Graphics Reference
In-Depth Information
raw video files, progressively scanned, and with YCbCr 4:2:0 color sampling. The
sequences were compressed with HEVC and AVC. For each sequence and codec,
four quantization parameters were selected, resulting in a total of 40 test stimuli.
Five training samples were generated using the Sintel39 sequence (its resolution
is 3,840 1,744) and manually selected by expert viewers so that the quality of
samples were representative of all grades of the rating scale.
The original sequences were cropped to the resolution of the monitor, keeping
only the central part, and the 10-bit sequences were clipped to 8-bit.
9.4.2.4
Training Session
Before the experiment, a consent form was handed to subjects for signature, and oral
instructions were provided to explain their tasks. Additionally, a training session was
organized to allow subjects to familiarize with the assessment procedure.
9.4.2.5
Test Session
Since the total number of test samples was too large for a single test session,
the overall experiment was split into two sessions of approximately 13 min each.
Between the sessions, the subjects took a 10 min break. The test material was
randomly distributed over the two test sessions.
Three dummy pairs (one with high quality, one with low quality, and one of
mid quality), whose scores were not included in the results, were included at
the beginning of each test session to stabilize the subjects' ratings. To reduce
contextual effects, the stimuli orders of display were randomized applying different
permutation for each group of subjects, whereas the same content was never shown
consecutively.
A total of 18 naive subjects (6 females and 12 males) took part in the experiments.
They were between 18 and 27 years old with an average of 23.4 years of age. All
subjects were screened for correct visual acuity and color vision using Snellen and
Ishihara charts, respectively.
9.4.2.6
Analysis of the Results
The subjective results were processed by first detecting and removing subjects
whose scores appeared to deviate strongly from others. The outlier detection was
performed according to the guidelines described in Section 2.3.1 of Annex 2 of [ 14 ].
In this study, one outlier was detected. Then, the mean opinion score (MOS) was
computed for each test stimulus as the mean across the rates of the valid subjects, as
well as associated 95 % confidence interval (CI), assuming a Student's t -distribution
of the scores.
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