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When saving PSD files with transparency, the transparent regions get premultiplied with white, resulting in a white fringe around objects when importing the PSD files into other applications.
No, that is incorrect. PSD is a straight color format - no premultiplication involved (because Photoshop uses straight color and only deals with premultiplication for file formats that require it).
What other applications are getting fringing when reading PSD? My guess would be that they are making a mistake in how they read the file.
When opening PNG files that are saved with straight alpha, Photoshop interprets the files as one transparent layer (which is great) but then simply ditches all the information stored in the transparent regions.
Photoshop opens the PNG transparency as transparency. Any color data in 100% transparent regions is still there, just 100% transparent. Try using the Layer Mask from Transparency command.
Also, you asked for alpha channel editing in the title, but all your comments are referring to transparency.
Those are 2 different things.
Wouldn’t the technique Mr. Harrington proposes in the video result in a possibly darkened/blackened fringe of soft edges that are not part of the shadow?
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Also, Mr. Harrington makes a handful of mistakes wrt. alpha vs. transparency and how different file formats support alpha channels and transparency.
That tutorial may be doing more harm than good.
Wow! Thanks a lot for the quick response!
You're right. I didn't distinguish between alpha channel and transparency. My bad. I'll try and fix the post title if possible.
As for the straight color in PSDs:
I definitely have the blending to white in Unity 3D (a 3D engine that has a very nice PSD to 3D texture workflow), and the very same behavior in Autodesk Maya. I also googled it beforehand and found a couple of videos like the one I posted so I assumed it was a problem of the file format. Thanks a lot for the clarification! I can't reproduce it in After Effects, Flash or Nuke (which I thought was where I encountered the problem aswell).
As for the PNGs:
This image shows what I mean. The "Alpha channel from transparency" also removes the color information in the most part of the completely transparent regions of the image. (original size here: http://img18.imageshack.us/img18/7479/photoshoppngbehavior.jpg and the image used: http://rapidshare.com/files/433225305/png_transparency_test.zip)
AE, Flash and other Adobe products should be importing PSD correctly (if not, we can fix that :-).
It sounds like Nuke may be as well, but Unity and Maya may not be reading it correctly.
You should file a bug report with them, and have them talk to us about it. (we keep trying, but don't seem to get much traction)
The PNG image is coming from Maya with the clipped background.
Yes, I checked the file. You can see the same result in other image editors when opening that PNG.
PNG is a straight color format, and the extra channel is defined as transparency. So technically anything behind 100% transparency won't matter and some applications will clip that data, as Maya did, to improve file size.