Adobe PHOTOSHOP 5.0, User Manual, Page: 77 / 400

69 ADOBE PHOTOSHOP 5.0
User Guide
Converting between bit depths
In most cases, RGB, grayscale, and CMYK images
contain 8 bits of data per color channel. With three
channels in an RGB image, this translates to a
24-bit RGB bit depth (8 bits x 3 channels), an 8-bit
grayscale bit depth (8 bits x 1 channel), and a
32-bit CMYK bit depth (8 bits x 4 channels).
Photoshop can also read and import 48-bit RGB,
64-bit CMYK, and 16-bit grayscale images (each
with 16 bits of data per color channel).
Note: A16-bit-per-channel image provides finer
distinctions in color, but its file size is much larger
than an 8-bit-per-channel image.
Photoshop supports these tools and commands in
16-bit-per-channel images: the marquee, lasso,
crop, measure, zoom, hand, pen, eyedropper,
color sampler, and rubber stamp tools; and the
Duplicate, Feather, Modify, Levels, Auto Levels,
Curves, Histogram, Hue/Saturation,
Brightness/Contrast, Color Balance, Equalize,
Invert, Channel Mixer, Image Size, Transform
Selection, and Rotate Canvas commands. To take
full advantage of Photoshop’s features, you can
convert a 16-bit-per-channel image to an 8-bit-
channel image.
To convert from 8 bits per channel to 16 bits per
channel:
1 Flatten the image you want to convert, as
explained in “Flattening all layers” on page 269.
2 Choose Image > Mode > 16 Bits/Channel.
To convert from 16 bits per channel to 8 bits per
channel:
Choose Image > Mode > 8 Bits/Channel.
Converting images from one
mode to another
The transition between converting an image from
one mode to another permanently changes color
values in the image. For example, when you
convert an RGB image to CMYK mode, RGB color
values outside the CMYK gamut are adjusted to
fall within the CMYK gamut. Consequently,
before converting images, it’s best to do the
following:
•
Do as much editing as possible in the image’s
original mode (usually RGB from most scanners,
or CMYK from traditional drum scanners or if
imported from Adobe Illustrator or from a Scitex
system).
•
Save a backup copy before converting. Be sure to
save a copy of your image that includes all layers in
order to edit the original version of the image after
the conversion.
•
Flatten the file before converting it. The inter-
action of colors between layer blending modes will
change when the mode changes.
For information on converting to CMYK, see
“Converting to CMYK” on page 97, “About
calibration” on page 80, and “Adjusting the
separation type and black generation” on page 90.
For converting open images to a different color
space, see “Converting the color space of open
images” on page 100.
To convert an image to another mode:
Choose Image > Mode and the mode you want
from the submenu. Modes not available for the
active image appear dimmed in the menu.