Image analysis – Pattern recognition – Feature extraction
Reexamination Certificate
2000-10-17
2004-03-09
Mariam, Daniel (Department: 2621)
Image analysis
Pattern recognition
Feature extraction
C382S266000, C348S192000
Reexamination Certificate
active
06704451
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to video quality and, in particular, to an objective assessment of the quality of coded and transmitted video signals.
BACKGROUND OF THE INVENTION
With the development of digital coding technology savings in transmission and/or storage capacity of video signals have been achieved and a large number of new muti-media video services have become available.
Savings in transmission and/or storage capacity by digital compress technology generally depend upon the amount of information present in the original video signal, as well as how much quality the user is willing to sacrifice. Impairments may result from the coding technology used and limited transmission channel capacity.
Video quality assessment can be split into subjective assessment by human observers providing their subjective opinion on the video quality, and objective assessment which is accomplished by use of electrical measurements.
It is the general opinion that assessment of video quality is best established by human observers which is, however, a complex, costly and time consuming approach. Accordingly, there is a need to develop objective visual quality measures, based on human perception, that can be used to predict the subjective quality of modern video services and applications.
Studies in the framework of the American National Standards Institute (ANSI) and the International Telecommunication Union (ITU) have led to a plurality of algorithms for objective video quality assessment.
As will be appreciated by those skilled in the art, calculation of quality indicators of video signals on a pixel bases, for example, requires a large amount of processing. As disclosed in a conference publication by S. D. Voran “The development of objective video quality measures that emulate human perception”, Globocom '91 conf. publ. vol. 3, pp. 1776-1781, 1991, an important class of disturbing distortions in a video signal are those that destroy, soften, blur, displace, or create edges or signal transitions in the video image.
In a further conference publication by S. D. Voran and S. Woff “An objective technique for assessing video impairments”, IEEE Pacific RIM Conference on Communications, Computers and Signal Processing, Proceedings Volume 1 of 2, pp 161-165, 1993, an objective technique is described, which is based on digital image processing operations performed on digitized original and impaired video sequences. The technique implies a features extraction process in which so called impairment measurements of perceptual video attributes in both the spatial and temporal domains are determined. The spatial impairment measurement is based on a Sobel filtering operation or, alternatively, a “pseudo-Sobel” operation, in order to enhance the edge content in the video image, and consequently in the spatial impairment measurement. The spatial impairment measurement is based on normalised energy differences of the Sobel-filtered video frames using standard deviation calculations conducted over visible portions of the pixel arrays of the original and impaired video signals. The impairment measurements thus extracted from the original and impaired video sequences are then used to compute a quality indicator that quantifies the perceptual impact of the impairments present in the impaired video sequence. The patent publication U.S. Pat. No. 5,446,492 discloses a similar technique in which the feature-extraction processes on the original and impaired video sequences are carried out at distantly apart source and destination locations. The features extracted from the original video sequence are such that they can be easily and quickly communicated between the source and destination locations via a separate low-bandwidth transmission path, i.e. the bandwidth of the source features is much less than the bandwidth of the original video sequence. To this end the feature-extraction process additionally includes a statistical subprocess which subjects the output of the Sobel filtering operation to a statistical processing, i.e. the computation of the standard deviation of the pixels contained within a so called region of interest for which the video quality is to be measured.
A drawback of these known techniques is the fact that the feature-extraction process is based on standard deviation calculations. One thing and another means that image distortions having contrary effects in the Sobel frames, e.g. blurring vs additional noise or false edges, can not always be detected. A further drawback is that the known techniques use a relative distance measure for the quality of perception, which consequently is sensitive for relative effects of very small size and as such of small visibility.
SUMMARY OF THE INVENTION
The present invention aims to provide objective quality measures that can be used to assess the subjective quality of video signals, dealing with the higher levels of cognitive processing which dominate the perception of video quality.
It is a further object of the present invention to provide such measures applicable for standardisation.
It is a still further object of the present invention to provide a method, an arrangement and equipment for objective quality assessment of degraded video signals for measuring the quality of video coding equipment and algorithms, video transmissions and other multimedia video services, and which among other things do not have the above mentioned drawbacks.
These and other objects and features are achieved by the present invention in a method of obtaining quality indicators for an objective assessment of a degraded or output video signal with respect to a reference or input video signal by quantifying the strength of edges or signal transitions in both the input and the output video signals using edge or signal transition detection, which method comprises a first main step of generating image features of the input and output video signals, and a second main step of determining quality indicators from the generated image features, and for the definition of which method the prior art of document U.S. Pat. No. 5,446,492 has been used. The process of quantifying the strength of the edges will hereinafter be referenced by the term edginess.
The method according to the invention includes in the first main step the steps of:
a) detecting edges in the input and the output video signals; and
b) calculating the edginess of the input and the output video signals, providing input and output edge signals; and in the second main step the steps of
c) establishing introduced edges in the output edge signal by comparing the input and output edge signals of corresponding parts of the input and output video signals, introduced edges being edges which are present in the output edge signal and are absent at corresponding positions in the input edge signal;
d) establishing omitted edges in the output edge signal by comparing the input and output edge signals of corresponding parts of the input and output video signals, omitted edges being edges which are present in the input edge signal and are absent at corresponding positions in the output edge signal;
e) obtaining normalised values of the introduced edges relative to the output edge signal adjusted by a first normalisation factor;
f) obtaining normalised values of the omitted edges relative to the input edge signal adjusted by a second normalisation factor;
g) calculating a first quality indicator by averaging the values obtained in step e); and
h) calculating a second quality indicator by averaging the values obtained in step f).
The method according to the invention is based on human visual perception, charactrised in that spatial distortions like the introduction and omission of edges or signal transitions have a great impact on the subjective quality of the video signal. Further, it has been found that the introduction of an edge is more disturbing than the omission of an edge.
This has been taken into account, in the method according to the invention, by obtaining normalised values of the introduced edges and the
Beerends John Gerard
De Caluwe Franciscus Elisabeth
Hekstra Andries Pieter
Koenen Robert Hendrik
Koninklijke KPN N.V.
Mariam Daniel
Michaelson Peter L.
Michaelson & Associates
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