Rendering a still with an alpha channel in v8.5 Pro

jack 4D

Hi. Just installed Carrara v8.5 Pro on my Mac Pro (10.6.8) and I'm not able to render a PSD still with an actual alpha channel. The background is rendered as transparent but there is no alpha channel. The only still format that does render an alpha channel is TGA but that's a large file, especially for an image sequence.

I also notice that a photoshop still rendered from v8.5 Pro with no alpha channel is more than twice as large as a photoshop still rendered from v6 Pro that does contain an alpha channel. That doesn't seem right.

I've tried using Multi-Pass and am able to generate a layer mask but no actual PSD alpha channel. Is this the way Carrara works now?

DUDU

I always use .targa files therefore I'm not the right person to answer but there is nobody else at the moment...

What do you mean with "The background is rendered as transparent but there is no alpha channel", is it a separate layer in PS ?

 

jack 4D

Thanks for your reply, DUDU. To use a very simple example, if I render, e.g., a still of a cone primitive as a PSD file w/ "Render Alpha Channel" checked, the resulting render contains a single layer showing the cone silhouetted against a transparent background. If I use Mult-Pass and add "Background" w/ "Embedded" checked, an additional layer is rendered showing the shape of the cone as solid black against a transparent background. But in either case, no actual alpha channel is rendered that shows up in the channels palette along w/ the individual RGB channels and composite channel. That only happens if I save as a TGA.

3DAGE

Hi Jack 4D

The "silhouette" Black and white image can be used as a "Selection " or "mask" in photoshop, or any image editor which uses layers.

Save as PNG (the same image quality as TIFF or TGA,) with lossless compression , and an alpha channel

Photoshop PSD will also save an alpha channel and as you mention this shows up as a transparent background,. allowing you to select and isolate either the object or the background.

You can also crrate a B/W "mask from that alpha.

 

jack 4D

Hi 3DAGE: Thanks for your reply and all the info. Yes I'm aware of how the "silhouette" can be used for masking, etc. I'm just finding it puzzling that an image rendered and saved as a Photoshop PSD in Carrara 8.5 Pro generates an image w/ a transparent background but does not generate an actual alpha channel, a separate channel apart from the actual image layer that defines which part of an image is transparent and which part is opaque. It did do it in Carrara 6 Pro but is not doing it in Carrara 8.5 Pro. Only the TGA format is generating a separate alpha channel. That's what I wanted to verify in order to make sure my app was not malfunctioning.

jack 4D

3DAGE, I guess my concern about an alpha channel not being generated in a PSD render as a separate channel in the channels palette (the way I'm used to seeing it) is not a big deal because, as you say, the "silhouette" can be used as a mask or selection (which can be saved as an alpha channel). Plus, v8.5 Pro generates a lot of other interesting varities of layers using Multi-Pass which I need to experiment w/ more. And I can export stills in PNG w/ transparency (which wasn't an option in C6 for an image render). So all looks good. Thanks for you help.

JoeMamma2000

I tend to avoid using PSD files, especially with Carrara.

Use Targa. Use TIFF. Use PNG. But don't use PSD.

Targa, TIFF, and PNG all give a fine alpha channel when rendered in Carrara.

I am convinced there is a bug in how Carrara deals with PSD files, especially when you have "Render Alpha Channel" selected. In some apps, when you attempt to import a PSD which was rendered in Carrara with an alpha channel, you get an "end of file" or some such error. And as you say, it appears that the alpha information has been premultiplied (transparency appears in the image, but without a separate alpha channel), even if you didn't select the premultiply option.

So I avoid them whenever possible. They do have their uses, and can work fine in some cases, but if other options are more reliable then why bother?   

MarkIsSleepy

Yeah - I've actually had Photoshop CS5 give error messages and be unable to open PSDs generated by Carrara on several occasions. Most of the time it works fine but every once in a while the file comes out corrupted somehow. It's not often enough to be reproduceable so I've never submitted it as a bug.