[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 ...

[PA3K]

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).

Patrik

[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.

[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 ...

[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.