Image-editing terms: A cheat sheet
Artifacts: Random image distortions, often introduced by too much compression, or by
saving a file in the wrong format.
Channel: The portion of an image containing a single color's tonal information. For exam-
ple, an RGB file has channels for each primary color: red, green, and blue.
Curves: A feature to fix color range problems. It allows you to change the number of red,
green, and blue pixels in an image, and their relative brightness.
Levels: A feature to fix value problems. It generates an interactive histogram so you can
adjust the number of pixels at each brightness level.
Mask: An electronic frisket, or defined area, that protects portions of an image from
change, or determines by its level of opacity how much change will be applied. Masks can
be created from selections, and saved for future use as a channel.
White balance: An in-camera lighting calibration that determines pure white in the current
lighting and balances the internal camera settings for the shot accordingly. If white is cor-
rectly displayed, all other colors in the spectrum should also display correctly.
Brightness and contrast
Most image-editing software has an automatic levels function that can do a
decent job on an image. For a portfolio, however, decent is not always good enough.
The automatic function tends to increase contrast at the expense of maintaining
detail—not a good feature for a photographer's portfolio, or for a designer whose
work includes textures and subtle ink variations. Most of the time you should use the
manual levels function, which gives you much more control. Curves allow even finer
adjustments in color and tone, but they also require more experience and understand-
ing of color management than levels do.
Color printing depends on fine-screened overlays of dots to mimic continuous-
tone color. Unfortunately, the dots are created with screens that do not have a
one-to-one correspondence to pixels. When screens and pixels collide, you get wavy,
distracting patterns. Most scanners come with software that includes a “Descreen” set-
ting, which usually does a very good job on the problem. But sometimes moiré per-
sists, or the scanner you're using is too old to have software that includes Descreen.
If you scan and discover that the file has moiré, the first thing to do is to
rescan it. You can often eliminate or decrease moiré simply by finding a sweet spot—