B&W alpha problem

[teabag]

Hello,

I have bumped into this problem recently with an interior render. i have i camera looking over the room and out of the window which are double glazed.

I have standard glass setting, fresnel reflections, glass is 99%refractive, with just a little blue tint in it through fog option, affect alpha checked....

Now....both in vray render elements and plain TIFF output (with alpha preserved) i get the ALPHA BLACK AND WHITE!!! although i suppose it should be greyscale to preserve the reflections on it.......

so whn i use this alpha map in PS as an alpha mask co i wanted to put some bacground behind the windows....then due to blacke and white alpha all the reflections from the glass panes disappear ..

Can anyone please suggest where am I going wrong here?

Thankx

Martin

[dlparisi]

Render out your image with a black background (i.e. your view through the glass is black) so all you see for the glass is a slight color tint and the reflections, set your background layer in PS to Linear Dodge and it should do the trick. It has to be over black though to work.

[teabag]

ok thanks for hint.

but should not vray handle this automatically? all i have in the background is vray sky.

i dont believe that such an advanced program cannot do this. There must be a way how to get alpha in grayscale rather than black and white. Or can someone prove me wrong?

[joconnell]

Yeah this is discussed in this thread - http://www.chaosgroup.com/forum/phpB...ic.php?t=17765 - Best thing to do is render a separate reflection pass or use the reflection render element and comp it back in over your new background - Reflections don't really need an alpha as such, they're normally just added or screened over whatever background you're comping on, their brightness will control how visible they are.

[dlparisi]

Be sure you don't have any glossiness set on the refraction. I'm not sure if it's a bug or not but it'll give you pure white in the alpha (I'm not sure if you're getting pure white or pure black in your alpha for the glass). For the blue tint in the windows, it's probably easier just to give the refraction a slight blueish tint. The fog options will simulate a depth to the glass (i.e. thicker areas get more opaque towards the fog color) and I doubt you need this through windows.

As for vray handling this automatically, it already does but it appears that you are trying to do conflicting things. Those two things being adding a sky background in post and rendering it with the Vray sky as the background - you can't have both. Put the Vray sky into the environment override slot and make your background pure black and it'll work.

[teabag]

Thanks for explanation i got it now guys Until now i have always used fog color to color my architectural glass...it works perfect from exteriors

I will use your technique and see

Best regards

Martin