In the prior version [3.4], I could change the material ‘affect alpha’ from 1 to 0, and then the material would not be saved (as long as you used a PNG or similar.) It was GREAT for adding new backgrounds in Photoshop.
Now, that lovely option seems to be missing completely from V-Ray Next .
How do I achieve this effect?
OK, I found one workaround .. or maybe this is the new method that is not listed in the documentation.
1] Make sure you are in advanced mode when looking at the material. There is an icon at the top right of the material’s layers.
2] Turn on the Raytrace Section. It is off by default.,
3] Under the specific material, go to the Raytrace Properties > UNCHECK ‘Visible to camera’
If you save the file as a PNG, it will work just like the old ‘alpha = 0’ process.
I sure wish the documentation kept up with the newly added features. That only took about an hour of my life. 
Hi,
Specifically for an Emissive material, you could use the Raytrace Properties rollout as you have done already. Another workflow would be use the V-Ray Wrapper material and slot the material there. With the V-Ray Wrapper material you have the option to control the Alpha Contribution and generally this is the recommended approach for most materials.
For more detailed information, please visit Chaos Docs
I should have mentioned that am teaching a class and the last time I used the wrapper material, it was a complicated multi-step and non-intuitive mess. I need SIMPLE & repeatable processes to teach students who do not use V-Ray every day. Sorry, the wrapper material is WAY too complicated.
Plus, you guys changed a functionality that was in multiple prior versions and provided no guidance.
Finally, the solution above ALSO works for other non-emmissive materials as well. That’s why I like it. Its simple and you actually have a shot at remembering it.
@Micha ,
For glass materials, you could adjust the Affect Channels parameter to All Channels, and this way the correct alpha will be produced in transparent areas. The option is available in the Reflection and Refraction rollouts of the material (Advanced settings).
I might have misunderstood the case. I thought what was sought after was for a background to be visible behind transparent objects?
Could you please specify the exact workflow and results expected?
Thanks for the feedback!
In this case, since you are using Opacity to produce transparency, there is no way to control the alpha contribution in the manner you seek. However, you could set up the material with Refraction (change the IOR to 1 if necessary) and adjust the Affect Channels to color only.
Regarding the V-Ray Wrapper material, the Alpha Contribution can be adjusted with a script to a value greater than 1, however, this is strongly not recommended and in the case with a reflective layer, you would not get realistic results!
Thank you for the workflow suggestion. I will try this.
For the long run it could be nice if you could set on the wish list that the wrapper alpha can overwrite the base material with 1 like it can overwrite with 0.
nice found for quick switch, but at the end it requires 2 renders in total for 1 scene.. I’ll focus on wrapper’s solution.
thanks georgi.georgiev it works awesome!
OK, a final update for those who want a simple alpha channel solution (like in the prior version.)
The good news is that the matte-shadow is much easier to use in V-Ray Next. I’ve listed the steps on how to create a backdrop (floor and or wall) that will not save with the file.
- Create a ‘backdrop’ material that you like.
- Next, create a brand new matte-shadow material.
- In the matte-shadow material settings, there is an area where you can select a ‘sub-material.’ Pick the ‘backdrop’ original.
- I suggest re-naming the new wrapper material so it is obvious; I use ‘backdrop-matte-shadow.’
- Assign the new ‘backdrop-matte-shadow’’ to the wall or floor geometry
- When you render, the areas will not render and be transparent. Easy!
- Just make sure to save the rendered image file in a format that has transparency, like .PNG
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?
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.
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
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?
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.
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
Hello @iva_mancheva , here’s the packed file.
Thank you in advance.
Kind regards,
Zlata
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
