Image analysis – Image compression or coding – Gray level to binary coding
Reexamination Certificate
1998-02-09
2001-11-13
Couso, Jose L. (Department: 2621)
Image analysis
Image compression or coding
Gray level to binary coding
Reexamination Certificate
active
06317519
ABSTRACT:
The technical field to which the present invention relates is that of the coding of digital video image sequences. The current problem within this field is that of compressing the visual information by exploiting a set of parameters for regulating the cost and quality of this compression. It is thus possible to comply with a certain number of constraints which depend essentially on the intended application. Quality criteria may compel the information to be compressed without impairing its final reconstruction. Such is the case, for example, in teledetection or production applications and also storage applications. On the other hand, the quality requirement may be less demanding, although greater than a required minimum, giving way to the need to compress the information in accordance with the capacities of a given transmission channel. Such is, for example, the case in videophone applications (over ISDN standing for Integrated Services Digital Network or STN standing for Switched Telephone Network), or communication applications on battlefields. Finally, the most common case amounts to balancing the degradations in quality with the performance in compression. Such is the case in broadcasting applications, or again video distribution applications (video on Compact Disc such as
Digital Video Disc
). Added to this there are constraints of a practical nature.
The main coding methods may be viewed as being a combination of several techniques used on the basis of their properties.
Thus, there may be distinguished:
coding by prediction which consists on the one hand in providing an estimate and on the other hand in correcting it by taking into account the estimation error;
coding by transform which enables the information to be made more concise, by decorrelating it through a change of representation space;
coding by approximation which replaces information with other predefined or at least simplified information.
The latest developments in this field rely on prediction through motion. On the one hand the Discrete Cosine Transformation (DCT) makes it possible locally to reduce the redundancy of an image in intra mode. On the other hand the objective of motion compensation is to reduce temporal redundancy. The motion information corresponds to the local similitude ties between a so-called “reference” image and that currently being investigated, and is interpreted through the concept of the physical motion of the underlying scene. Thus, consider a partition of an image into blocks; for each of these blocks, a search is made in the other image investigated for the most similar block. The motion (horizontal, vertical) is then the difference in location between the blocks (line-wise, column-wise) in relation to the location of the first, and is coded once per block. This motion information is supplemented with the data regarding the residues from this prediction (values of the error in the prediction through motion). This is processed in a manner similar to the data of intra images.
The most recent schemes for coding image sequences exploit the motion data through a prediction. The MPEG2 approach is a good example of this.
The improvements obtained in data compression may however be deemed inadequate and better image quality may be desired for a given bit rate or a smaller bit rate, and hence coding cost, for a given final quality.
The invention which is the subject of the present patent application is aimed at remedying the aforementioned drawbacks.
To this end, the subject of the invention is a process for coding digital data from a sequence of video images carrying out a coding of the difference in luminance between an image segment to be coded and a corresponding segment of an image predicted from a so-called reference image, characterized in that the prediction is made as a function of a luminance compensation of values of luminance of the reference image.
Its subject is also a device for coding digital data from a video sequence for the implementation of the process comprising a motion estimation circuit for calculating vectors of motion between two images, a motion compensation circuit based on a reference image for calculating a predicted image, a subtractor for subtracting the predicted image from the current image for calculating a residue to be coded, characterized in that it comprises a circuit for estimating luminance between the two same images and a luminance compensation circuit receiving the information from the luminance estimation and motion estimation circuit for calculating the predicted image.
Its subject is also a device for decoding digital data coded according to the preceding process, for calculating an image reconstructed from a residue and a predicted image, characterized in that it comprises a circuit for compensating luminance as a function of luminance vectors which are luminance estimation information items, for calculating the predicted image.
The process consists in enhancing the prediction through motion compensation, with a so-called luminance compensation.
It also makes the possible to replace an intra image by a so-called auto-compensated image using the same mechanism as inter compensation based on motion, luminance and residue data.
The proposed invention makes it possible to reduce the amount of information contained in the residue, irrespective of the method of motion analysis used before the estimation of luminance. The enhancing of the model of motion by a so-called luminance approach significantly reduces the residue, and this reduction in residual information being larger than the corresponding addition of luminance information, the novel coding of inter images is more powerful.
Moreover, the invention proposes a unified coding scheme where the difference between the approach for intra images and that for inter images is small. The implementation of the invention is simplified and hence cheaper owing to the fact that it utilizes elements of existing schemes differently, by incorporating therewith a method which is of low complexity from a hardware point of view.
The invention is independent of the particular coding of each type of information item (motion, luminance, residue). The method used can be incorporated into the MPEG2 coding schemes, even though the data regarding luminance is coded In addition to the standard data.
These luminance compensation techniques are especially effective when there is a change of scene lighting which, in the prior art, would customarily give rise to expensive intra coding.
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M. H. Hayes, Iterated function systems for image and video coding, at the International Workshop on Image Processing: Theory, Methodology, Systems and Applications, Budapest, Hungary, Jun. 20-22, 1994, ISSN 0866-5583, Journal on Communications, May-Jun. 1994, Hungary, pp. 11-19.
Couso Jose L.
Kurdyla Ronald H.
Laks Joseph J.
Thomson Licensing S.A.
Tripoli Joseph S.
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