Unwanted visibility in alpha channel

[atyss]

My question regards rendering and alpha channels

I want to render some elements separately from my original scene. These elements are placed in background, and are partially masked by foreground objects. I don't want those masking foreground objects to be part of the alpha channel.

I tried many settings, like putting the material of the masking objects in Shadow Color, but they become invisible in the RGB too (in other words, they cease to mask the back objects).

I also tried to put their material in a black Constant Color, but this time they are visible in the alpha channel.

I tried different settings in Info > Selection > Render Setup, changing the Primary Rays, Secondary Rays and Shadows.

None of these tests gave me the wished results. Is there a way to acheive this?

Thanks
Bernard Lebel

[atyss]

For those it may concern! After digging even more in the problem, I found the solution, and I want to share it with you, if you ever stumble on the same problem.

In the material of the foreground object (those who mask the background ones), put a black Constant Color and apply a material shader called alpha. This shader can be found in:
Shader_Lib/Example_SRC/alpha
Then edit the shader and decrease the Alpha value to zero.

Bernard Lebel
This shader can be found