Aperture Alpha Channel Aberrations

by Ben Long

A lot of Aperture's functionality comes not from Aperture itself, but from Core Image, lower level operating system features that Aperture can tap into. For example, the Mac OS has built-in routines for opening and displaying various file types such as JPEGs, TIFFs, GIFs, PNGs, and Photoshop documents. While this helps integrate Aperture with other OS features, it also means that, as Aperture users, we're sometimes subject to the bugs and errors of other OS functions.

Consider this image, which I shot in London.

FIGURE 1.jpg

Because I wanted to perform a separate set of edits on the sky, I used Photoshop to create an alpha channel mask so that I could manipulate the sky and the building separately. I used a number of normal Photoshop masking operations to create this alpha channel.

Figure 2.png

When I import this Photoshop document into Aperture, though, I see this in the thumbnail:

figure 3.png

and this in the Viewer pane:

Figure 4.png

The problem is that OS 10.4.8 has a bug in its Photoshop routines that causes some Photoshop documents that have alpha channels to render improperly (this bug does not affect Photoshop itself, as it uses its own routines for interpreting files). If you open this same image in iPhoto or Preview or any other app that relies on the OS-level Photoshop renderer, you'll see the same results. (This bug has been around for a few OS revs now. If you're using Aperture 1.5, you must be running 10.4.8.)

To get around this, use Photoshop to save a separate version of your document, delete any alpha channels from this document, and import the new version into Aperture. I like to keep the old version in Aperture also, just in case I want to perform additional Photoshop edits that depend on my mask. It's easy enough to keep them both stacked together, giving me quick access to the Aperture-friendly version, and the complete, masked version.

Figure 5.png

Note that layer masks attached to a Photoshop layer do NOT cause this problem. Only stand-alone alpha channels.


James Duncan Davidson
2006-12-13 21:42:43
Excellent! And thanks for the reminder that you can use Aperture to group and manage files that you use external editors on. This is just the trick for when you need to render a RAW file with ACR or some other external RAW conversion utility when Aperture doesn't provide the rendering you want.
Jan Steinman
2006-12-14 14:56:01
Thanks! I've seen this on a number of my images, and now I know why!

It's great to know layer masks don't do this. I don't fancy just throwing out my masks, but I'm willing to convert them to "do nothing" adjustment layers so they can stay with the image, but not cause the display problem.

Ryan Miglavs
2006-12-14 17:58:57
I've seen this on a number of my pictures, and gotten a little cheesed with the OS X image handling of the alpha channels. To this point, I've just left them in Aperture looking funky, but I like the idea of stacking them with alpha-free versions. Not ideal, but it will work until Apple gets around to fixing that.
Joseph Mostefaoui
2007-03-31 01:58:56
As it seems release 10.4.9 does not fix this problem. It's get very annoying. I hope Leopard will do something for it.