Section III Digital Postprocessing
416
described earlier in this text, the images displayed
on your computer screen or television are RGB
images: all the colors you see are combinations of
varying amounts of red, blue, and green light. On
the screen, tiny phosphor dots of red, blue, and
green glow when struck by a beam of electrons,
transmitting the color. Light Emitting Diode
displays (LEDs) and Liquid Crystal Displays
(LCDs) operate on different principles, but have
the same visual effect. Red, green, and blue are
referred to as the additive primaries, since their
light is added together to make a color.
Images that are printed on paper or
other opaque base material cannot depend on
transmitted light to convey colors to the eye.
Instead, they make use of reflected light. A
different set of colors (cyan, magenta, and yellow)
is used for printed images. Each color blocks,
or subtracts, a specific color and reflects others.
Yellow absorbs the blue wavelengths of light but
reflects red and green, magenta absorbs green
but reflects red and blue, while cyan absorbs red
and reflects blue and green. For this reason, cyan,
magenta, and yellow are called the subtractive
primaries. (Refer to Chapter 4, Light and Exposure
for additional information.)
Alter Contrast Using Curves
1. With the image open, click on the
Curves
icon in
the
Adjustments
panel.
2. Click on the midpoint of the diagonal line to
anchor it at that point. A small black dot will
appear at the center of the graph line.
3. Click on the line at a point midway between
the center and the lower-left corner, then drag
it a small distance diagonally down and to the
right. Note that the line is now a curve from the
midpoint to the lower-left corner. The line from the
midpoint to the upper-right corner has assumed
a matching (but opposite) curved shape,
Figure 15-28.
4. The image will have become more contrasty, with
the tonal range extended. The degree of contrast
change depends on the distance the point was
moved—the farther the line curves away from the
diagonal, the greater the change.
5. To decrease contrast, the curve is moved in the
opposite direction, curving upward from the lower
left corner to the midpoint and downward from
the midpoint to the upper-right corner. The tonal
range is decreased, flattening the image.
Figure 15-28. With the curve anchored in the
middle, a change in the lower portion of the line
is mirrored in the upper portion. This mildly
S-curved line represents a moderate increase in
the contrast of the image. If a color image (or a
desaturated RGB file like this one) is being adjusted,
changes can be made independently to each of the
three color channels.
When working with a color image, contrast
of the red, green, and blue color channels can
be adjusted individually for greater control.
The desired channel can be selected using the
Channel
box above the tone graph.
RGB
is the
default; when it is displayed, contrast adjustments
are applied to all three channels simultaneously.
Contrast also can be adjusted locally (in a
selected area). Such a change might be made to
bring up detail in the somewhat dark foreground
of a landscape. This is equivalent to the
conventional darkroom process of using different
contrast filters to expose selected areas of a print.
First, a selection is made, and then the contrast
adjustment is carried out. The change will affect
only the selected area.
Correcting and Adjusting
Color
Working with color images, as compared to
black-and-white, can be considered three or four
times as complicated, since color images consist
of either three channels (red, blue, and green)
or four (cyan, magenta, yellow, and black). As