> Anyone have the spec for the new PalmOS 4.0 image compression algorithm
> called "PackBits"?
Yet another technology they didn't invent. It's been around for years
and years in TIFF files (as you probably know, from fax machines at Xerox).
Packbits compression seeks repeated data values. Packbits is
considered an RLE (run-length encoding) compression scheme because it looks
for "runs" or repeated values, and tallies their number, or "length." While
its name implies that runs of bits are "packed" together, it is actually runs
of bytes. It is very similar to the Macintosh Packbits compression used by
MacPaint, except the Packbits compression used for a TIFF will allow the
dimensions of the image to be variable. It works by reducing repeated strings
of the same characters into two components: the "run count" and the "run
value." The count and value are stored in one byte each. Each two-byte
grouping is referred to as an RLE packet. It is not a good compression scheme
for images with large color ranges, as these will not tend to have many runs
of the same color.
Section 9 here:
http://www.google.com/search?q=cache:aUAwG8cek6I:partners.adobe.com/asn/developer/pdfs/tn/TIFF6.pdf+%2B%22tiff%22+%2B%22specification%22+%2B%22packbits%22&hl=en
and..
http://www.dspguide.com/datacomp.htm
Each byte (eight bits) from the input file is replaced by nine bits in
the compressed file. The added ninth bit is interpreted as the sign of
the number. That is, each character read from the input file is
between 0 to 255, while each character written to the encoded file is
between -255 and 255. To understand how this is used, consider the
input file: 1,2,3,4,2,2,2,2,4, and the compressed file generated by
the PackBits algorithm: 1,2,3,4,2,-3,4. The compression program simply
transfers each number from the input file to the compressed file, with
the exception of the run: 2,2,2,2. This is represented in the
compressed file by the two numbers: 2,-3. The first number ("2")
indicates what character the run consists of. The second number ("-3")
indicates the number of characters in the run, found by taking the
absolute value and adding one. For instance, 4,-2 means 4,4,4; 21,-4
means 21,21,21,21,21, etc.
An inconvenience with PackBits is that the nine bits must be
reformatted into the standard eight bit bytes used in computer storage
and transmission. A useful modification to this scheme can be made
when the input is restricted to be ASCII text. As shown in Table 27-2,
each ASCII character is usually stored as a full byte (eight bits),
but really only uses seven of the bits to identify the character. In
other words, the values 127 through 255 are not defined with any
standardized meaning, and do not need to be stored or transmitted.
This allows the eighth bit to indicate if run-length encoding is in
progress.
/d
