Cycles: Invisible objects with transparent BSDF requires many transparent light bounces #115858

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Josef-Nilsen · 2023-12-06T19:22:35+01:00

Josef-Nilsen commented

2023-12-06 19:22:35 +01:00

System Information
Operating system: macOS Sonoma 14.1.2
Graphics card: Integrated, MacBook Pro 2023

Blender Version
Broken: 4.0.1
Worked: (newest version of Blender that worked as expected)

Meshes where the light is passing through multiple sheets of glass/plastic can create black artifacts in Cycles. This is related to the "Max transparent light bounces", and increasing the number of bounces will remedy the issue.

This makes a lot of sense for an object that has an alpha value which is not strictly 0, but when the alpha is 0 and the object is supposedly completely invisible, this limitation still holds. This becomes a bit cumbersome with complex meshes where you have to bump the transparent light bounces to 16+, as I imagine spending a lot of time ray tracing completely invisible objects is wasted.

Exact steps for others to reproduce the error
The stacked glass sheets on the left in my example are animated from opaque to fully transparent using a mix shader. To rule out any issues with that, I also included a stack of sheets to the right, using only the Transparent BSDF directly.

Since these meshes are not complex, I have reduced the maximum number of transparent light bounces to 2 to illustrate the problem.

Is Cycles handling the Transparent BSDF this way to avoid any abrupt disappearances of objects in Blender when animating (going from a low alpha to strictly 0), or why is this the case?

The only workaround without a performance impact I can think of right now is rendering the fade out with a higher number of maximum transparent light bounces, and then excluding the fully transparent object from rendering altogether and switching the number of bounces down for the rest of the scene?

**System Information** Operating system: macOS Sonoma 14.1.2 Graphics card: Integrated, MacBook Pro 2023 **Blender Version** Broken: 4.0.1 Worked: (newest version of Blender that worked as expected) Meshes where the light is passing through multiple sheets of glass/plastic can create black artifacts in Cycles. This is related to the "Max transparent light bounces", and increasing the number of bounces will remedy the issue. This makes a lot of sense for an object that has an alpha value which is not strictly 0, but when the alpha is 0 and the object is supposedly completely invisible, this limitation still holds. This becomes a bit cumbersome with complex meshes where you have to bump the transparent light bounces to 16+, as I imagine spending a lot of time ray tracing completely invisible objects is wasted. **Exact steps for others to reproduce the error** The stacked glass sheets on the left in my example are animated from opaque to fully transparent using a mix shader. To rule out any issues with that, I also included a stack of sheets to the right, using only the Transparent BSDF directly. Since these meshes are not complex, I have reduced the maximum number of transparent light bounces to 2 to illustrate the problem. Is Cycles handling the Transparent BSDF this way to avoid any abrupt disappearances of objects in Blender when animating (going from a low alpha to strictly 0), or why is this the case? The only workaround without a performance impact I can think of right now is rendering the fade out with a higher number of maximum transparent light bounces, and then excluding the fully transparent object from rendering altogether and switching the number of bounces down for the rest of the scene?

transparency.blend

5.1 MiB

Josef-Nilsen added the

labels 2023-12-06 19:22:36 +01:00

Richard Antalik commented

2023-12-06 20:57:34 +01:00

I would assume this is working as expected, but it may be something, that can be optimized. If whole object is transparent, can't you just disable it's visibility for rendering?

Josef-Nilsen commented

2023-12-06 22:51:26 +01:00

The scene is not static, but part of an animation, where early on the object is indeed visible, but later on it is not.

Richard Antalik commented

2023-12-07 18:02:47 +01:00

You can animate visibility as well. When the object is visible but transparent, you would need to use more bounces anyway though.

As far as optimizing out object if it is fully transparent, I can only notify @brecht to consider this feature. But we don't accept feature requests here. Use https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests for feature requests instead.

You can animate visibility as well. When the object is visible but transparent, you would need to use more bounces anyway though. As far as optimizing out object if it is fully transparent, I can only notify @brecht to consider this feature. But we don't accept feature requests here. Use https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests for feature requests instead.

Brecht Van Lommel added

and removed

labels 2023-12-08 20:31:15 +01:00

Brecht Van Lommel commented

2023-12-08 20:33:28 +01:00

In the future we could add an optimization that check if all materials can be simplified to a single 100% transparent BSDF, and then mark that object invisible. But there is no bug here as far as I can tell.

The workaround for now would be to manually add a driver for object visibility.

In the future we could add an optimization that check if all materials can be simplified to a single 100% transparent BSDF, and then mark that object invisible. But there is no bug here as far as I can tell. The workaround for now would be to manually add a driver for object visibility.

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Cycles: Invisible objects with transparent BSDF requires many transparent light bounces #115858