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52
Adobe Acrobat Help
Acrobat Distiller Options
Using Help
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52
quality. However, using 4-bit ZIP compression with 8-bit data can affect the quality,
since data is lost.
Note:
Adobe implementation of the ZIP filter is derived from the zlib package of Jean-loup
Gailly and Mark Adler, whose generous assistance we gratefully acknowledge.
•
The JPEG (Joint Photographic Experts Group) compression method is suitable for
grayscale or color images, such as continuous-tone photographs that contain more
detail than can be reproduced on-screen or in print. JPEG is
lossy
, which means that it
removes image data and may reduce image quality, but it attempts to reduce file size
with the minimum loss of information. Because JPEG eliminates data, it can achieve
much smaller file sizes than ZIP compression.
Acrobat provides five JPEG options, ranging from Maximum quality (the least compression
and the smallest loss of data) to Minimum quality (the most compression and the greatest
loss of data).The loss of detail that results from the Maximum and High quality settings are
so slight that most people cannot tell an image has been compressed; at Minimum and
Low, however, the image may become blocky and acquire a mosaic look. The Medium
quality setting usually strikes the best balance in creating a compact file while still
maintaining enough information to produce high-quality images.
•
The CCITT (International Coordinating Committee for Telephony and Telegraphy)
compression method is appropriate for black-and-white images made by paint
programs and any images scanned with an image depth of 1 bit. CCITT is a lossless
method.
Acrobat provides the CCITT Group 3 and Group 4 compression options. CCITT Group 4 is a
general-purpose method that produces good compression for most types of
monochrome images. CCITT Group 3, used by most fax machines, compresses
monochrome images one row at a time.
•
Run Length is a lossless compression option that produces the best results for images
that contain large areas of solid white or black.
About resampling
Resampling
refers to changing the pixel dimensions (and therefore display size) of an
image. When you
downsample
(or decrease the number of pixels), information is deleted
from the image. When you
resample up
(or increase the number of pixels), new pixels are
added based on color values of existing pixels. You specify an
interpolation
method—
average downsampling, bicubic downsampling, or subsampling—to determine how
pixels are added or deleted.
Note:
Distiller can only downsample; it cannot resample up.
To resample an image, Distiller combines pixels in a sample area to make one larger pixel.
You provide the resolution of your output device in dots per inch (dpi), and Distiller
combines pixels as needed to reduce the image’s resolution (ppi) to the specified dpi
setting:
•
Average downsampling averages the pixels in a sample area and replaces the entire
area with the average pixel color at the specified resolution.
•
Bicubic downsampling uses a weighted average to determine pixel color and usually
yields better results than the simple averaging method of downsampling. Bicubic is the
slowest but most precise method, resulting in the smoothest tonal gradations.