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In-Depth Information
4. The video sequences were presented to each viewer between one and three times
and the viewers were asked to assess the relative quality of the side-by-side
encodings according to a 5
ı
scale:
left better, left slightly better, no preference,
right slightly better
or
right better
.
3
The viewers were provided approximately
30 seconds between the presentation of each of the spliced video sequences to
record their preferences. A total of seven (7) video pairs were presented and each
viewing session had a duration of approximately seven (7) minutes.
9.5.1.3
Test Sequences
The four video sequences described in Table
9.11
were used in the subjective exper-
iments. The YCbCr 4:2:0 8-bit 4K video sequences have a variety of characteristics
typical of what might be encountered in the context of a streaming application, and
they were selected from those that are generally available and used for video coding
test purposes. All the sequences have a native frame rate of 50 fps. The 4K sequences
were displayed at 30 frames per second; the highest frame rate at which the monitor
is capable of displaying 4K content. The order of presentation followed that shown
in Table
9.11
, from top to bottom.
9.5.2
Subjective Quality Assessment Results
Tab le
9.12
shows the results of subjective video quality assessments. For each of the
video sequences, the average bit rates of the HEVC and AVC encoders are shown
along with the viewers' assessments of the video sequences, according to the 5
ı
scale described in Sect.
9.5.1.2
. Each video sequence was encoded using a fixed QP.
The QP values for the encodings were selected to yield good, but not especially high
video quality, in order to avoid viewing scenarios where either both encodings would
yield indistinguishably excellent quality or both encodings would yield substantial
coding artifacts.
9.5.3
Results
The subjective results presented in Table
9.12
show that the viewers either had no
preference or favored the HEVC encoded video at a bit rate that was approximately
3
A5
ı
scale was selected for this study in lieu of more commonly used 7
ı
and 3
ı
scales, because
it is known to work well to prioritize video quality improvement work during commercial encoder
development.
