138 Image and Video Compression
Human beings are the nal recipients in many image-processing applica-
tions; images are assessed by subjects (observers). This method is based on
the mean of the opinion given by observers. Even though correct judgment
is possible, this method is fairly expensive and slow. This approach is called
subjective image quality assessment (IQA).
Predication of the perceived image quality accurately based on computa-
tional models is another way. The prediction is supposed to be in correlation
with subjective assessment. This approach is called an objective IQA.
Thus, image quality assessment techniques are classied mainly into (1)
subjective and (2) objective. Further objective quality assessment is classied
into the following:
1. Full reference (FR)—In this type of assessment, the original image
before processing (compression) is used as reference. The decom-
pressed or distorted image is compared with reference, typically
using distance criteria. This method is still used by the researchers
to evaluate performance of the compression scheme developed.
2. No reference (NR)—In this approach, there is an image available as
reference. Certain artifacts/features in an image like blockiness and
ringing are measured and given to a well-trained prediction model
to get the quality score. The model uses subjective assessment dur-
ing training. The task is difcult but is drawing the attention of
many researchers to develop no reference quality metrics.
3. Reduced reference (RR)—Certain features like edge information and
pixel values at a xed position are extracted and attached as side
information along with the compressed image. For assessment, the
features are compared and reduced so that a reference quality met-
ric can be developed. This approach can also be used for image qual-
ity improvement (repair).
Subjective IQA is discussed in the next sections, and all objective IQA
techniques are discussed in further sections along with MATLAB programs
and comparison between various quality metrics.
7.2 Subjective Image Quality Analysis
Subjective image quality analysis may be the only best method for quantify-
ing visual image quality. The mean opinion score (MOS) is a widely used
method for subjective assessment of image quality. In this method, ve
or more grade scales are used. Experts and nonexperts are asked to give
grades for images as 5 for “Excellent,” 4 for “very good,” 3 for “good,” 2 for
“bad,” and 1 for “very bad.” The average of the grades given to that image is