Comments on: Utilizing Alpha Channels When Placing Extracted Images

By: Alan G

Alan G — Wed, 31 Dec 2008 21:22:34 +0000

I mostly avoid the Magic Wand because of its “all or nothing” approach to selecting pixels. In this case Select>Color Range would work very well (it allows for partial selections, which preserves anti-aliasing in the image), and the selection dialog is interactive so you can see what you’re getting.

But for me the fast approach to a complex image like this is usually to duplicate the highest-contrast channel in the channels palette, reverse it, and then make a quick Levels adjustment. Problems with jpeg compression or variations of background shade or texture become very easy to see and control, and the alpha channel is easy to fine tune with the dodge and burn tools. (Set Dodge to “Highlights” and Burn to “Shadows” with a 25-50% exposure setting.)

Katrin Eisman’s excellent and thorough “Photoshop Masking and Compositing” is a terrific resource for this sort of thing.

Another point worth making is that InDesign handles an alpha channel in a PSD much more accurately than the same channel in a TIFF. I’ve never bothered to find out why; I just always use PSD for placed images.

By: David Blatner

David Blatner — Tue, 25 Nov 2008 19:27:14 +0000

Just to back up Ted here: Absolutely! Using alpha transparency in imported images is one of the best reasons to use InDesign. You can also sort of make your own transparency masks in InDesign with any object.

By: Ted LoCascio

Ted LoCascio — Tue, 25 Nov 2008 14:45:57 +0000

@Lee – Yes, you really can use transparency through alpha channels in InDesign. I’ve never had a problem. It is not a solid path.

By: Lee

Lee — Mon, 24 Nov 2008 16:17:20 +0000

Let’s just get one thing straight: you can’t really use alpha channels in InDesign, can you?

I mean it will make a solid path based on the 50% threshold of the channel, but you can’t actually use the transparency?

When I moved over to InDesign from Quark, alpha channels was one of the things I was looking forward to using. But after trying many times, I could never get them to work. The only alpha channel support seems to be just a cheap way of getting a clipping path.

By: Eugene

Eugene — Fri, 21 Nov 2008 22:42:00 +0000

Ah, Klaus you’ve triggered a memory. Indeed, you are correct. It was at the time of sending EPS from Quark through a RIP. Argh, obviously blocked out that memory Ok, I have bad memories of Alpha Channels through RIPs.

Ok, I’m convinced, Ted this is a fantastic post and I’m convinced. In the back of my mind I always kept thinking, “NO TO ALPHA CHANNELS” but now, as Klaus points out, in PDF they pose no “threat”".

Great stuff!

By: Klaus Nordby

Klaus Nordby — Fri, 21 Nov 2008 20:44:46 +0000

@Eugene: “The reason I am not convinced is that some RIPs I’ve dealt with don’t handle Alpha Channels very well, or not at all.” If one prints directly from ID to the RIP, that could be an issue — but surely, if one creates a PDF from ID, either flattened or with live transparency, then all those alpha channels should become harmless in the PDF, and hence not a danger to ill-behaving RIPs? Can anyone confirm or deny this theory?

By: Eugene

Eugene — Fri, 21 Nov 2008 20:24:22 +0000

I’m not entirely convinced of this. Although it’s an interesting approach to removing a background. The reason I am not convinced is that some RIPs I’ve dealt with don’t handle Alpha Channels very well, or not at all.

By: Toby Fairchild

Toby Fairchild — Fri, 21 Nov 2008 19:59:30 +0000

I agree with Ted. In addition to what Ted said regarding alpha channels being applicable to the whole image rather than just a layer as with layer masks, you can use a layer mask (or selection) to then create an alpha channel to be stored with the image.

By: Ted LoCascio

Ted LoCascio — Fri, 21 Nov 2008 12:51:48 +0000

An alpha channel is a mask that is applied to the entire image (all layers) rather than a single layer. You can only apply one layer mask to a single layer, but you can create multiple alpha channels. For example, I could create another alpha channel based on a selection of the flower without the stem.

You can create as many alpha channels as you want, store them in the image, and then apply them in InDesign. Layer masks are non-destructive and I use them all the time, but you can’t control them in InDesign like you can with alpha channels.

I think creating a selection and using Refine Edge is much easier than working with Calculations. The latter is a legacy feature which is far less intuitive.

By: Jerome Gantner

Jerome Gantner — Fri, 21 Nov 2008 11:21:31 +0000

With that image one of the fastest ways to get a nice outlined images appears to be the calculations command. Use it to create an alpha channel that is “grey” multiplied into “grey”, or try some other combinations. A little refining and you should be able to come up with a good greyscale mask which can be turned into a layer mask for the image.