How to get the correct alpha channel for this problem

[3dkobi]

Hi,
I have a problem in a very complex scene under NDA so I simplified it for the sake of the question - I have 2 objects in the scene and a camera animation (rotating around those objects).
My problem -  I need to produce a render sequences that has only the visible part of the blue sphere visible in the render, and everything else is Alpha - Including the green part of the torus that is in front of the sphere. Like in the second attachment - only what I highlighted should be visible in the render, everything else should be Alpha
how can I achieve that ?

[Beanzvision]

You could try setting the material you want excluded to black in the advanced alpha mode settings. ;)

[3dkobi]

Thanks, I'll try that

[BigAl3D]

I would enable the Multipass feature, add a Mask, change to Include Selection and drag the Sphere in that. Depending on how you save it, it could be a PNG sequence with alpha and you're done. You can also use a separate alpha sequence and Trackmatte it out in AE or another compositing app. If you're not sure how to do this, post back and I'll illustrate it visually.

[3dkobi]

Thanks, I followed Beanzvision advice and it works great