Everything related to Maxwell Render and General Stuff that doesn't fit in other categories
By ki_cz
Thanks a lot for having a look, hopefully somebody will give us some insight :)

It's a strange problem, almost as if when using IBL, Maxwell is using the FIRE engine automatically or something ...
Hi ki_cz,

As I wrote before, but here is the simple explanation.
I can only attach pictures from maxwell render V3, but I am almost sure, that nothing will be different in V5.1.
As I wrote before, this is a "classic" - caustic through dielectrics problem.
You can see in my simple pictures how it works. Every "shadow" of transparent material is considered as caustic. Caustic can be refracted or reflected. Caustic can be seen directly or indirectly (refracted through another dielectric material or reflected through a mirror). The most difficult caustic is indirectly seen caustic of almost unidirectional beam light sources as is Sun (0,5°) or any small light sources shining from a big distance. There is nothing wrong with Your HDR images as You can see, there is good enough contrast between the shadow of a lit object and another unshadowed part. If there will be a problem with HDR images (dynamic range), there will be much less contrast or the edge of the shadow will be much softer. There is also no problem with the resolution of HDR as You can see.
You can see in the attached pictures, that the caustic is there, but in much higher sampling levels. Another point is that in the physical sun and sky scenario, there is a problem just with indirect seen refracted caustic. In the pictures with HDR, there is the same bad-looking directly (my surprise too!) and indirectly seen refracted caustic. And You can predict from small preview images, that the caustic will be there, but clean in SL 40 or even above. In the Fire engine, there is all caustic disabled (correct me please if I am wrong).

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By ki_cz
Like, I suppose that I understand that the caustics are more complicated with IBL, but for a render engine to be unable to calculate a transparent single pane of glass in an IBL environment without reaching "SL 40" is a bit absurd to me.

I also find it weird that when using an MXI or HDR image on a spotlight or an emitter, then the caustics/transparency seems to work as it should. The problem seems to only exist when using an image for global illumination, which makes me think that there is something else going on behind the scenes other than it simply being too complicated of a calculation.
By ki_cz
After a break I'm back to rendering and still wondering if anybody has any suggestions for how to deal with this issue? Simple single panes of glass taking until SL40 to be transparent with IBL seems kind of unacceptable to me, especially when it is so fast with emitters or physical sky ...
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By Mark Bell
Maybe start a fresh file and create some new AGS glass and go from there. I've had instances in the past where I've accidentally turned on/off a setting or adjusted a figure without realising it which has had a big impact on the render, but wasn't able to find what setting had changed for a long time.
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By Artistus
OMG! Now I understand the problem of darkening of the contents inside the glass bottle, like the cream inside the glass or huge amount of transparent grainy materials like salt, sugar etc. Because of this BUG (or maybe unsolvable issue), which takes A LOT more time to render caustics back through the dielectric it is impossible to use Maxwell for such projects. The render can take easily a week for 1 image for such a project. That's not serious at all.

There is single sheet of low grade glass 15*25 cm over the grey surface and 1*1 cm plane of light in attachment. If you try to render a camera lens this way, you'll get a lot of noise inside lens system. What I can see in the image below - it's ridiculous!

Maxwell team, please, check if this can be solved.
By ki_cz
Thank you very much for adding some input :) Let's hope we hear something from NL soon, I agree that this makes a lot of projects impossible to complete.
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By Artistus
seghier wrote:
Thu Jan 28, 2021 1:48 pm
Check this

I'm afraid that this is not the solution for this kind of projects here.

I have tested the same scenario in Blender + LuxCore render.
1. With the default Path engine in LuxCore the situation is even worse, getting the result like fully dark shadow behind the glass sheet (HDRI map illumination in Maxwell).
2. When I change engine in LuxCore to Bidirectional and leave default Sobol sampler the issue looks the same as in Maxwell with physical sun or light plane for illumination.
3. But when I switch to Metropolis Light Transport sampler (which is available only in Bidir engine on CPU, the issue looks like to have optimization or some sort of workaround, so the caustics is getting back through the glass with a good grain of noise, which hopefully reduces MUCH MUCH faster than in any of previous options.

I'd like to have similar optimization available in Maxwell (like on 3rd example from LuxCore).

By ki_cz
Thanks for the continued investigation, I will run a few more different tests once I have a chance as well, hopefully to help narrow down the issue for the NL team.
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By Artistus
Well, I have an update for this investigation. It seems like the issue is in the light size compared to the object size. When the light is relatively small (radial size from object's perspective), then it's very time consuming to get caustics from glass (it's shadow) back through that glass object. If the light source is relatively big, the issue magically disappears, but on the other hand caustics in this scenario will be less crisp.

You can compare the results in attachment. Expected result when light plane size is 10 cm, while sheet size is 5 cm. And odd result when light plane is 1 cm in size.

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