Channels

page two
The default setting for channels is to have black indicate the areas that are masked, or which would be excluded from a selection made from the mask. White indicates the areas that are not masked, and which will be included in the selection when the channel is loaded as a selection.
     However, you can reverse this order if you like. I have created a new channel mask on the photograph, this time creating various shapes with selection, and painting tools, and then filling them with white.

If I double click on the Alpha channel’s name in the Channels palette, its Channels Options dialog box will open.

If I choose the Selected Areas radio button, as shown, my mask will then display like this:

This is relevant because the latter is the way Photoshop displays color information in the CMYK color channels which automatically appear when you open a color image in that mode. Darker areas are where there is more of the color. Lighter areas are where there is less of the color.
     Either way, the selection which results when the channel is loaded as a selection is the same. I did that, then chose Select > Inverse to choose all areas other than the shapes I had drawn. I then chose Edit > Fill and chose Black from the menu. If you have the default colors in your toolbox colors boxes (with black as the foreground color), you can press Alt-Backspace to fill with the foreground color. Here is what I got. The selection edges are also shown.

Older Tutorials
Quick Mask
Adjusting Contrast
Using Curves
Scanning Negatives
Rubberstamping
More Adjustments
Sharpening
Filters
Color

The shapes were created with different tools which had different edge effects. I also added feathering to several, and anti-aliasing was selected for three of the forms.

Details are as follows:

1. Created with a large soft brush by stroking with white on the mask using the paintbrush tool. Detail below.
   
2. Created with the pencil tool by drawing with white on the mask. This tool makes hard edged strokes with no blend at the edges. Detail below.
   
3. This was drawn be dragging a selection on the mask with the rectangular marquee tool. I had zero feathering set in its options bar, and the rectangular marquee tool does not use anti-aliasing. I filled the shape (on the mask) with white.
   
4. Same as number three, but this time I added a 3 pixel feather to the rectangular marquee tool’s options bar settings before dragging the selection on the mask. Detail below.
   
5. Ditto the above, but this time I used a ten pixel feather setting.
   
6. This one was a text outline selection created with the Type Mask tool in exactly the same way as the two red letters at the start of this tutorial. Shown below is a detail from the middle of the right side of the letter.
   
7. This is the letter D, created in the same way as the preceding letter A, but with a 2 pixel feather added to the selection after it was created. I chose Select > Feather and chose 2 pixels before I filled the shape with white on the mask. It didn’t show up well in the image, so I recreated the effect with a red letter on white.
   
8. This is a shape created on the mask with the Custom Shape tool. When using the Shape tools on a mask, the only option available for type is the Create Filled Region. I kept anti-aliasing checked in the options bar. The shape was filled with white on the mask.
   

I will repeat the same graphic information I showed you on page one for one of the mask shape outlines to make the point, once again, that the channels, and the masks they store are image maps. They tell Photoshop “How Much?” to affect the existing pixels with any new colors, adjustments, filters or effects that you add while a selection created from that mask is active.
     Look at the detail shown for item number four in the numbered graphic above. Here is the mask detail that created the (inverted) selection that was filled with black.

Here is a numbered graphic of that mask.

And here are RGB readings taken from this magnified screen capture. Remember, readings taken from the mask itself would be in K values as it is a grayscale image, not RGB.

1. Pure black. In the inverted selection, this area is 100 % affected by the colors added, in this case, black.
2. 
3. 
4. 
5. 
6. Pure white. Since I inverted the selection created from this mask, the base, or existing image pixels in these areas were 100 % unaffected by any changes made while the inverted selection was active.

I have repeated this demonstration to try and make it very clear that the mask is a map. It is a map of “How Much?” instruction for Photoshop to use when a selection is created from that mask, and colors, adjustments, filters, or effects are added. It tells Photoshop exactly how much to affect the existing pixels with whatever you add.
Continue on page three

Copyright © 2002 by Jay Arraich.
All rights reserved.
All photographs copyright ©2002 by Jay Arraich
jay@arraich.com