Masks and Selections (Part 1 of 3)
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.
In Part 2 of this series, we�ll take a look at how masks work.
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