background image
ISO/IEC 10918-1 : 1993(E)
TISO0670-93/d003
Application environment
A
Compressed image data, including table specifications
Application environment
B
Figure 3 Interchange format for compressed image data
FIGURE 3 [D03] 9,5cm = 371 %
These figures are also meant to show that the same tables specified for an encoder to use to compress a particular image
must be provided to a decoder to reconstruct that image. However, this Specification does not specify how applications
should associate tables with compressed image data, nor how they should represent source image data generally within
their specific environments.
Consequently, this Specification also specifies the interchange format shown in Figure 3, in which table specifications are
included within compressed image data. An image compressed with a specified encoding process within
one application environment, A, is passed to a different environment, B, by means of the interchange format.
The interchange format does not specify a complete coded image representation. Application-dependent information,
e.g. colour space, is outside the scope of this Specification.
4.2
Lossy and lossless compression
This Specification specifies two classes of encoding and decoding processes, lossy and lossless processes. Those based on
the discrete cosine transform (DCT) are lossy, thereby allowing substantial compression to be achieved while producing a
reconstructed image with high visual fidelity to the encoder's source image.
The simplest DCT-based coding process is referred to as the baseline sequential process. It provides a capability which is
sufficient for many applications. There are additional DCT-based processes which extend the baseline sequential process
to a broader range of applications. In any decoder using extended DCT-based decoding processes, the baseline decoding
process is required to be present in order to provide a default decoding capability.
The second class of coding processes is not based upon the DCT and is provided to meet the needs of applications
requiring lossless compression. These lossless encoding and decoding processes are used independently of any of the
DCT-based processes.
A table summarizing the relationship among these lossy and lossless coding processes is included in 4.11.
The amount of compression provided by any of the various processes is dependent on the characteristics of the particular
image being compressed, as well as on the picture quality desired by the application and the desired speed of compression
and decompression.
14
CCITT Rec. T.81 (1992 E)
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