Adobe PHOTOSHOP 6.0 Manual page 123

Although Grayscale is a standard color
model, the exact range of grays represented can
vary, depending on the printing conditions.
In Photoshop, Grayscale mode uses the range
defined by the working space setting that you have
specified in the Color Settings dialog box.
(See "About working spaces" on page 125.)
These guidelines apply to converting images to
and from Grayscale mode:
You can convert both Bitmap-mode and color
•
images to grayscale.
To convert a color image to a high-quality
•
grayscale image, Photoshop discards all color
information in the original image. The gray levels
(shades) of the converted pixels represent the
luminosity of the original pixels.
You can mix information from the color
channels to create a custom grayscale channel
by using the Channel Mixer command.
When converting from grayscale to RGB, the
•
color values for a pixel are based on its previous
gray value. A grayscale image can also be converted
to a CMYK image (for creating process-color
quadtones without converting to Duotone mode)
or to a Lab color image.
Duotone mode
This mode creates duotone (two-color), tritone
(three-color), and quadtone (four-color) grayscale
images using two to four custom inks.
(See "Printing duotones" on page 384.)
Indexed Color mode
This mode uses at most 256 colors. When
converting to indexed color, Photoshop builds a
color lookup table (CLUT), which stores and
indexes the colors in the image. If a color in the
original image does not appear in the table,
the program chooses the closest one or simulates
the color using available colors.
By limiting the palette of colors, indexed color can
reduce file size while maintaining visual quality—
for example, for a multimedia animation appli-
cation or a Web page. Limited editing is available
in this mode. For extensive editing you should
convert temporarily to RGB mode. (See
"Converting to indexed color (Photoshop)" on
page 118.)
Multichannel mode
This mode uses 256 levels of gray in each channel.
Multichannel images are useful for specialized
printing—for example, converting a duotone for
printing in Scitex CT format.
These guidelines apply to converting images to
Multichannel mode:
Channels in the original image become spot
•
color channels in the converted image.
• When you convert a color image to multi-
channel, the new grayscale information is based on
the color values of the pixels in each channel.
Converting a CMYK image to multichannel
•
creates cyan, magenta, yellow, and black spot
channels.
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