Converting To Indexed Color - Adobe PHOTOSHOP 5.0 User Manual

72 CHAPTER 4
Choosing a Color Mode
Diffusion Dither
error-diffusion process, starting at the pixel in the
upper left corner of the image. If the pixel's value
is above middle gray (128), the pixel is changed
to white—if below, to black. Because the original
pixel is rarely pure white or pure black, error is
inevitably introduced. This error is transferred to
surrounding pixels and diffused throughout the
image, resulting in a grainy, filmlike texture. This
option is useful for viewing images on a black-
and-white screen.
Original grayscale image, and 50% Threshold
conversion method
Pattern Dither conversion method, and Diffusion Dither
conversion method
Custom Pattern
grayscale image through a custom halftone screen.
This method lets you apply a screen texture, such
as a wood grain, to an image. To use this option,
you must first define a pattern (see "Filling and
stroking selections and layers" on page 211).
Converts an image by using an
Simulates the effect of printing a
You can create a pattern representing the texture
you want and then screen the grayscale image to
apply the texture. To cover the entire image, the
pattern must be as large as the image. Otherwise,
the pattern will be tiled. For example, if you apply
a 1-inch-by-1-inch pattern to a 4-inch-by-4-inch
image, the pattern appears as 16 squares. Adobe
Photoshop comes with several self-tiling patterns
that can be used as halftone screen patterns (see
the procedure to use a pattern from the PostScript
Patterns folder in "Filling and stroking selections
and layers" on page 211).
Because the Custom Pattern option simulates dark
and light colors by making the halftone pattern
thicker and thinner, it makes sense to choose a
pattern that lends itself to thickness variations,
typically one with a variety of gray shades.
To prepare a black-and-white pattern for
conversion, you can first convert the image
to grayscale and then apply the Blur More filter
several times. This blurring technique creates
thick lines tapering from dark gray to white.

Converting to indexed color

Converting to indexed color deletes all but 256
colors from the image—the standard number of
colors supported by many multimedia animation
applications and Web pages. Converting an
RGB image to indexed color lets you edit the
image's color table or export it to an application
supporting only 8-bit color. The conversion also
reduces the file size by deleting color information
from the image.