Refraction and "All Channels"

[SonyBoy]

I've noticed that it's necessary to have Dispersion enabled in order to get a multimatte element for a refracted object to work. Is this intended behavior, and if so what's the rationale behind it as dispersion and generating a matte seem like they are completely unrelated? This has confused me several times in the past, where I've been unable to get a matte element even with "All Channels" set on the refractive properties. At least now I know that this is why, but I'm curious to know why this is necessary as obviously there may be situations you don't want chromatic abberation and just want to get a refracted matte.

[vlado]

Is this for glossy refractions? I don't think the intention was ever to require dispersion to be enabled in order to propagate render elements though refraction, so it might simply be a bug.

Best regards,
Vlado

[SonyBoy]

Refraction glossiness is at .995, though again I am confused as to why even that would be relevant versus non-glossy refractions for the same reason -- surely glossiness like dispersion also has nothing to do with wanting to generate a refracted matte? If it's not intended behavior then yes this does seem like a bug to me.

[vlado]

The code for glossy refractions is different than the code for perfect refractions; I will have to check if it takes care of propagating the channels. If not, we'll add it.

Best regards,
Vlado

[yolov]

Can you get us a scene that shows this behavior?

[SonyBoy]

Yes, will send something over ASAP.

[SonyBoy]

Here's a much simplified scene that shows the problem. With dispersion on the jar's glass shader off, the cube inside the jar does not output a matte into the red channel of the multimatte. When on, it does. Note that the glass shader has been left with its two upstream bump shaders intact just in case this is relevant, but previously this shader was working anyway in Vray 3.6 so this is perhaps related to Next.

[SonyBoy]

Can you get us a scene that shows this behavior?

Hi Yolov just wanted to make sure you see the attached scene. Thanks.

[yolov]

Hey, yes, I had a chance to look at the scene. It seems a bit strange.
I'll make a note for the devs to look into this.
In the meantime, here's a few findings:
- if you disable 'affect all channels' for relfections on the same material - the object inside will be masked properly for multimatte without enabling dispersion.
- if you use cryptomatte - the masks will be there regardless of settings (but I don't know what compositing app you use, so keep in mind that cryptomatte only works for bucket rendering to multichannel exr files and the cryptomatte plugin is available for nuke, fusion and since recently for after effects).

I'll update the thread once there's some progress on this.

[SonyBoy]

Ah, perhaps previously I had had All Channels on reflections turned off, as it had been working as expected. I thought I had tested cryptomattes as well and found that this pass was also dependent on having dispersion turned on, but maybe I didn't. But those will not ultimately get me the mattes I need anyway -- I need to generate some custom per vertex ID mattes. Either way, as you say it doesn't seem right the way it's currently behaving. Thanks for looking into it.