Alpha channel and glass...

[Parker1]

A quick question for you guys...

I am doing an interior scene and my camera is facing toward the glass entry system. The clear glass I am using looks great and has some very nice reflections on it. I want to render the scene and then comp in the exterior background in photoshop separately. The issue is that when I select the alpha channel in PS and delete it I am deleting the glass as well and thus I lose the reflections that were there.

How do you go about using clear glass and still have the ability to comp in a layer behind it in photoshop?

[instinct]

There a many ways i guess. One would be rendering the glass as a VrayMtlSelect Element. Then remove it in the RGB how you described. Put the bg behind the main layer and simply plus the mtlselect element over it.

Kind Regards,
Thorsten


P.S. if i recall right plus is called "linear dodge" in PS but am not really using PS a lot.

[dlparisi]

Another way is to play with your environment overrides to produce a black background and use the image alpha to place the background just in the windows. Set this background to Linear Dodge (Add) in photoshop.

I don't remember the exact steps but I believe it's placing a vraysky (or whatever your using for sky illumination) in the GI override, placing the same map in the reflection override and turning on the refraction override but leaving it black. I don't think the max environment has any effect at this point. I also seem to remember unchecking "visible in reflections" in the vray properties window for the glass in the windows. The reason for all of the overrides is to ensure that the view directly through the windows is black but reflective objects in your scene (floor, mirrors, other windows) have the proper reflections of a bright exterior. Again I may be forgetting a check box here or there but this is the gist of it.

[zoranm]

Bryan,

Go to "vray object properties" for glass object(s). Under Matte Propperties change "alpha contribution" to something like 0.1. You might need to experiment a bit with this value until is to your liking. If reflections are not OK, render them sepparately as a reflection pass and comp them in PS.


Good luck

Zoran

[MikeHampton]

Hmm... maybe I am not undestanding, but why would you be selecting the alpha channel in PS and "deleting" it? Why wouldn't you select the alpha channel and then paste your background into the selection?

[tehamnes]

I use the alpha channel as a mask which preserves the glass and reflections and give you the transparency set in the material. Just select the alpha channel and click the mask button on your image layer, then create a new layer with the background image - works for me. I suppose the more advanced methods will perhaps give better reflections and tuning options in post.