Feathering
The description of feathering from the Adobe manual is, “Blurs edges by building a transition boundary between the selection and its surrounding pixels. This blurring can cause some loss of detail at the edge of the selection.”
Feathering is set by a radius measurement in pixels. This means that the feathering occurs in both directions, or on both sides of the selection boundary. I think many people think of feathering as working outward, only.
Since feathering is set by pixels, a feather value will have a very different effect on images of very different resolutions. A seven pixel feathering of a selection in a 72 dpi image would be ten times as large as a seven pixel feathering of a selection in a 700 dpi image.
The examples below should help you see what is happening when you use
different feathering values. A black and white image of two pears on
a light to white background was selected, and colorized. The image
on the left was selected with the magnetic lasso with feathering
set to zero. The selected pears were then colorized. I then backtracked
in the history palette, and reselected the image with a ten pixel
feather setting, and colorized it, producing the image on the right.
If you look closely at the bottom of the pears on the right, you can
see the pink coloring spreading out onto the white background.
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Center, is an edge from the same image, colorized when the pears were selected with a five pixel feather. (No full size image is shown of the five pixel feathering)
Above, right was colorized with a ten pixel feather setting for its selection. This is a magnified view of the full size photo shown above, right.
Now, I'll show you what happens if you move a portion of an image which has been colored with a feathered edge, and place it on a different background. I selected a copy of the original black and white pears with zero feathering. I then inverted the selection, and filled the background with a light green color. After that, I repeated the colorizing that is shown in the illustrations above. Everything was the same except this time, I had a green background.
Next, using the same selection boundary that was used to colorize the
image, zero pixels, and ten pixels respectively, I moved the pears onto
a white background.
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You should bear this in mind when using feathering on portions of images you intend to move to a new background.
Anti-aliasing is similar to feathering. They work in somewhat the same manner, but anti-aliasing is much more subtle. Shown below are comparisons of the two.
In the group of curves in the illustration at right above, the settings were, from left to right, anti-aliasing turned on with no feather, next has no anti-aliasing with a three tenths feather setting, next has anti-aliasing turned on with no feathering (same as the first curve), and last has four tenths feathering and no anti-aliasing. I used the three tenths, and four tenths settings in the Select > Feather dialog box since they were the closest approximation of anti-aliasing.
You have to look very closely at the illustration on the right to see the difference, but it’s there.
For more information on anti-aliasing, see my page at Anti-aliasing.
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