Graphics Reference
In-Depth Information
Fig. 9.9
DSIS basic test
cell (BTC)
Original
Coded
Vote N
1 sec
10 sec
1 sec
10 sec
5 sec
Time
9.4.1
Test Methodology
First, we provide a brief tutorial about some frequently used subjective quality
assessment methods. In general, there are two broad methods to carry visual
evaluation tests: double stimulus and single stimulus. In double stimulus test
subjects rate either the quality or change in the quality between two video clips
reference (original) vs. impaired (coded). In single stimulus test, subjects rate
the quality of the impaired (coded) video clip, only. We will now describe two
examples of the former, namely, double stimulus impairment scale (DSIS), and
double stimulus continuous quality scale (DSCQS).
9.4.1.1
DSIS (Double Stimulus Impairment Scale)
This method is used when the material to be evaluated shows a wider range of
visual quality covering all quality scales (and not of the impairments). There are
two variants of DSIS: Variant I and Variant II. The structure of the Basic Test Cell
(BTC) of Variant I, is shown in Fig.
9.9
. It consists of two consecutive presentations
of video clips. Original (reference) video clip is presented first followed by the
presentation of the impaired (coded) version of the video clip. A message is then
displayed for 5 s requesting viewers to vote.
Viewers are expected to mark their visual quality score on an answer sheet with
quality rating over a defined scale e.g., scale that is made of 5 levels—ranging
from “1” (very annoying) to “5” (imperceptible). In Variant II of DSIS, the pairs
of original (reference) video clip and impaired (coded) version of the video clip are
presented twice before voting. For visual test evaluations conducted in Sect.
9.4.2
,
Variant I of DSIS methodology, as described earlier was chosen.
9.4.1.2
DSCQS (Double Stimulus Continuous Quality Scale)
Double Stimulus Continuous Quality Scale (DSCQS) is used in cases when it is
not possible to present the full range of quality scales. In this method, the original
(reference) and the coded (impaired) samples of a video clip are presented twice
and, in random order, for each BTC. At the end of the second presentation, the
viewers are asked to grade each of the two original and the two coded video clips,
separately. It should be noted that because of the random presentation order, viewers
do not have an
apriori
knowledge of whether a video clip shown belongs to the
original or to the impaired one.
