Issues with glass shader

[nicola.danese]

Hi

we're having some issues rendering glass objects in mtoa version 2 and arnold core 5

First when you load the glass preset it doesn't change the transmission value, so you have to remember to do it manually. That's not a huge deal, but still confusing since when you create a new standard surface and apply a glass present you get a black object

Second and more important is the lack of shadows from the glass object to the surface below it, I noticed that if you set transmission to 1 the object doesn't cast any shadow, while decreasing the value of transmission let the shadows coming out, but the glass gets dark, too dark...

I'm trying to render a clear transparent glass with proper shadows, light this reference

http://explore.patternity.org/wp-content/uploads/2014/01/Patternity_Glass-Shadows_Zoe-Ghertner.jpg

thanks!

[Anonymous]

Just throwing in my opinion here. That image you're referencing is a heavy caustic effect. Those shadows are there because the light is being bent away to the brighter areas (the caustic bits). I've read a few times that Arnold and caustics don't play nice with each other yet.

I don't want to send you on the wrong path but perhaps try playing with scatter and see if that creates some sort of shadow.

I've not been able to get caustics like that in Arnold for a LONG time.

[zenop]

Did you test out your shader in an extremely simple lighting environment? E.g. a good quality HDR with decent dynamic range? The shader setup is so simple that it is more likely to be the lighting that is off.

This render has opaque off, transmission pushed to 1. There's no caustics as mentioned previously, but shadows shouldn't be a problem.

If you could illustrate your problem we will be able to help you better.

[nicola.danese]

hey guys, thanks for your replies!

I'm testing on a very simple env, with one area light, here's the result of my render with transmission at 1, no shadows at all from the glass...no-shadow-glass.png

[zenop]

What are the shape attributes of your object? Is shadow casting on?

[nicola.danese]

yep, is on, here's a render with transmission set to 0.8, I got shadows, but the glass is extremely dark

[Robert_Mastny1]

Hi Nicola!

It would be of help if you could upload your Maya test file and the HDR you are using (if you can share it); it's easier to look into it to determine where the error lies.

[nicola.danese]

Hi Robert, here's the scene made with maya2017 that shows the issue I'm having

https://we.tl/OODh1shhy4

thanks for your help!

[Robert_Mastny1]

Thanks for adding! Hopefully someone else other than me also looks at that.
I could reproduce your "problem". It's under quotes because it could well be just the type of geometry that does not cast a (visible) shadow and that there is nothing wrong.

I inserted a fairly complex object into the scene and assigned the same glass shader and got proper shadows. So I would say it has nothing to do with the shader or scene itself. But I certainly would expect a visible shadow from that object.

Maybe someone else can weigh in on this and can explain why this is happening.

[nicola.danese]

@Zeno Pelgrims can you please share with me the scene you used to get that shadow so I can get more hints about it...cheers

[lee_griggs]

Caustics are disabled by default in the Standard Surface shader. They can be enabled under the Advanced section.

Here is a tutorial.

[lee_griggs]

Are you still getting the Transmission issue with the latest build of MtoA?

You can, of course, create your own material preset to get around this issue.

[nicola.danese]

Hey Lee

we are using MtoA 2.0.1, so not the latest. I'll check as soon as we upgrade

thanks!

[nicola.danese]

so it looks like if you want shadows from glass with transmission set to 1 you have to keep opaque checked. this way you get refractions and shadows. problem is if your glass has color, cause the shadows is going to be black, so you have to uncheck opaque, and you have some colors projected from the glass, but still no shadows. I really don't get the correct workflow for glass rendering, it looks to me like a bunch of hacks to get a nice render. To get caustics I produced a caustic map from another renderer, and projected it into the emission of the object that is receiving the caustics...it would be fantastic to have a nice tutorial on this topic!

[lee_griggs]

Lowering the Transmission Weight to something like 0.95, will give you colored shadows.

Please share your caustic results.

[nicola.danese]

Hey, here you can find some test we made about caustics...

[lee_griggs]

Nice work Nicola. Is that using an Emissive object?

[nicola.danese]

hey, you mean the caustics on the table? I exported the object in houdini and made a top-down render of the object lit by a caustic light (you can save the photons map, then disable the source light so you still get the caustic without the original light. Then I took the texture and mapped it into the emission of the shader attached to the table...) it's a painful job to get that out...especially if you have that caustics on more objects at the same time...

[sulkybunny]

hey guys anything new on this front? Is there a way to get these nice caustics cast from a glass object? Without going to comp and work around? I have an animated glass character and I am eager to get these nice looking light effects all over the place..

[nicola.danese]

hey Saul

I don't think there's any improvement on that side, afaik arnold isn't a bidirectional path tracer, so technically arnold can't produce caustics, but I'd be very happy to be proven wrong 😉

If your work is heavily based on caustics I suggest you another approach (I mean another renderer...)