CHAPTER 3
86
Syntax
The JPEG filter implementation in Acrobat products does not support features of
the JPEG standard that are irrelevant to images. In addition, certain choices have
been made regarding reserved marker codes and other optional features of the
standard. For details, see Adobe Technical Note #5116,
Supporting the DCT
Filters in PostScript Level 2.
In addition to the baseline JPEG format, beginning with PDF 1.3, the
DCTDecode
filter supports the progressive JPEG extension. This extension does not add any
entries to the
DCTDecode
parameter dictionary; the distinction between baseline
and progressive JPEG is represented in the encoded data.
Note:
There is no benefit to using progressive JPEG for stream data that is embed-
ded in a PDF file. Decoding progressive JPEG is slower and consumes more memory
than baseline JPEG. The purpose of this feature is to enable a stream to refer to an
external file whose data happens to be already encoded in progressive JPEG. (See
also implementation note 11 in Appendix H.)
3.3.8 JPXDecode Filter
The
JPXDecode
filter
(PDF 1.5)
decodes data that has been encoded using the
JPEG2000 compression method, an international standard for the compression
and packaging of image data. JPEG2000 defines a wavelet-based method for
image compression that gives somewhat better size reduction than other methods
such as regular JPEG or CCITT. Although the filter can reproduce samples that
are losslessly compressed, it is recommended only for use with images and not for
general data compression.
In PDF, this filter can be applied only to image XObjects, and not to inline images
(see Section 4.8, “Images”). It is suitable both for images that have a single color
component and for those that have multiple color components. The color
components in an image may have different numbers of bits per sample. Any
value from 1 to 38 is allowed.
From a single JPEG2000 data stream, multiple versions of an image may be
decoded. These different versions form progressions along four degrees of
freedom: sampling resolution, color depth, band, and location. For example, with
a resolution progression, a thumbnail version of the image may be decoded from
the data, followed by a sequence of other versions of the image, each with
approximately four times as many samples (twice the width times twice the
height) as the previous one. The last version is the full-resolution image.