Efficient detection of error blocks in a DCT-based compressed video sequence

Abstract: A method of efficient and reliable detection of error blocks in a DCT-based compressed video sequence extracts edges from an image that correspond to the DCT blocks. These edges are processed to determine an edge energy value. The edge energy value is compared with a threshold to provide an alarm for an error block when the threshold is exceeded. The edge energies for each block may be summed and compared with an overall threshold value to generate the alarm, or the edge energy for each edge may be compared with an edge threshold value to determine which are good and which are suspect, with the alarm being set when at least three of the edges are suspect (two edges for corner blocks).

Description: BACKGROUND OF THE INVENTION The present invention relates to video compression, and more particularly to a method of efficient and reliable detection of error blocks in a DCT-based compressed video sequence. One noticeable defect in video sequences that have been subjected to a DCT-based codec, such as MPEG-2 video compression, is the incorrect displaying of DCT blocks of data that actually belong to a different field, usually due to coding or buffer overrun errors. US-6,795,580-A and corresponding EP-0986270-A entitled "Picture Quality Measurement Using Blockiness", by Bozidar Janko and Steven Maurer, describe a method of detecting the amount of blockiness in an image. Vertical and/or horizontal edges are detected and cross-correlated with a grid having a kernel equal in size to the DCT block sizes used in the particular compression scheme used. The average magnitude of the accumulation buckets used for the cross-correlation is compared with the maximum accumulation value from one of the buckets to determine the amount of blockiness. If incorrect codec coding or buffer overruns cause error blocks throughout the image, a general blockiness measurement is sufficient to detect their presence. However, a general blockiness measurement doesn't reliably detect small numbers of error blocks because their blockiness signature is often masked by the noise in the overall picture content. A single error block in an image, otherwise devoid of blockiness, is often not enough to cause the overall blockiness measure to exceed acceptable tolerances, even though the defect is easily and often annoyingly obvious to the human eye. Pickering M.R. et al: "A STATISTICAL ERROR DETECTION TECHNIQUE FOR LOW BIT-RATE VIDEO" Brisbane, Australia, Dec. 2-4, 1997, New York, NY: IEEE, US vol. CONF. 12, 2 December 1997, pages 773-776, XP000898158, ISBN: 0-7803-4366-2 discloses a statistical error detection algorithm for detecting 8 x 8 pixel blocks in a decoded image that have the wrong DC value. For blocks that are dominated in appearance by a single DCT basis function, the algorithm (1) calculates a standard deviation over a single frame of DCT coefficient magnitudes at each spatial frequency and (2) detects an error if a coefficient in a following frame is found to be some number of standard deviations from the mean for that spatial frequency. The detection of error therefore requires the processing of two consecutive frames, i.e. the current frame and the following frame. Chung J Kuo et al: "ADAPTIVE POSTPROCESSOR FOR BLOCK ENCODED IMAGES", IEEE TRANSACTIONS ON CIRCUIT AND SYSTEMS FOR VIDEO TECHNOLOGY, IEEE INC., NEW YORK, vol. 5, no. 4, 1 August 1995, pages 298-304, XP000528331, ISSN: 1051-8215 discloses an adaptive postprocessor for block encoded images to remove grid and staircase noise which uses a space-variant low pass filter to remove grid noise at the block boundary, and an adaptive regional or directional low pass filter to remove the staircase noise and smooth the variation of pixel values at either side of an edge. What is desired is a method of efficient and reliable detection of error blocks in a DCT-based compressed video sequence that more reliably detects small numbers of error blocks in images in the video sequence which would not otherwise be detected via an overall blockiness measure. BRIEF SUMMARY OF THE INVENTION According to this invention there is provided a method as claimed in claim 1 herein. The present invention provides a method of efficient detection of error blocks in a DCT-based compressed video sequence by extracting edges with a periodicity equal to the DCT block kernel of the compression scheme being used, as detected by an overall blockiness measure. The extracted edge energies corresponding to the DCT block edges may be summed and compared with an overall threshold to determine if a particular block is an error block. Alternatively each edge energy may be tested against an individual edge energy threshold, and those blocks which have a specified number of edges, typically three, that exceed the threshold are determined to be error blocks. Either method may further be refined by subtracting from the edge energy the moving average of localised energy around the edge being measured to reduce false positives from spatially noisy