Spectral rendering

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lacilaci
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Spectral rendering

Post by lacilaci » Tue Jan 21, 2020 8:02 pm

There is an interesting talk about turning cycles into spectral renderer
https://devtalk.blender.org/t/thoughts- ... r/2192/332

There is also a bit scary comparison on how much differently the RGB renderers handles colors and lights.

I know luxcore can do spectral effects like dispersion and that old lux was full spectral renderer.

So my question is, are there benefits to render spectraly and is there any plan or motivation to make luxcore into a spectral renderer?

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Re: Spectral rendering

Post by sarmath » Tue Jan 21, 2020 8:22 pm

lacilaci wrote:
Tue Jan 21, 2020 8:02 pm
There is an interesting talk about turning cycles into spectral renderer
https://devtalk.blender.org/t/thoughts- ... r/2192/332

There is also a bit scary comparison on how much differently the RGB renderers handles colors and lights.

I know luxcore can do spectral effects like dispersion and that old lux was full spectral renderer.

So my question is, are there benefits to render spectraly and is there any plan or motivation to make luxcore into a spectral renderer?
I am gonna quote Dade again from this forum :D
http://ompf2.com/viewtopic.php?t=1981

"In my opinion, aside from considerations on the rendering quality where spectral rendering is likely to be always better, you have to factor also the impact on rendering times: not because it is computational more expansive than RGB/XYZ rendering but because it generates more noise. After all, you have yet another dimension to sample (i.e. wavelengths)."
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Dade
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Re: Spectral rendering

Post by Dade » Tue Jan 21, 2020 9:59 pm

Spectral rendering is also a bit useless in pure path tracer like Cycles. For instance, dispersion is pointless without light tracing.
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lacilaci
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Re: Spectral rendering

Post by lacilaci » Wed Jan 22, 2020 5:27 am

Dade wrote:
Tue Jan 21, 2020 9:59 pm
Spectral rendering is also a bit useless in pure path tracer like Cycles. For instance, dispersion is pointless without light tracing.
Well they do show how spectral rendering gives quite a bit different result with colored light and saturated materials.
The RGB rendering also looks almost broken in extreme situations.

Thiago also compares luxcore and cycles to octane which is spectral and both luxcore and cycles looked pretty bad while spectral cycles and octane retained colors way better.

Once they make a blender build I'd like to test in practical situations how much and if any difference there is. Especially in situations where there are important color shifts. Like overexposed materials or saturated sky and sunset etc. These are usually extremely hard to make look good.

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Re: Spectral rendering

Post by Sharlybg » Wed Jan 22, 2020 9:19 am

For me spectral should be more considered now. It should be an option at some point. From my artist POV i was always shocked by the additional realism that Maxwell Indigo and Thea render are able to provide.

The main reason that lead people to forget about Spectral was noise/speed issue. But don't forget that we were doing it on very slow hardware ( Old dual core /quad core ) and also engine was not optimized as today.

Now 32 to 128 threads CPU is something we can afford. And also GPU are now faster and integrate hardware acceleration for rendering. We also have denoiser that become more powerfull day after days.

With online learning method Bidir engine can be the first to test this features while being faster.
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lacilaci
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Re: Spectral rendering

Post by lacilaci » Wed Jan 22, 2020 11:23 am

Sharlybg wrote:
Wed Jan 22, 2020 9:19 am
For me spectral should be more considered now. It should be an option at some point. From my artist POV i was always shocked by the additional realism that Maxwell Indigo and Thea render are able to provide.

The main reason that lead people to forget about Spectral was noise/speed issue. But don't forget that we were doing it on very slow hardware ( Old dual core /quad core ) and also engine was not optimized as today.

Now 32 to 128 threads CPU is something we can afford. And also GPU are now faster and integrate hardware acceleration for rendering. We also have denoiser that become more powerfull day after days.

With online learning method Bidir engine can be the first to test this features while being faster.
I keep hearing this from many people. That the lighting etc just works much better in those renderers. Also if you look at the comparisons in devtalk, the rgb rendering looks like it's even broken somehow. I assume that situations like saturated sky, sunset, overexposed saturated materials might get messed up with just rgb rendering...

It would be cool if there was a switch in luxcore to do rgb or full spectrum so we can do direct comparisons right on spot. But I kinda don't think it's that easy to do :D

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Re: Spectral rendering

Post by Sharlybg » Wed Jan 22, 2020 12:36 pm

Look like it is possible to convert RGB to a kind of fake spectral without affecting performance :

https://radiance-online.org:447/radianc ... erfect.pdf
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lacilaci
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Re: Spectral rendering

Post by lacilaci » Wed Jan 22, 2020 12:44 pm

Sharlybg wrote:
Wed Jan 22, 2020 12:36 pm
Look like it is possible to convert RGB to a kind of fake spectral without affecting performance :

https://radiance-online.org:447/radianc ... erfect.pdf
well, as the quote from dade says. It's not making rendering slower but generates more noise.
Now, while I'm always looking to speed up rendering I also want to see good renders. Anyways, I asked the guy from devtalk "smilebags" the one who is doing the cycles conversion to spectral if there will be a build so hopefuly today... I probably won't be able to make a direct comparison against luxcore though...

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lacilaci
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Re: Spectral rendering

Post by lacilaci » Wed Jan 22, 2020 12:55 pm

The big problem that made me think there is more to spectral rendering than fancy effects is how those saturated colors interact. Also Dade, you said dispersion is pointless for pathtracers but how do you do diamonds, and some thin film dispersion effects.. All spectral renderers can do those effects, yes they might not be able to do caustics but rest looks correct and there is no easy "fake" to do dispersion. There are some fakes but look like dogshit.

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Sharlybg
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Re: Spectral rendering

Post by Sharlybg » Wed Jan 22, 2020 1:05 pm

I also wonder if the additional photorealism look in other rendering isn't achieve with far higher like bounce by default ?

Btw Radeon prorender is doing spectral rendering on viewport with vulkan : http://www.cgchannel.com/2019/12/radeon ... rendering/
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