Imprecise Alpha on rendering

Hello everyone !

I’ve been having a problem for quite some time on Create, and I wanted to share it with the community (and especially with the developers).

During a rendering (whether in Real-Time or in Path Tracing), the alpha channel which is saved in the output files is…very imprecise…

Insofar as I integrate my renderings into compositings, it often (or even systematically) happens that the outline of these sequences (therefore in the foreground) have a very ugly outline (or a very apparent outline in the event of a big difference between the background and the elements in the foreground).

I am attaching 3 images that will better describe what I want to express :

In my example, I simply embedded an object (Darth Vader’s helmet, WIP, don’t judge…) in an empty scene, with a red Dome Light, to maximize the contrast between the background and the object.

Here you see the raw rendering (without alpha layer).
Then you will find below the same rendering, but with the “Save Alpha” box checked and, if you look closely, you can see a red “line” around the object.

This is what I call “Imprecise Alpha”

On the following image, I made, under Maya, a simple green screen to separate the object from the background, and I composited the whole thing (raw rendering under Create and green screen rendering under Maya), we can clearly see that the red “line” has disappeared !

I think with these images, it will be much clearer.

Is there, in your opinion, a way to “refine” the generation of this alpha ? I can always use the green screen technique, but it’s longer, more complicated and it generates a lot more files…for nothing !

Any help will be welcome !

Thank you for your time and expertise !

Best Regards,

Hi BillyJohn! Thanks for sharing this issue and reference images - I’ll see if the team has any ideas.


P.S. The Omniverse documentation site has more info and links to all tutorial videos (under the “Learning and Feedback” category on left nav bar): Omniverse Platform Overview — Omniverse Developer documentation

he / him | LinkedIn | Discord ID: Prof E#2041
Omniverse: “A New Era of Collaboration and Simulation” (video)
Omniverse Create’s latest features show & tell: Twitch livestream.
As seen in VentureBeat | Physics Showcase

Omniverse Resources:
Omniverse | Documentation | Tech Requirements | Videos | Roadmap | Twitter | Discord | Twitch

PLEASE NOTE: If you are reporting a bug/issue, please provide OS, GPU, GPU Driver, the version of the app, and full log file (if applicable). For crashes, please zip and provide a link to your logs → C:\Users\ [YOUR NAME] \ .nvidia-omniverse\logs

Hi billyjohn! If you want to render elements for later compositing with an alpha channel, you need to render them over black so that your background is not baked into the pixels at the edge of the object. In the case of a dome light you can do this easily but unchecking “Visible in Primary Ray” in the dome light’s properties.

Hi @emendizabal ! Hi @anders_langlands !

Thank you for your answer !
It’s true ! I didn’t think to test disabling the “Visible in Primary Ray” for the backgrounds, which makes (partially) logical that the alpha clipping appears slightly in the renderings !

I will try this right away !

Thank you once again !