Adobe PHOTOSHOP 5.0 User Manual page 82

74 CHAPTER 4
Choosing a Color Mode
Previous
previous conversion, making it easy to convert
several images with the same custom palette.
For a color illustration of color palettes, see
figure 4-7 on page 224.
Specifying the bit depth
Using the Uniform or Adaptive palette, you can
specify the bit depth of an indexed-color image—
the number of bits of color information per pixel.
Bit depth, also known as color depth, determines
the number of colors used to display (or print) an
image. For example, if you choose 4 bits per pixel,
the image is composed of 16 colors; 6 bits per pixel,
64 colors; 8 bits per pixel, 256 colors. (The number
of colors used is displayed in the Colors text box.)
In addition, you can specify the exact number of
colors to be displayed (up to 256) by choosing
Other for Color Depth and entering a value for
Colors.
The options in the Indexed Color dialog box
control only how the indexed color table is created.
Adobe Photoshop still treats the image as an 8-bit,
256-color image.
Specifying dithering options
Unless you're using the Exact color table option,
the color table may not contain all the colors used
in the image. To simulate colors not in the color
table, you can dither the colors. Dithering mixes
the pixels of the available colors to simulate the
missing colors. You can choose from three
dithering options:
Uses the custom palette from the None Does not dither colors but instead uses the
color closest to the missing color. This tends to
result in sharp transitions between shades of color
in the image, creating a posterized effect.
Uses an error-diffusion method that
Diffusion
produces a less structured dither than the Pattern
option.
Uses a halftone-like square pattern to
Pattern
simulate any colors not in the color table. This
option is available only when you're using the
Mac OS System palette, the Web palette, or the
Uniform (216 colors) palette.
Specifying color matching options
You can choose from three options to match colors
in the original image when dithering:
Finds the best possible match for each pixel.
Best
Although this option takes longer to process, it can
provide a better image. This option is only
available if you choose None or Diffusion for the
dithering option.
Faster Takes less time than Best, but may not find
the best color match for each pixel. This option is
only available if you choose None or Diffusion for
the dithering option.
Prevents colors that are
Preserve Exact Colors
already in the chosen palette from being dithered.
This is useful for preserving fine lines and text for
Web images. This option is only available if you
choose Diffusion for the dithering option.