If you choose Pattern Dither or Diffusion Dither, Canvas
screens the image, rendering its tones as patterns of tiny
dots. Pattern Dither screens an image using a fixed pattern
similar to a traditional halftone screen. Diffusion Dither
screens an image using a process that creates a random pat-
tern effect.
If you choose Threshold, Canvas converts the image to a
high-contrast black-and-white image. Pixels of lightness val-
ues from 0 to 128 become black, and pixels of lightness val-
ues from 129 to 255 become white.
When you paste a selection into a Black & White image, Canvas uses
diffusion dither on the selection.
Grayscale image mode
Grayscale mode is appropriate for images scanned from black and
white photographs, or when the image will never be printed in color.
In Grayscale mode, pixels use 256 brightness levels to represent a
range of shades from pure black to pure white. Grayscale uses 8 bits
per pixel and requires less memory than most color modes.
If you convert a color image to Grayscale mode, Canvas discards all
color information.
Indexed image mode
Indexed color mode uses a palette of 256 colors for images.
Because it stores fewer colors, it requires significantly less memory
than RGB Color and CMYK Color modes, both of which support
millions of colors. Smaller memory requirements make Indexed
mode especially useful for images used on Web pages.
An Indexed image includes a color table, or palette, of colors used
in the image. When you convert an existing image to Indexed
mode, you can specify the number of colors from the image to
include in the color table.
Most image filters, effects, and opacity controls arent available to
be applied to Indexed images, except the Offset and De-Interlace
filters, as well as some third-party plug-in filters.
To convert an existing image to Indexed mode
1
Select paint objects with the Selection tool and choose Image >
Mode > Indexed. The Indexed Color dialog box appears.