Expert: LizaL - 3/16/2005

Question
I have seen many Photoshop professionals using channels to modify images as opposed to using selection tools or filters to extract certain parts of images using alpha channels or change differant parts of images using alpha channels and the like as well. Can you descibe and provide any other information you know about channels? That would be greatly appreciated. Thank you for your help.

I have in the past just used selection tools to extract images. This is odviously not very effective as it can cause pixeltion etc. I would like to be able to extract images effectively with curves.

I am using Photoshop CS, on a Windows Xp Pro computer.

-Jeff

Answer
Hi Jeff,

I'm sorry I didn't answer you sooner; I didn't get your notification email until this morning.

Anyway, you're right, in many cases channels are more effective in making selections than selection tools. But the reason is not so much pixelation than in causing extra work. Selection tools can sometimes leave artifacts behind, causing the user to have to do a lot of cleanup.

Also, using channels is effective in that the selections stay isolated, and can also be turned on and off and used whenever needed.

Channels are great -- but it's a topic that we could spend literally hours discussing. So I'm going to encapsulate some basic information for you here:

Some channels basics --

-- Channels are used to store information about the image. For instance, a color RGB picture will have four channels: red, green, blue, and the fourth channel is those colors combined into a composite RGB channel.

-- Channels appear as grayscale images in their palette, yet they can store color information.

-- When you make a selection, it can be stored as a channel.

-- Some functions of Photoshop, such as Quick Mask, let you create channels "on the fly."

Quick Mask is a fast way of creating both a selection that could be made into a permanent channel, or just a temporary channel.

Here's an example of using Quick Mask. Let's say I want to change the color of the shutters on a house. To use Quick Mask for this purpose, I'd follow these steps:

* Double-click on the enter Quick Mask mode button, which is located just below the big color swatches in the tool box. It's the button on the right.

* Make sure the Selected Areas selection is ticked.

* Then choose a brush best suited to "painting" the area that I plan to change.

* Paint. When finished, click the Exit Quick Mask mode button, which is just to the left of the Enter Quick Mask button.

Now I have a selection made from that Quick Mask edit, and I can modify it any way I want.

Here's a visual example:

http://www.little-works.com/all_experts/quick_mask3.mov

(You might have to wait a minute to make sure the movie loads completely.)


As you can see, Quick Mask gives you a quick way to create a new mask -- which is also a channel -- and make adjustments to just a particular part of an image. You see, it isolates the area you're working on from the rest of the  image so you're not modifying the whole thing.


Here's another little movie to watch, that sort of builds on the Quick Mask demo, yet makes the channel permanent.

http://little-works.com/all_experts/save_channel.mov

In this movie I used the active selection I'd created with Quick Mask, and, by accessing the Select pulldown menu, chose Save Selection, and named that selection "shutter." Not only did this make it a "permanent" selection for that image, but it made it a channel, as you can see from the Channels palette.

You can also see that when I clicked on the eye icon to make the shutter channel active, the rest of the picture turned red. This means that I'm working on only the shutter channel -- that the rest of the image is masked out.

It's an extreme example, but I made the shutter very purple. Just wanted to show you that once you have a channel activated, you can do any sort of adjustment you need.


Finally, here are some things you can read,and see online:

1. This tutorial is for Photoshop 6, but the theory is the same.

http://www.arraich.com/ps6_tips_channels1.htm



2. Alpha channels and the channels palette:

http://graphicssoft.about.com/od/photoshop/l/bllps504h.htm



3. This is directly from Photoshop's Help section:

     Channels are grayscale images that store different types of information:
         * Color information channels are created automatically when you open a new image. The image's color mode determines the number of color channels created. For example, an RGB image has four default channels: one for each of the red, green, and blue colors plus a composite channel used for editing the image.

         * You can create alpha channels to store selections as grayscale images. You use alpha channels to create and store masks, which let you manipulate, isolate, and protect specific parts of an image. In addition to supporting alpha channels from Photoshop, you can save, load, and delete selections as alpha channels in ImageReady.

         * You can create spot color channels to specify additional plates for printing with spot color inks.

     An image can have up to 56 channels. The file size required for a channel depends on the pixel information in the channel. Certain file formats, including TIFF and Photoshop formats, compress channel information and can save space. The uncompressed size of a file, including alpha channels and layers, appears as the rightmost value in the status bar at the bottom of the window when Document Sizes is chosen from the pop-up menu.

     Note: As long as you save a file in a format supporting the image's color mode, the color channels are preserved. Alpha channels are preserved only when you save a file in Adobe Photoshop, PDF, PICT, Pixar, TIFF, or Raw formats. DCS 2.0 format preserves only spot channels. Saving in other formats may cause channel information to be discarded.



Hope this helps! Again, sorry for the delay, and if I can help you further, please don't hesitate to post back.

Lisa