areas. The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Fig. 1 is a functional block diagram view of a method of efficient detection of error blocks according to the present invention. Fig. 2 is an illustrative view of the method of efficient detection of error blocks according to the present invention. Fig. 3 is a flow diagram view of the method of efficient detection of error blocks according to the present invention. DETAILED DESCRIPTION OF THE INVENTION Referring now to Fig. 1 a video input signal that has been compressed and decompressed at least once is input to a Component Separator 10. Generally the luminance component (Y) is extracted in the Component Separator 10 but, either to specifically detect error blocks in the color portion of decoding or for more computationally light-weight processing when it is known that the codec manifests identical errors in luminance and chrominance channels, one of the chrominance components (C) may be measured instead. The video component from the Component Separator 10 is input to an Edge Extractor 12 , which generally is in the form of an edge-enhancing filter such as a Sobel filter or a boxcar filter as specified in the above-mentioned co-pending U.S. Patent Application. Horizontal edge transitions are extracted, and vertical edge transitions may also be extracted when necessary. The extracted edge transitions are converted to positive differences, i.e., the transitions are subject to absolute values. The positive differences are than input to a Block Identifier 14. The Block Identifier 14 processes the positive differences to find a spatial period (size) and spatial phase (location) corresponding to the DCT blocks. The processing of the input video sequence to this point is essentially identical to the algorithm described in the above-mentioned U.S. Patent and European Patent Application. When the periodicity of the DCT blocks is unknown, this may be determined through the use of the overall blockiness measure as specified in said Application. This is accomplished by repeatedly measuring video images at different spatial periods, obtaining the overall blockiness measure for each one. Any period that consistently and repeatedly reports the highest value across numerous images in the video sequence is the period of the DCT blocks. This method works even if the DCT block impairments are much too faint to be detected by the human eye. All differential energies, and the spatial phase and period of the DCT blocks, are input to a Feature Extractor 16 , regardless of whether the overall blockiness measure is high enough to cause an alarm. The Feature Extractor 16 processes the edges on the identified DCT block boundaries and, in one embodiment, sums them. If the sum exceeds a user definable combined block threshold value, the block is reported to an Error Block Reporter 18. The Feature Extractor 16 may also test each edge to see if it exceeds a user definable edge threshold value and, if more than a specific number of them do, report the block to the Error Block Reporter 18 . The number of edges necessary to cause an alarm is typically three, but may be set to four to cause the Error Block Reporter 18 to be less sensitive or may be set to two for blocks on the corners of the image which only have two edges. The algorithm described above may be further modified to subtract from each edge the average differences surrounding the edge. By doing this, only the edge energy that is disproportionate to the local region remains, hence lowering the chance of false detection of error blocks in regions with high differential content, such as video snow, fields of grass, etc. The average differences are obtained by summing the differences in a line centering on and perpendicular to the edge being tested. Typically the number of differences that are averaged is equal to the detected spatial size of the blocks, but this may be reduced for the sake of computational efficiency. As shown in Fig. 2 an image is shown in a bit-map video representation that has a normal block and an error block. The image is processed to determine the edge differences that match DCT blocks, horizontally and/or vertically. The blocks with large edge transitions are error blocks, while the blocks with small edge transitions are normal blocks. As shown, the error block in this particular illustration has a sum of the edge differences equal to 629, which is substantially larger than the sum of the edge differences for a normal block, equal to only 83. Referring now to Fig. 3 the inputs to the error block detecting algorithm are the video stream, the field size of the images in the video stream, edge thresholds (sum and/or individual), and the period/phase of the DCT blocks according to the compression scheme used (if known). The video stream is separated into its components, one or more of which may be processed to detect error blocks. Each component is processed by an edge enhancement filter to determine edges. Based upon the edges determined and the DCT block kernel, the corresponding blocks are located in the images of the video