alpha channel

[Jan Verzelen]

i spotted an unrealistic black part on the side of the glassplate in this image and when i right-click the rendering window to show the alpha channels this is the result. Since i don't know much about alpha channels i wonder what's happening here.

[Roy Hirshkowitz]

Normally when this happens it means some light paths are finding there way to the background via transmission only.

 

There is a global switch you can try.  It's on the Advanced Lighting tab and it's called No Transparent Object Alpha.  It will prevent you from doing alpha compositing through windows so it may not work for you.  (I need to add a local switch here-- either material or object prop.)  Let me know how it goes.

[Jan Verzelen]

"alpha compositing through windows"? Don't know what that means. Anyway, checking the switch doesn't change a lot. Only the alpha channel is no longer transparent, part of the face stays black as night. Changing te view angle gives te same result. You can view the final image in my albums as "livingroom2" I cheated by using the alpha channel to "correct" this in PS

[Roy Hirshkowitz]

Ah, that makes some more sense.  Turning up the refractive bounces may help (try 16?).  You've got some total internal reflection going on.

 

You shouldn't get any alpha for this in the very latest build so the cheat won't work anymore (> 271, I think).

 

*****

 

By alpha compositing through windows I mean the ability to see a color background (like a photo) behind a window with glass in it.  By default, nXt for AutoCAD is set up to provide alpha channel for this.  You lose that ability when you check the switch.

 

[Jan Verzelen]

Turning up the refractive bounces doesn't change anything but i saw some strange behaviour in the drawing. I'll check earlier backups to see if the drawing file could be corrupted.

[Roy Hirshkowitz]

Still very likely to be total internal reflection.  The edges of tables are notorious for this.  If it's what I think it is, it's working like a fiber-optic cable where the light can't escape.  If you can't manage to get it out of there by adding extra bounces, then post-processing may be the only option.

 

It can, of course, be some other problem.

[Jan Verzelen]

I have to stick with post-processing. The problem remains. Thanks anyway Roy.

[akinlolu olugboji]

Have you tried rounding the edge of the glass top, or using another material like plastic on the edge?

[Jan Verzelen]

Yep. Actually the glass solid is filleted. I'm affraid that changing the material won't satisfy me. I also have to explode the solid to be able to do that,no?

[akinlolu olugboji]

Was any solution found for this apart from post processing, for those of us not Gurus in PS or any other for that matter.Note the lamp In front of the white wall, or is Nxt saying the background would be reflected in the lamp shade too, because of the material?

[akinlolu olugboji]

This illustrates my point better.

[Roy Hirshkowitz]

I added a "No Transparent Alpha" switch in the Object Properties a while back.

[akinlolu olugboji]

Got it takes, works like a dream.