Alpha Issue with Color+Alpha in VRayMtl

[jordan_dubosc]

Hi Team,

Happy New Year!

We are encountering some unexpected results when testing feather alpha under the following setups:

- V-Ray 6 Update 1.2 (6.10.09 build 32197) with 3ds Max 2023
- V-Ray 6 Update 2.1 (6.20.06 build 00000) with 3ds Max 2024

Test Setup

- A basic gray VRayMtl with a single VRayBitmap in the opacity slot (set to None/Raw) with color only (no refraction applied).


- A basic gray VRayMtl with a single VRayBitmap in the opacity slot (set to None/Raw) with color + alpha (no refraction applied).





We have internal tools that automate shader creation, where the refraction mode is systematically set to Color+Alpha. This has worked without issues when no refraction is applied. We also use Color+Alpha for refracted materials to ensure proper alpha output, as we don’t render skies or visible DomeLight. The sky is added in post-production.

Initially, I suspected the texture itself, as I couldn’t reproduce the artifact with a radial gradient that’s purely black and white, but when imported in Nuke the alpha is correct.

Question

Should we exclusively use the Color Only mode when no refraction is set (except for refracted materials)?

To my understanding, this artefact shouldn’t occur since no refraction is applied.

Looking forward to your insights!

Jordan

[Pixelcon]

Did you try fiddling with the opacity mode for your material? Clip/stochastic?

[hermit.crab]

Setting the Opacity mode (VRayMtl > Options) to "Clip" should yield correct results.

[jordan_dubosc]

Yes, Clip mode does eliminate theartifact but it also alters slighly the feather look, we have to remain with stochastic mode

[hermit.crab]

Yes, Clip mode does eliminate theartifact but it also alters slighly the feather look, we have to remain with stochastic mode

From what I tested, it seems when both Refraction and Opacity (stochastic/normal so that mid-tones are present) affect the Alpha channel, it gets messed up. Not sure if this isn't a limitation. I've logged the issue (internal bug-tracker id: VMAX-14694) for dev investigation and will keep you in touch. For now, the only workaround is either to use Opacity's clip mode or to disable Affect Alpha in the Refraction.

[jordan_dubosc]

Thanks! for now we will try to track if there is refraction we use color + alpha else we stay in color only