User Guide

ARTS PDF Aerialist Professional User Guide
Images
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8.2.2. Undo
Generally, one level of undo is supported for all Image functions except those functions
performed on all, or a selected range of, images in a PDF document. Undo is activated
by the standard “Control + Z” (Windows) or “Command+Z” (Macintosh) key sequence.
Repeating the keystrokes does a redo.
8.2.3. Compression Methods
Figure 81. Compression Methods
When you use the Images feature to add or replace an image in a PDF document, you
will be asked to select one of Acrobat’s supported compression filters. These include ZIP,
LZW, JPEG, Run Length, or None. Following is a brief overview of these compression
types. Users will need to experiment to determine which method gives them the best
combination of quality and file size.
The compression filters can be broken down into two classes: lossless and lossy. A
lossless filter (ZIP, LZW and Run Length) is one in which the process of encoding and
decoding results in no change of information. For a lossy filter (JPEG), the process of
encoding and decoding changes the information. Lossy filters can be used when the
resulting loss of information is not visually significant.
8.2.3.1. ZIP
ZIP encoding, like LZW encoding, discovers and exploits many patterns in its input
data. Thanks to its cascaded adaptive Huffman coding, ZIP-encoded output is usually
substantially more compact than LZW-encoded output for the same input. ZIP and
LZW decoding speeds are comparable, but ZIP encoding speed is considerably slower
than LZW encoding speed. Effective compression of monochrome images depends
upon the compression filter used and the properties of the image, but reductions of
2:1 to 8:1 are common. LZW or ZIP compression of graphics results in compression
ratios of approximately 2:1. The PDF specification also refers to ZIP encoding as
Flate encoding.