Light Tracing Too 'Powerful'

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Beinsezii
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Light Tracing Too 'Powerful'

Post by Beinsezii » Mon Oct 28, 2019 7:37 pm

When using the 'Add Light Tracing' setting in the path renderer, the generated effects eventually become way too bright and sometimes blow out given enough samples. Best explained with photos, so here they are:
Result of about 2500 samples. Way too bright.
Result of about 2500 samples. Way too bright.
Certain settings cause it to explode. IDK why.
Certain settings cause it to explode. IDK why.
Result of about 1000 samples. What you'd expect.
Result of about 1000 samples. What you'd expect.
Bidir HQ reference. Closest thing to ground truth. 30 minutes @4k + OIDN
Bidir HQ reference. Closest thing to ground truth. 30 minutes @4k + OIDN
And the blend file, which somehow became 18mb and is too big to attach.
Render and wait till about 1700 samples and it'll start getting brighter till ~ 2400 samples.

The issue persists in CPU if you run enough samples into it. Setting the light/camera ray ratio to something really high like 95% will cause it to embrighten or explode very fast.
Issue is not present on Bidir, at least not that I've been able to find.

I'm here instead of Github for now because I know basically nothing about 3d software outside of my playing around with shaders for fun, so it could be user error. Issue always happens at the exact same sample in the render, so I doubt its cosmic rays.
My hypothesis is eventually the large values of caustic light cause some sort of overflow in the normalization/tonemapping area.


Off-topic: any good ideas to simulate the discoloration in diffraction with cycles? The linked blend file also has my current experiment with 3 separate glass shaders for RGB at slightly different IOR's when using the cycles/eevee renderers.

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Dade
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Re: Light Tracing Too 'Powerful'

Post by Dade » Fri Nov 01, 2019 1:59 pm

I have tried your test scene up to 5000 samples but I can not reproduce your problem:

5000.jpg

What version are you using ? I have fixed a problem in the past that could explain what you are observing, can you try if it happens with a daily build too: viewtopic.php?f=9&t=736
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Beinsezii
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Re: Light Tracing Too 'Powerful'

Post by Beinsezii » Tue Nov 05, 2019 1:57 am

You're 100% correct. Switching from latest stable (2.2) to latest alpha (2.3) fixed the value bug and color accuracy is significantly closer to bidir.
I had searched through the github closed issues and couldn't find anything directly related, so I didn't even think to try the new dev build.

Thank you for your time and help.

Martini
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Re: Light Tracing Too 'Powerful'

Post by Martini » Sun Nov 17, 2019 12:19 am

I am seeing this problem too with last night's build of 2.3 :cry:

Here is CPU Path with Light at 20% ratio (to avoid weirdness from my GPU being so much faster than CPU)
thing01-cpupath+light.jpg
Here is BIDIR
thing01-cpubidir.jpg
Both rendered to 4096 samples. Sorry I don't have screenshots of intermediate sample counts. What I did notice was that it started out looking more-or-less correct, but by about 300 samples it was quite visibly noticeably too bright, and just gets worse with more samples. :(

Edit: here's the .blend file. You can get the exact HDRI I used from this HDRI Haven, or substitute your own.

Edit 2: if you want to render this, check the Performance section because I think I had set it to only use 6 threads as I also use this computer for SheepIt clients
Attachments
thing01-lux.blend
(811.5 KiB) Downloaded 6 times
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Martini
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Re: Light Tracing Too 'Powerful'

Post by Martini » Sun Nov 17, 2019 1:22 pm

Martini wrote:
Sun Nov 17, 2019 12:19 am
I am seeing this problem too with last night's build of 2.3 :cry:
OK so now that I'm able to run LuxCore UI (standalone), I have noticed that this problem only seems to manifest when rendering inside Blender. For the standalone renderer, the issue seems not to occur. Could there be an issue with tonemapping vs different sample counts or something like that? It feels like the total accumulated brightness is being divided by a number that isn't increasing fast enough.
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Dade
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Re: Light Tracing Too 'Powerful'

Post by Dade » Sun Nov 17, 2019 4:12 pm

The problem is I can never replicate this problem here. I have tried inside and outside Blender and with Linux and Windows but the scene just works fine:

bright.jpg

It may be a random problem and it may be also related to the hardware (CPU) used (i.e. this may explained why it never happen to me).

I have a vague idea of why it may be happening. I applied a tentative patch, can you check if the latest automatic build (when it is ready) has fixed the problem ?
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Martini
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Re: Light Tracing Too 'Powerful'

Post by Martini » Sun Nov 17, 2019 10:45 pm

Dade wrote:
Sun Nov 17, 2019 4:12 pm
The problem is I can never replicate this problem here. I have tried inside and outside Blender and with Linux and Windows but the scene just works fine:
Well, good news, now neither can I! :lol:
Brightness is now behaving as I'd expect
Brightness is now behaving as I'd expect
Dade wrote:
Sun Nov 17, 2019 4:12 pm
It may be a random problem and it may be also related to the hardware (CPU) used (i.e. this may explained why it never happen to me).

I have a vague idea of why it may be happening. I applied a tentative patch, can you check if the latest automatic build (when it is ready) has fixed the problem ?
Yes, confirmed, problem no longer occurs! :D

If you have time I'd love to know your thoughts on why this was happening on my CPU but not yours?? Very intriguing! :ugeek:
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Dade
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Re: Light Tracing Too 'Powerful'

Post by Dade » Mon Nov 18, 2019 10:36 am

Martini wrote:
Sun Nov 17, 2019 10:45 pm
If you have time I'd love to know your thoughts on why this was happening on my CPU but not yours?? Very intriguing! :ugeek:
When multiple cores write a floating point 32bit value, it is unknown who will win but only one will win, the values can not be mixed/corrupted (i.e. you can not have lower 16bit from one value and higher 16bit from another). I have the feeling this is not true for floating point 64bit, at least on some hardware, you can end with 32bit from one value and 32bit from another.
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Sharlybg
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Re: Light Tracing Too 'Powerful'

Post by Sharlybg » Mon Nov 18, 2019 11:00 am

When multiple cores write a floating point 32bit value, it is unknown who will win but only one will win, the values can not be mixed/corrupted (i.e. you can not have lower 16bit from one value and higher 16bit from another). I have the feeling this is not true for floating point 64bit, at least on some hardware, you can end with 32bit from one value and 32bit from another.
When i ear such kind of explanation i just wonder if you have some students or people around to learn from you. You are such a precious specimen on your own. We should clone you :mrgreen:
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Racleborg
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Re: Light Tracing Too 'Powerful'

Post by Racleborg » Mon Nov 18, 2019 12:00 pm

When i ear such kind of explanation i just wonder if you have some students or people around to learn from you. You are such a precious specimen on your own. We should clone you :mrgreen:
Ditto :)

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