Compositing refractions in Photoshop?

[DPS]

Wondering if someone can point me to a tutorial or make a suggestion on the above.

I'm integrating a render of a building into a backplate. My building has some glass components and I need to be able to see some of the surrounding buildings that have been photographed the backplate through the glass.

To get the correct refractions happening in the render I loaded my backplate onto a plane using screen mapping inside a vray light material. I created an alpha channel for the sky so that the sky in the backplate isnt affecting my refractions. Regarding the properties on the plane geometry: I set it to not affect alpha and visible only to refractions. I rendered against a black background and the render looks exactly like I want in Max frame buffer (minus the sky obviously)

I'm now trying to comp this on top of my backplate in Photoshop but the refractions are looking weird.

It's as if I need a method to make the my renders Alpha opaque anywhere that the glass is on top of my 'refraction plane', and still remain semi transparent in other areas where my photoshop sky replacement will be.

Easy method I guess would be to re-render without the alpha mask on my refraction plane - but I loose some flexibility here.

[DPS]

Here's a crop that shows the problem.

[Neilg]

Rendering the glass a separate layer will help. it's a workaround for your issue, but I wouldn't work any other way now.
switch the glass layer off, do a render, then:
select all, right click vray properties then check on matte object, set alpha contribution to -1, in the reflection/refraction/gi box set reflection and refraction amounts to 0 and uncheck no GI on other mattes.
Now turn your glass layer on, set the material to black fully reflective, turn 'cast shadows' off in object properties and render it with a fresnel falloff in an extra tex element. You can use that element as a mask to paste it back in.
We'll usually break up main glass and balcony into separate renders too. this is a really flexible way of dealing with so many minor issues that also gives a lot of control in post.

[m_hinks]

And to further help with this process, you could stick your layers of glass onto layers, and use the render mask, select layer to render each glass set, means you can leave all your "no Glass" geom as is, not faff with matte properties. I have noticed a slight fringe when using this method and highlighted to Vlado, but it seems correct.

Your glass renders will be mins, your no glass hours!