alpha not updating properly when using 'selected' mask

[Richard7666]

I have a window where I've changed the exterior somewhat, (hid my ground plane). I want to re-render just the glass, so I select my glass and set my render mask to 'selected'.
However, the alpha doesn't update to reflect the new geometry outside the window.

The alpha updates fine if I don't use the 'selected' mask and just re-render the whole region instead.

Any idea what's going on?
See attachment. If you render, adjust the ground plane (change its vray object properties to matte and -1 alpha or hide it or something), then selecct the window glass and re-render I'd be interested to know what happens

[piotrus3333]

heve you deleted all chanells from VFB before rendering windows with mask?

[Richard7666]

heve you deleted all chanells from VFB before rendering windows with mask?

I hadn't, because I was trying to only re-render the window instead of the entire room, but still keep the rest of the image.
Is this not possible?

[piotrus3333]

what is 'affect chanels' property of glass shader than?

can't open the file, I'm on older max.

[matanov]

Hey Richard,

Can you explain a little more what is the exact effect you are trying to achieve?

[piotrus3333]

I would say that is the corect result. you are rendering glass on top of previous alpha channell. plane will give you full white and glass will add its alpha to previous glass alpha (you end up with double alpha behind glass)

[piotrus3333]

use texture mask instead if simple region is not efficient.