component. If the period (size or kernel) of the DCT blocks is not known, the above steps may be repeated for different DCT block sizes to determine what the period of the DCT blocks in the images are. With the period and phase information, together with the edge information, the blocks in the images corresponding to the DCT blocks are located. At each edge boundary the edge energy is obtained and stored in a block edge array. For each block the total edge energy is computed and compared to an overall threshold value, as in Fig. 2. Those blocks that have a total edge energy greater than the threshold are reported as error blocks. An optional or supplemental methodology is, after locating the blocks in the images, to obtain the horizontal edge energies at block boundaries. The energy surrounding each boundary is averaged along a line through the center of the particular edge, and subtracted from the edge energy at the boundary. For each edge the energy is compared with an edge threshold value to determine whether the edge is "OK" (normal) or "suspect" (possible error block). If both edges are "OK", then this methodology returns to report the total edge energy when it exceeds the overall threshold, as in the first methodology. If one or both of the edges is "suspect", then the steps are repeated vertically to determine if the orthogonal edges are "OK" or "suspect". If at least three of the edges of any block are determined to be "suspect", then that block is reported as an error block. Thus the present invention provides a method of efficient and reliable detection of error blocks in DCT-based compressed video by obtaining edge energies for blocks in the image that correspond to DCT blocks, and evaluating the edge energies to determine whether they exceed a given threshold that indicates an error block.

Claims: A method for detecting error blocks in a DCT-based compressed video sequence using a single frame comprising the steps of: locating blocks (12, 14) in images of the video sequence corresponding to DCT blocks having a given block size; obtaining edge energies (16) corresponding to the block edges for each block of each image, averaging (16) differences surrounding each of the block boundaries in each dimension; and subtracting (16) the averaged differences from the said edge energies to determine noise reduced modified edge energies wherein the step of averaging differences comprises summing differences in a line centred on and perpendicular the block edge for which the edge energy is obtained; and reporting (18) as an error block each block where modified edge energies for the block meet a given criteria. The method as claimed in claim 1, further comprising the step of determining (14) from the images in the video sequence the given block size. The method as recited in claim 2 wherein the determining step comprises the steps of: enhancing (12) edges that occur in the images of the video sequence; correlating (14) the edges in the images with a plurality of DCT block sizes; and selecting (14) as the given block size the DCT block size having the best correlation from the correlating step. The method as recited in claim 1 wherein the obtaining step comprises the steps of: computing (16) differences across block boundaries in a first dimension as edge energies; and computing (16) differences across block boundaries in a second dimension orthogonal to the first dimension as edge energies. The method as recited in claim 4 , wherein the reporting step comprises the steps of: for each block summing (18) the edge energies for all edges of the block to produce a total edge energy; and indicating (18) the block as an error block when the total edge energy exceeds a given overall threshold value as the given criteria. The method as recited in claim 1 wherein the obtaining step comprises the steps of: computing (16) the differences across block boundaries in a first dimension as edge energies; comparing (16) the edge energy at each block boundary in the first dimension with an edge threshold value to determine whether the block boundary is a suspect edge; and repeating the computing, comparing and determining steps across block boundaries in a second dimension orthogonal to the first dimension when the results of the comparing step for either of the block boundaries produces the suspect edge. The method as recited in claim 6, wherein the reporting step comprises the steps of: totaling (18) the number of suspect edges for each block to produce a total suspect edge number; and indicating (18) that the block is an error block when the total suspect edge number is greater than or equal to a specified number as the given criteria. The method as recited in claim 7, wherein the reporting step further comprises the step of, for blocks that have only two boundaries in common with other blocks, indicating (18) that the block is an error block when both boundaries are suspect edges.

Inventors: MAURER STEVEN D, MAURER, STEVEN D.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1999
Main classification: H04N7
IPC codes: G06T9, H03M7, H04N5, H04N7