TIFF 6.0 Specification
Final—June 3, 1992
Section 22: JPEG Compression
Introduction
Image compression reduces the storage requirements of pictorial data. In addition,
it reduces the time required for access to, communication with, and display of
images. To address the standardization of compression techniques an international
standards group was formed: the Joint Photographic Experts Group (JPEG). JPEG
has as its objective to create a joint ISO/CCITT standard for continuous tone
image compression (color and grayscale).
JPEG decided that because of the broad scope of the standard, no one algorithmic
procedure was able to satisfy the requirements of all applications. It was decided
to specify different algorithmic processes, where each process is targeted to sat-
isfy the requirements of a class of applications. Thus, the JPEG standard became a
“toolkit” whereby the particular algorithmic “tools” are selected according to the
needs of the application environment.
The algorithmic processes fall into two classes: lossy and lossless. Those based on
the Discrete Cosine Transform (DCT) are lossy and typically provide for substan-
tial compression without significant degradation of the reconstructed image with
respect to the source image.
The simplest DCT-based coding process is the baseline process. It provides a
capability that is sufficient for most applications. There are additional DCT-based
processes that extend the baseline process to a broader range of applications.
The second class of coding processes is targeted for those applications requiring
lossless compression. The lossless processes are not DCT-based and are utilized
independently of any of the DCT-based processes.
This Section describes the JPEG baseline, the JPEG lossless processes, and the
extensions to TIFF defined to support JPEG compression.
JPEG Baseline Process
The baseline process is a DCT-based algorithm that compresses images having 8
bits per component. The baseline process operates only in sequential mode. In
sequential mode, the image is processed from left to right and top to bottom in a
single pass by compressing the first row of data, followed by the second row, and
continuing until the end of image is reached. Sequential operation has minimal
buffering requirements and thus permits inexpensive implementations.
The JPEG baseline process is an algorithm which inherently introduces error into
the reconstructed image and cannot be utilized for lossless compression. The
algorithm accepts as input only those images having 8 bits per component. Images
with fewer than 8 bits per component may be compressed using the baseline pro-
cess algorithm by left justifying each input component within a byte before com-
pression.
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