I think this question was asked a lot in the last years since vray 3, Is this thing hard to implement, companies that Do online configurators with pre*rendered CG assets rely heavily on this feature, and using GPU is the next logical thing in speeding up render time, I have to use 2 scenes one with CPU only where i can use this feature to make the alpha controbuion work and another one with GPU for rendering the asset. In vray 4 i wasn’t even able to get a proper matte on an shadow or reflection with CUDA. Do we need to wait until Vray 6 for this to happen? I have to resort to all type of gimmiks in order for it to work.
The feature is not yet supported on the GPU, I’m afraid - it isn’t trivial to implement.
I am so confused.
If this supposedly isn’t supported on GPU and doesn’t work, as I thought until just now trying to find a workaround, then why does it also clearly work?
This image is rendered using a dome light with an hdri, with cast shadows off as well as specular/diffuse/reflections also turned off.
The floor is a plane set to alpha-1, with a wrapper also set to -1 (though that doesn’t make any difference on or off).
It is still of course unusable in this case, as the window refractions are totally screwed, which is known about and seemingly unfixable, although it is ‘being looked at’ for more than 3 years now 
Pardon, I must have meant Alpha Contribution control (value 0) was not yet supported on V-Ray GPU. @andrei_lazar You should send the scene via the contact form so we can take a look. Mention this thread in the message.
Hi,
Just checking that alpha contribution is still not yet supported by GPU? I am trying to use alpha contribution -1 and its not working 
-1 definitely works, here at least. If you have your scene objects we can solve it I’m sure
Hi, I have sent you a PM.
Thanks.
What’s the trick getting alpha contribution to work on GPU? Setting it 0 or -1 has absolutely no effect whether it’s within “V-ray properties” on the object level, or within a MtlWrapper.
A value of 0 doesn’t work but -1 definitely does, as in the above image. You can post a simple scene where it is not working for you and we can see what’s up.
Today is a day for solving problems
That’s the spirit! 
Here’s what I’m seeing on my end. I’ve tried both Cuda and RTX with the same results.
GPU_alpha_max.zip (88.6 KB)
@studiotribe You need to activate “Matte Object”, “Shadows”, “Affect Alpha” and optionally “Matte for refl/refr” for the effect to take place. Read more about those properties here. Also, there is a toolbar button, which automatically sets those values, called “Make V-Ray shadow catcher”.
Thanks. I’m aware of that functionality but that’s not what I’m using it for. I need the beauty pass to remain the same but force the alpha to black on certain objects. Imagine a product rendering on a white background. I need a mask that isolates the product from the background and instead of rendering a separate pass, it’s easier to force the alpha for the backdrop to black.
What should do the trick is setting up a VRayExtraTex pass with a white VRayColor as a texture, where the objects you wish to mask out are excluded.
It`s really disappointing that alpha contribution -1 still isn`t working in GPU mode without matte turned on..
This feature is requested for years from several users and would have a big benefit for lots of usecases.
Is this really in the development and will be available someday by the dev team ?
The suggestes workaround with VrayExtraTex isn`t working in cases, where an object ( which should have no effect on the alpha ) is covering an underlying object ( which shall contribute to the alpha )…any suggestion ?
@roboxx Just so you know, our developers are informed about the matter. In the cases you describe, maybe separately rendering a correct alpha (with using invisible to camera, etc.) is the way to go.
Will it ever work? Without any tricks. Without settings object to matte? Just as it should work? This thread started 2 years ago…
By the way setting alpha contribution to 0 has no effect also
Hello,
Are you testing with V6 ?
This topic is near the top of our current priority list, Alphas for objects behind glass were improved in V6. The workflow should be better
There are still some cases that need attention but this is workable now at least
Best,
Muhammed
This is still not working…
V-Ray 6
Vray 7 and still no progress on this topic?