Rendering Background with Alpha using Arnold renderer

[cg67MLF]

Hello,

I am new in this forum. So hello everybody 🙂 !!

Sorry for my English, I am French.

I am trying to render an scene using Arnold renderer with an Alpha channel about 50% on background.

If I render an object without background, the Alpha channel is correct : white on object shape, black on background.

Now I want the Alpha channel to be gray on background, so I have added a simple plane behind my object, material 100% black and 50% transparent.

When I render it with Arnold, the Alpha channel on object shape is still white but the Alpha channel on plane is also 100% white, I was expecting it to be gray (as plane object is 50% transparent). This method worked fine on my previous projects with Mental Ray renderer.

I could edit the Alpha channel with another tool but I should be able to make it work with Arnold renderer.

Any idea ?

Thanks in advance for your answer.  

[10DSpace · Accepted solution]

@cg67MLF 

If you are using the Physical Material you need to put a 50% Gray Map in the Cutout (Opacity Slot) of your plane.  see image below of gray alpha channel done with that.

You may have just set the Transparency slot to .5, but that is not the same thing.   That just tells Arnold that some light can pass through it, but does not affect the alpha channel.  

[cg67MLF]

Hello 10DSpace,

Thank you very much for your solution, it works perfectly !! I still have some work to fully understand Arnold !!

Cheers