and adjustments to the directional light and path tracing settings are made dynamically. All lights are on realtime.
Has anyone else encountered this issue with HDRP path tracing and transparent materials?
Is there a setting or workaround that might resolve this, or could this be a potential bug in HDRP?
Thank you for your help and insights!
I just noticed that volumetric clouds are also visible through transparent materials, which shouldn’t normally happen with Path Tracing. This behavior definitely makes me think it might be a bug or limitation in how HDRP handles transparency with Path Tracing.
This is not really a bug, eventhough the path tracer won’t support volumetric clouds if you look directly at the sky, it will sample the sky texture after a ray miss (when the ray bounce on something and then ends up on the sky). And the clouds are written into that texture so that we can see them in the reflection for exemple.
The same thing happens if you have a reflective sphere for exemple, with path tracing on, you should be able to see the clouds in the sphere reflection but not when looking directly at the sky.
And this is most likely the same reasons why the Affect Physically Based Sky thing happens in your first post. You could report something about it but I’m not sure how it could be solved easily !
Thank you for the explanation; I understand the principle (even though, in my case, it’s not really a bounce but rather rays passing through the glass and ending up in the sky texture). What I don’t understand is why the directional light disk is written into this texture when it’s never actually enabled (“Affect Physically Based Sky” is always unchecked).
I mentioned the volumetric clouds issue just to add context, but it’s not really essential for me. My main goal is to make this directional light disk disappear. Do you have any “out of the box” ideas for a workaround, maybe?
Thanks again for your help!
I see what you mean, are you it’s not juste the sky texture not being “updated” when you change the settings (like checking Affect Physically Based Sky). As a test can you make sure if the issue reproes if this is checked on your side ?
And for this, a bouce here is just a ray hitting something, it doesn’t matter if it’s direction change or not 
Yes, I’m sure about this because “Affect Physically Based Sky” is unchecked by default, and I never enable it. I only make a few adjustments at runtime, such as rotating the directional light and tweaking the angular diameter, but that’s it. Also, my Update Mode is checked and set to “Realtime.”
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I see, I haven’t been able to repro on my side. Do you have a repro project to share or a bug you could log using the bug reporter ?
Hi chap,
On my side, I managed to reproduce the bug very easily. In a test scene, I simply set up a camera, a transparent cube, and a directional light. I aligned the light and adjusted its settings so that, if there were a Physically Based Sky with the display option enabled on the directional light, we’d see the disk. However, in this scene, I didn’t even add a Physically Based Sky in the volume; only the path tracing is enabled.
As you can see from the images, the disk still appears with path tracing active, even without the sky setup.
Here’s a Unity package file of the test project:
test_HDRP.unitypackage (80.6 KB)
Let me know if there’s anything else you’d like me to test or clarify!
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Thanks, I logged it here if you want to follow it
Have a nice day.
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Hello,
I’m following up on this topic because this bug is still blocking my project. The ticket is still marked “Under Consideration,” and I already posted a comment asking for more information, but haven’t received any response yet.
This is particularly important because I’m working on a professional project for one of my clients.
Is there a chance it will be addressed in the near future, or should I start exploring alternative solutions to work around this issue?
Thank you in advance for your time and help!
