Using masks rather than the selection
tools is somewhat like working in grayscale instead of
bitmap mode. Masks offer much finer control of how filters,
effects, and colors are applied.To
continue the analogy, in bitmap color mode, a pixel is
either black or it is white. In grayscale, you have 256
levels of color, ranging from black through gray to white.
With a basic selection, a pixel is either inside the
selection marquee or outside. Two simple marquee selections
have been made, without feathering or anti-aliasing.
When you make a basic selection and, for
example, apply a color, those pixels inside the selection
receive the color, those outside do not.
With the selections shown in the first
image, I pressed D for default colors, then X to swap the
foreground and background colors. Pressing Delete (Backspace
on PCs) fills the selection with the background color. Note
that this is an RGB image, although black and white are at
this point the only colors visible.
Zooming in, we see how the selection works.
Pixels within the “marching ants” have been colored black,
those outside the selection remain unchanged.
Altering the selection a bit, we check the
option for Anti-aliasing for the Elliptical Marquee tool,
leaving the Rectangular Marquee tool as a basic selection.
Now when we fill the selections, Photoshop uses
anti-aliasing to smooth the appearance of the curved
selection. (This circle does not have the jagged edges of
that seen in the second picture, above.)
Zooming in to the same area seen earlier,
you’ll see how gray pixels were added, both inside and
outside the selection, to visually smooth the transition
from black to white.
Now we re-select with the Rectangular
Marquee, this time set to feather the selection 3 pixels.
Feathering also offers a transition between the selected and
unselected pixels. Rather than producing a smoothed curve,
feathering is designed to allow you to fade the edges of a
selection.
Another zoom to 800% and we can see up-close
the effects of feathering.
Although the feathering was set to three
pixels, the effect on either side of the selection marquee
is more extensive. Using a maximum zoom (1600%) along the
selection border shows the extent of the feather.
In the following figure, the lower part of
the Info palette is visible, showing the color values for
the four Color Samplers. Number 1 is black, number 4 is
white, while numbers 2 and 3 show the extent from the
selection border to which the feathering was applied.
Remember that the Marquee tool’s options were set for a
three pixel feather, yet the color is changed for a distance
of seven pixels on either side of the selection marquee.
The following graph shows the distribution
of color values of the fourteen shades of pixels. From the
neutral gray at the selection board (in the middle of the
graph), to the extreme color changes at the left and right
ends of the graph (represented by Color Samplers 2 & 3 in
the preceding image), the graph shows how Photoshop uses
feathering to blend the colors.
Remember that these simple examples of
selection, anti-aliasing, and feathering carry over to more
than just black and white. Just as the
black-to-gray-to-white transition used numerous levels of
gray, so to would a filter applied to a feathered edge use
varying values. As just one example, the mage below shows
how feathering softens the effect of the Add Noise filter.
Next week, in Part 2 of this series, we’ll
take a look at how masks work.