CHAPTER 3
84
Syntax
The resulting PDF image object, then, contains the page information segment
and the immediate text region segment and refers to a
JBIG2Globals
stream that
contains the symbol dictionary segment.
3.3.7 DCTDecode Filter
The
DCTDecode
filter decodes grayscale or color image data that has been
encoded in the JPEG baseline format. (JPEG stands for the Joint Photographic
Experts Group, a group within the International Organization for
Standardization that developed the format; DCT stands for discrete cosine
transform, the primary technique used in the encoding.)
JPEG encoding is a lossy compression method, designed specifically for
compression of sampled continuous-tone images and not for general data
compression. Data to be encoded using JPEG consists of a stream of image
samples, each consisting of one, two, three, or four color components. The color
component values for a particular sample must appear consecutively. Each
component value occupies an 8-bit byte.
During encoding, several parameters control the algorithm and the information
loss. The values of these parameters, which include the dimensions of the image
and the number of components per sample, are entirely under the control of the
encoder and are stored in the encoded data.
DCTDecode
generally obtains the
parameter values it requires directly from the encoded data. However, in one
instance, the parameter might not be present in the encoded data but must be
specified in the filter parameter dictionary; see Table 3.11.
The details of the encoding algorithm are not presented here but are in the ISO
specification and in
JPEG: Still Image Data Compression Standard,
by Pennebaker
and Mitchell (see the Bibliography). Briefly, the JPEG algorithm breaks an image
up into blocks that are 8 samples wide by 8 samples shigh. Each color component
in an image is treated separately. A two-dimensional DCT is performed on each
block. This operation produces 64 coefficients, which are then quantized. Each
coefficient may be quantized with a different step size. It is this quantization that
results in the loss of information in the JPEG algorithm. The quantized coef-
ficients are then compressed.