DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 38.12 Compression
ratio control in JPEG is
effected by altering the
quantizer matrix: The larger
the entries in the quantizer
matrix, the higher the
compression ratio. The
higher the compression ratio,
the higher the reconstruction
error. At some point,
compression artifacts will
become visible.
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JPEG performance is loosely characterized by the error
between the original image data and the reconstructed
data. Metric such as mean-squared error (MSE) are used
to objectify this measure; however, MSE (and other
engineering and mathematical measures) don’t necessarily
correlate well with subjective performance. In
practice, we take care to choose quantizer matrices
according to the properties of perception. Imperfect
recovery of the original image data after JPEG decompression
effectively adds noise to the image. Imperfect
reconstruction of the DC term can lead to JPEG’s 8×8
blocks becoming visible – the JPEG blocking artifact.
The lossiness of JPEG, and its compression, come
almost entirely from the quantizer step. The DCT itself
may introduce a small amount of roundoff error; the
inverse DCT may also introduce a slight roundoff error.
The variable-length encoding and decoding processes
are perfectly lossless.
Compression ratio control
The larger the entries in the quantizer matrix, the higher
the compression ratio. Compression ratio control in
JPEG can be achieved by altering the quantizer matrix,
as suggested by the manual control sketched in
Figure 38.12. Larger step sizes give higher compression
ratios, but image quality is liable to suffer if the step
sizes get too big. Smaller step sizes give better quality,
at the expense of poorer compression ratio. There is no
easy way to predict, in advance of actually performing
the compression, how many bytes of compressed data
will result from a particular image.
CHAPTER 38 JPEG AND MOTION-JPEG (M-JPEG) COMPRESSION 457
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VLE -1