Signal Processing - March 2016 - 122
SOI (Start of Image)
APP1 (Exif-Marker)
APP0 (JFIF-Marker)
DQT (Quantization Table for Base Layer)
APP11
ftyp-box (File Type Box)
APP11
TONE-Box (Inverse TMO Box)
SPEC-Superbox
APP11
APP11
RTRF-Box (Residual Color Trafo)
LTRF-Box (Base Color Trafo)
LCHK-Box (Legacy Data Checksum)
SOF (Start of Frame for Base Layer)
RESI-Box
APP11
SOI (Start of Image, Residual Layer)
SOF (Start of Frame, Residual Layer)
APP11
DQT (qnt Matrix, Residual Layer)
DHT (Huffman Table, Residual Layer)
SOS (Start of Scan, Residual Layer)
APP11
Entropy Coded Data (Residual Layer)
APP11
EOI (End of Image, Residual Layer)
DHT (Huffman Table, Base Layer)
SOS (Start of Scan, Base Layer)
Entropy Coded Data (Base Layer)
EOI (End of Image, Base Layer)
figure 3. The JPEG XT file format: APP markers contain the boxes structure. Blue syntax elements were defined
in the legacy JPEG standard, yellow elements were defined in later parts. The green Exif marker is defined outside
of JPEG. The red APP11 markers are not part of the legacyJPEG standard and will thus be ignored by legacy JPEG
readers. In contrast, JPEG XT readers will be able to interpret the payload data in the boxes that contain relevant JPEG
XT information. Since the size of an APP marker is limited to 64 kB, a single box may extend over several markers; the
RESI box carrying the residual code stream is a typical example. Otherwise, an APP marker or a series of APP markers contains a single box. This box may be, however, a superbox whose payload again consists of boxes. The SPEC
box defining the instructions how to reassemble the HDR image from base and exension image is a typical example.
122
IEEE Signal Processing Magazine
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March 2016
|
mai11, and 5) a local operator with
strong contrast enhancement [12] mantiuk06. To fully understand the implications of the TMOs and JPEG XT
parameters, all possible combinations
of these parameters were tested. We
used the combination of ten base quality levels # ten extension layer quality
levels # five TMOs # three profiles,
which results in a total of 1,500 conditions for each of the 106 images resulting in 159,000 tests. However, such a
large number of conditions clearly cannot be tested in a subjective experiment.
Therefore, from the total 106 HDR images, a subset of 20 images was selected by
experts for subjective evaluations and
these images were adjusted for viewing
on an SIM2 HDR monitor; see [13] for
more details on the subjective evaluations.
The results of subjective experiments
are crucial to select the right image quality metric and to provide ground truth reference. However, a subjective experiment
alone cannot cover the entire space of
parameters. Due the tedious nature of
those experiments, only a limited number
of images can be tested, which makes the
findings of such studies difficult to generalize. For that reason, we analyzed compression performances with respect to
HDR-VDP-2 [14], the best-performing
objective quality metric according to a
set of evaluations. The image quality
computed for a range of base and extension layer quality settings may result in
arbitrary bit rates, making the results difficult to aggregate. Therefore, the predicted quality values were linearly
interpolated to find the HDR-VDP-2
Q-scores for each desired bit rate. This
step was necessary to determine average
performance and confidence intervals for
all tested profiles.
In Figure 4, we compare the performance of the three profiles with popular
HDR image formats, including lossless
OpenEXR, Radiance RGBE, lossy JPEG
2000, and JPEG-XR (floating encoding).
OpenEXR and Radiance offer lossless
compression, however the loss happens
when converting images to their internal
pixel formats: 8-bit RGB channels and
shared 8-bit mantissa (E) for Radiance
RGBE; and 16-bit half-float (sign,
5-bit exponent, 10-bit mantissa) for
Table of Contents for the Digital Edition of Signal Processing - March 2016
Signal Processing - March 2016 - Cover1
Signal Processing - March 2016 - Cover2
Signal Processing - March 2016 - 1
Signal Processing - March 2016 - 2
Signal Processing - March 2016 - 3
Signal Processing - March 2016 - 4
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Signal Processing - March 2016 - 120
Signal Processing - March 2016 - 121
Signal Processing - March 2016 - 122
Signal Processing - March 2016 - 123
Signal Processing - March 2016 - 124
Signal Processing - March 2016 - 125
Signal Processing - March 2016 - 126
Signal Processing - March 2016 - 127
Signal Processing - March 2016 - 128
Signal Processing - March 2016 - Cover3
Signal Processing - March 2016 - Cover4
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