Where is material's alpha channel?

[aenc_randyc]

Can some one from Chaos please contribute to this thread? what is the new method that replaces the "alpha contribution" setting from previous versions the OP asked?

[nikoleta.garkova]

Hi aenc_randyc

As previously mentioned in this thread, the current and recommended method is by going with Wrapper material and slotting your desired material there. When you do so, you will be able to control the Alpha contribution to determine the appearance of the object in the alpha channel of the rendered image. You can read more in the Wrapper material docs page. If there is something specific you want to achieve, please do share more information so we can assist you further.

[konstantin_chaos]

Can some one from Chaos please contribute to this thread? what is the new method that replaces the "alpha contribution" setting from previous versions the OP asked?

Please also note that the old alpha parameter (and the whole underlying shader) used in V-Ray 2 was deprecated in V-Ray Core.
It was considerably slower to render and hard for us to maintain.
We did our best to migrate the underlying shader to a modern one and I consider this being a success (it remained unnoticed for most users).
And still there are few options that are no longer available after this migration (as the alpha contribution).
The good news is there is still a way for you to get the effect you're after (as pointed out by my colleagues).

Konstantin

[Z_Had]

Wrapper works, but not with GPU rendering. I'm using Sketchup 22, vray 6. With option "black alpha" or "ignore in alpha", and CPU rendering, everything's fine, but when I render with GPU, it turns white again.
Can you confirm that it is not supported, or am I doing something wrong?

[iva_mancheva]

Hello Z_Had,

Could you please confirm if you are using V-Ray 6 for Rhino or V-Ray 6 for Rhino, hotfix 1? There was an issue with newly created Wrapper materials when rendered on GPU with V-Ray 6.00.00, which is fixed in the latest hotfix - 6.00.01.
Please note that in order for the Wrapper material to work properly as a Shadow Catcher on GPU, the option for Alpha Contribution should be set to Black Alpha (-1). For better results also make sure to use Wrapper materials created with the latest hotfix of V-Ray 6 or V-Ray 5.

Kind regards,
Iva

[Z_Had]

Hello Z_Had,

Could you please confirm if you are using V-Ray 6 for Rhino or V-Ray 6 for Rhino, hotfix 1? There was an issue with newly created Wrapper materials when rendered on GPU with V-Ray 6.00.00, which is fixed in the latest hotfix - 6.00.01.
Please note that in order for the Wrapper material to work properly as a Shadow Catcher on GPU, the option for Alpha Contribution should be set to Black Alpha (-1). For better results also make sure to use Wrapper materials created with the latest hotfix of V-Ray 6 or V-Ray 5.

Kind regards,
Iva

Hello iva_mancheva,
I'm using latest V-ray for Sketchup v. 6.00.01 (hotfix 1). Issue persists, I've created new wrapper material (in new, empty file), set my background image material as base material, and set Alpha contribution to Black Alpha. It renders as black alpha on CPU but white on GPU.

[iva_mancheva]

Hello Z_Had,

I am not able to reproduce the issue so far. Could you please send us the scene file, so we can take a look? You can do this by packing your project from the SketchUp menu > Extensions > V-Ray > Pack Project and share the resulting archive through https://support.chaos.com/hc/en-us/requests/new (please make sure to mention the link to the Forum thread in the message).
Alternatively, if the .zip file is small enough you can share it directly in the forum thread or via forum private message.

Thank you!

Kind regards,
Iva

[Z_Had]

Hello iva_mancheva, here's the packed file.
Thank you in advance.
Kind regards,
Zlata
https://we.tl/t-hyNrK6sRTp

[iva_mancheva]

Hello, Z_Had,

Thank you for sharing your model.
I have identified two issues. The first one is - objects with Wrapper material with Alpha Contribution set to -1 have a white Alpha on GPU when Wrapper's Matte option is disabled. If the Matte option is enabled, the objects appear is black in the Alpha channel, as expected. The second one is - objects with Wrapper material appear white in the Alpha channel when looked through glass, regardless of the Alpha contribution and Matte option state of the Wrapper.
Both issues are logged and we will let you know if there is any progress in this regard.

Could you please tell me more about what exactly would you like to achieve with the Wrapper material, so I could try to think of a workaround?

Kind regards,
Iva

[Z_Had]

Hello, Z_Had,

Thank you for sharing your model.
I have identified two issues. The first one is - objects with Wrapper material with Alpha Contribution set to -1 have a white Alpha on GPU when Wrapper's Matte option is disabled. If the Matte option is enabled, the objects appear is black in the Alpha channel, as expected. The second one is - objects with Wrapper material appear white in the Alpha channel when looked through glass, regardless of the Alpha contribution and Matte option state of the Wrapper.
Both issues are logged and we will let you know if there is any progress in this regard.

Could you please tell me more about what exactly would you like to achieve with the Wrapper material, so I could try to think of a workaround?

Kind regards,
Iva

Hello Iva.
What I'm trying to achieve is simple - I want a material that is visible in render, but not in alpha (specifically, not "black alpha" - rather "ignore in alpha"). So, a way to control Alpha contribution of a specific material.
Previously, nikoleta.garkova suggested that a way to achieve this was by using Wrapper material. It worked, but only on CPU rendering. If I understood correctly, you also confirmed this.
Currently, my goal is to render background image as seen from window (through glass) but to be able to replace it in Photoshop if I'm not satisfied (sometimes it is overexposed or underexposed - I don't know if there's a way to exclude it from light source). I do a lot of photo-montage work, so I need to position background correctly.
That is one situation that comes to mind, but not the only one. Ideally, we would have the option to tick/untick "affect shadows" and "alpha contribution" for every material.
Kind regards,
Zlata

p.s.
I'm finding that Wrapper material achieves this on CPU regardless of Matte state or if it's looked through glass. On GPU - you are right - if the Matte option is enabled, the objects appear is black in the Alpha channel, as expected, but not through glass.

[iva_mancheva]

Hello Zlata,

Generally the way to control the Alpha contribution of a material is to slot it in a Wrapper material. Unfortunately making a material visible in the render, but not in the alpha channel is currently achievable only with the CPU engine.
You can disable casting shadows for objects using a specific material by adding a Raytrace Properties attribute and disabling the "Cast Shadows" option.

As a workaround for your purpose you can render the image without the background image (using Sun and Sky or a Dome light) and then position it accurately in an image editing software. You can also place the Background image in a Background layer in VFB, for better results in this case you can use an invisible Dome light for the exterior lighting. Currently there are no transform options for the VFB Background layer image, so an accurate positioning of the image will be difficult, but it is a good option when this is not an issue.

Kind regards,
Iva