Cannot capture alpha through glass

[Anonymous]

materialsetup.pngobjectparameters.pngI'm running 3ds Max 2019 with Arnold 2.0.931 and cannot capture alpha information for transparent materials. The object is assigned the Arnold Properties modifier and Opaque is unchecked. The material is a Standard Surface with transmission at 1.0 and Transmit AOVs checked on. I've assigned a Transmission AOV in the Render Setup dialog and am saving that and the main file as .TIF. Light source is an Arnold Skydome with the Physical Sky map applied. It does not appear to matter what I change, I cannot get the glass to show its alpha channel. See attached reference images. I had to convert the original .TIF images to .PNG to upload them.cubeaov.pngcubeaov-transmission.png

This is a show stopper for us and will mean going to Vray if we can't get it fixed. If it is a bug, let us know. If there is some untried work around, let us know. But don't give me the "just use a transmission AOV" answer because that is not working.

[lee_griggs]

Could you upload the scene, please?

[Anonymous]

See link below. This is a simple test scene but I'm certain thattesting.zip once it is resolved here I will be able to move it across the rest of our work.

[JONBERGER3572]

Have you figured this out, yet? I'm having the same problems.

[lee_griggs]

The skydome always returns a solid alpha. A workaround is to reduce the camera contribution of the skydome light.

[stephengabriel]

Setting the skydome contribution to 0 for camera had no effect regardless of what materials or lights we are using. Even a transmission AOV produced a fully white (opaque) result. Clearly, it is calculating transparency properly, it just is not saving it to the output channel whether writing to EXR, TIF or PNG.

The workaround is to put a background image into the Arnold Renderer tab in the Render Setup dialog. While this works, it requires a bit of time to get the position and lighting right and is a pain to do. It is easier and faster to do in Photoshop IF we can save alpha channel date during output of the rendered image. But so far, we cannot.

[Stephen.Blair]

There's a background that is visible to transmission (in that testing scene from 2018)

[stephengabriel]

I did some experimenting with a variety of settings using the suggestions above as well as the Arnold Help files and found that four settings contribute to losing the alpha channel for transparent objects against a background.

1) Using a transparent Arnold Standard Surface material, check the box for Transmit AOVs.

2) In the Render Setup Dialog, Arnold tab, change the Background(backplate) to None. This will still calculate the lighting from the skydome but any ray that sample the skydome will return with an alpha value.

3) In the skydome settings, Shape Rendering rollout, uncheck Light Shape Visible. If this is checked, it returns a value from the skydome even with the setting in 2 above and renders opaque.

4) In the skydome settings, Contribution rollout, set the Transmission to 0.0.

Once these are all set, flush all of your Arnold caches and you should be able to render out through glass with an alpha channel to allow you to composite over other images.

Thanks to Lee Griggs and Stephen Blair for their help with this.

[dave56VJH]

I was not using a skydome, but 2) was the one that fixed the issue (together with 1 of course). Thanks to you all!