Did a long render with all sorts of caustics et al turned on.  And the result was unusable.  Why?  Turns out that in C6pro, running on OS 10.4, when you check "alpha channel" in the render options you don't get one.  Instead it adds the alpha channel to the color channels.

Did a bunch of test renders and opened them in PhotoShop to investigate.  In each case, the "alpha" for the document was multiplied into the RGB channels.  It could not be selected, separated, or otherwise manipulated or removed.

And the end result was unusable.

When did this happen?  I've been rendering with alpha channel and g-buffers since, I dunno, C3.

I have noticed oddness in Carrara's alpha channels for a while -- definitely since C5P.  I find, though, that the alpha channel is still there, but it's not accessible in Photoshop.  I have taken the animations into After Effects and have been able to use them just fine.  If you don't happen to have AE, it's more problematic.

Out of curiosity, what were your render settings?  Did you have "premultiply" selected?  It sounds like your description of the file as viewed in photoshop indeed is premultiplied.  Maybe C6 has changed how it handles premultiplied alphas, but I've always checked premultiply in my renders because it makes for cleaner composites (in my experience).

When next I'm in front of Carrara, I'll run a couple tests and see if it's a premultiply issue.  I seem to recall that there was a plug in available for Photoshop called unmult; unfortunately the version I found with a quick google search (http://www.redgiantsoftware.com/unmult.html) doesn't seem to do what I recall, which was to extract the alpha from a premultiplied file.  You might give it a try.

Let me know if that works.

I didn't have premultiplied clicked.  Is that how that function is supposed to work?

I find it an interesting question if there is something about the alpha channel that is too non-standard and/or advanced for PhotoShop to "see."  I was able to see the other g-buffers normally, but the color channels all contained this extra element.

Well, I can do everything I'd normally do with alpha channel with object index or another of the g-buffers instead.  Still annoying, though.

I am on XP and see that same problem Only when I turn on the G-buffers & render without the alpha channel checked.  
If I have just the alpha or the alpha plus the G-buffers on it renders the channels correctly.
Pre-multiplied on or off does not effect the G-buffers showing up in the color channels.
The 2 G-buffers that cause the bleed over into the color channels when the alpha is left off are 3D Position & Normal Vector.
Again this is on the windows build, it "works" the same way in C5 pro.

wayne k
guam usa

Eep!  I guess I'll have to stop using G-buffers, too, at least until this is fixed!   (And as long as I'm doing 2-hour renders, that is).

I might not have gotten my point across very well.
Both the alpha and G buffer channels work correctly - as long as I have the alpha enabled when those two G-buffers are also on. 

wayne k
guam usa 

Hrm.  My issue is the reverse; g-buffers appear to work normally, but turning on Alpha Channel messes up the render.  I watched it render a couple of times; the render was normal until almost done, then the alpha channel washes over it, destroying it.

Just did another render with alpha channel off, one g-buffer in.  G-buffer shows up normally in PhotoShop (object index) and appears correct.

HOWEVER, I can not be entirely certain the g-buffer isn't being added to the image.  I suspect it is not, as I am using a plane with a transmap on it, and I would think the original shape as displayed in the object index would give itself away....

 Mmm. Strange. It works for me. In Carrara it looks like the alpha channel is in the actual rendering (everything turns black with a tine light outline). But when I save it is an alpha channel called 'Mask' when I open it in Photoshop.
I must say I deleted some renders because I thought I had set the background color to black or something. Only after opening a file in Photoshop to 'see what I could make of it because no time for new render' I discovered all blackness was gone.

Are you saving a .psd document? Or maybe a tif or something that mis-interpretets (huh?) the information while making the resulting file. It sounds like some of the information that Carrara puts on screen gets transferred to the actual saved file.

I do not seem to be able to replicate this situation.

Yup.  Saving as a .psd and opening that in PhotoShop -- same way I've been doing it for years with full success.    I don't think I remember the alpha channel being displayed in the render room before, either.  I seem to recall finding it when I opened the saved document in a paint program.

I'd do a little lab if I thought it was worth the time.  Better just to say "okay, one more thing Carrara doesn't do right" and just move on.