User Guide

ARTS PDF Aerialist Professional User Guide
Images
| CONTENTS | INDEX |
 Page 157 of 200
8.2.3.2. LZW
LZW compression can discover and exploit many patterns in its input data. The
compression obtained using the LZW method varies from file to file; the best case (a
file of all zeros) provides a compression approaching 1365:1 for long files, while the
worst case (a file in which no pair of adjacent characters appears twice) can produce
an expansion of approximately 50%.
8.2.3.3. JPEG
Using JPEG compression, color and grayscale images can be compressed by a factor
of 10 or more. The JPEG filter supported by PDF is a lossy filter. JPEG encoding
provides very significant compression of color and grayscale images, but because it
is a lossy compression it is not appropriate in all circumstances. Screenshots, in
particular, are often unacceptable when JPEG encoded. This happens because each
pixel in a screenshot is usually significant, and the loss or alteration of just a few
pixels can drastically alter the appearance of the screenshot. Unlike screenshots, the
effect of JPEG encoding on continuous-tone images is typically acceptable,
particularly when high compression is not demanded.
8.2.3.4. Run Length
Run-length encoding is a compression technique that works best for black-and-white
or line art images. It works by replacing “runs” of the same color with a single
character. The more runs there are, and the longer the run sequence, the greater
the compression.
The compression achieved by run-length encoding depends on the input data. In the
best case, a file of all zeros, a compression of approximately 64:1 is achieved for
long files. The worst case, the hexadecimal sequence of alternating <00 FF 00 FF
…>, results in an expansion of 127:128.