Transmit AOVs option is not working - 3ds Max 2019

[ZDruien]

Hello,

I'm trying to render a glass material with a "baked in" transparent alpha channel when exported as a .png for compositing different backgrounds behind. I'm currently using an Arnold Standard Surface material and bumping the Opacity property's color to gray to fake this, but it's not giving me the desired effect, and it's increasing my render times significantly. I need the alpha channel to be tied to the Transmission property if that makes sense.

I'm coming from Mental Ray where rendering out an Arch & Design Glass material did this automatically.

If I'm interpreting the tool tip correctly, Transmit AOVs should produce this effect, but it doesn't. One guess is that, not being able to disable the (deprecated) Background (Backplate), is making the renderer think the background isn't transparent, as the tool tip states it must be for Transmit AOV's to work. The glass material is rendered and saved out with the Background (Backplate) color instead of transparency. It's entirely possible that Transmit AOVs doesn't actually do what I think it's supposed to, though.

Transmit AOVs Tool Tip:

"When enabled, Transmission will pass through AOVs. If the background is transparent, then the transmissive surface will become transparent so that it can be composited over another background. Light path expression AOVs will be passed through so that for example a diffuse surface seen through a transmissive surface will end up in the diffuse AOV. Other AOVs can also be passed straight through (without any opacity blending), which can be used for creating masks for example."

My scene | 3ds Max 2019 - MaxtoA version 2.0.937.0

• My scene is using an .hdr file in the Environment Map slot of the Environment and Effects dialog for lighting.
• I've disabled the Transmission option in the Emit/Affect section of the Skydome Light Parameters in the Render Setup dialog.
• Transmission Samples are set to 2 and Ray Depth is set to 8.
• The glass objects in the scene have Opaque disabled in the Arnold Properties modifier.
• Alpha Channel is enabled in the PNG Configuration dialog when exporting the image.
• The standard surface material has Transmission set to 1 and Transmit AOV's is checked.
• I've tried setting the Background (Backplate) Source to all possible options in the dropdown to no avail.

My work requires regularly rendering transparent glass and plastic materials that can easily be composited onto different backgrounds in post. If anyone has advice on how to do this in Arnold, that doesn't require opacity blending, I would be very grateful.

Thank you so much.

[Stephen.Blair]

Your guess looks to be correct. There's always a background sky shader that's visible to camera and transmission rays. I'm more familiar with Arnold than 3ds max, so I had to export an ASS file to see what's really going on, and that's what I saw.

[Anonymous]

+1 .. same same same thing happens to me, like @Zack Druien

[Slava_91]

Try to render without Background (Backplate) for test If you dont need you Backplate in refraction you need in aiSkyDome set transmission level to 0

[ZDruien]

Thank you for your suggestion, Slava. There doesn't seem to be an option to turn off the Background (Backplate) in 3ds Max. There are only options to choose which source: Scene Environment, Custom Color, Custom Map and Custom Arnold Shader. I have tried all of these options.

I do need a scene environment for HDRI lighting, reflections, and refraction. A temporary work-around has been making the Backplate a Ray Switch RGBA material with its Specular Transmission set to red for a separate pass and then using that color in post to identify areas that need to be given an alpha channel manually.

[Stephen.Blair · Accepted solution]

maxtoa 2.0.938 allows you to render without a background shader

so there'll be no background visible to transmission rays

[ZDruien]

Thanks for the update, Stephen.

If anyone else is trying to use a background (hdri) for Environment Lighting & Reflections (IBL) you should set that image in the Rendering > Environment dialog. Then in Render Setup > Environment, Background & Atmosphere > Mode: Advanced > set Background (Backplate) source as Custom Arnold Shader and set the Custom Shader map to a (Float to RGBA) map with the A (alpha) set to 1.

You should then be able to render a transparent surface material with an alpha channel for compositing over different backgrounds. Just make sure you have Transmit AOVs enabled.

Thanks

[rad43]

I'm currently having this same problem in Maya with Arnold. Does anyone know if there is a similar way to fix this?

[Stephen.Blair]

Please ask in the Maya space.