METHOD FOR ANALYSING THE CONTENT OF A VIDEO SIGNAL

Abstract: A method for analysing differences in the content of successive frames of a digital video sequence to create a profile that predicts the likely perceptual significance of and allows separation and classification of different type of signal component.

Description: Method for Analysing the Content of a Video Signal This invention relates to a method of analysis which yields a description of a video signal (hereinafter referred to as a profile or statisϋcal profile), which relates the signal content to its likely perceptual significance Proven applications of the profile analysis include noise filtration, data compression, moϋon detection, object tracking and video special effects As a background to the invention, it will be understood that digital video is a major growth area, both in home entertainment and in business Digital video is at the heart of up and coming technologies such as video conferencing and video telephony The mam problem with digital video is the amount of data needed to represent even a small number of frames Hence, processes that compress this data or improve the performance of other data compression techniques will play a crucial role m its development and successful apphcauon This mvenuon proposes a method of analysis which yields content and significance information in the form of a statisϋcal 'profile' This information can be used directly by CODECS to regulate the selection of blocks, regions or other image components contained within a video signal, or the selection of appropπate coding techniques Alternaϋvely it can be used to control a filtration process which selecuvely es noise from a video signal Within any digital video signal, there is a considerable amount of noise This is likely to be true even in high quality broadcast systems but is a particular problem in low cost soluuons such as those which might be used, for example, in a video telephone Noise significantly degrades the compression performance and the quality of the output from most video compression CODECs The method of the invention is intended to complement rather than necessarily replace existing compression, filtration, image processing and analysis techniques According to the invention, there is provided a method of analysing a digital video signal to generate a statistical profile thereof, according to which corresponding pixels in consecutive frames and/or neighbouring pixels in the same frame are examined for relative bπghtness and/or relative colour and stored as a function of the amount of change and the number of and/or pattern of changed neighbouπng pixels, thereby to produce a pixel map, and the mapped pixels are categorised into classes ranging from zero score for zero change to the highest score for the greatest change, enabling the classification, 1 e statistical profile, to be utilised during further processing of the digital video signal The method may be performed by means of dedicated hardware or by programmed computensed apparatus Further features of the invention will be apparent from the following descπption, making reference to the accompanying drawings, in which - Figure 1 illustrates application of the invention to noise filtration, Figure 2 illustrates application of the invention to video compression, and Figure 3 illustrates application of the invention to video motion detection In more detail, Figure 1 illustrates a noise filtration system based on a statistical profile and shows how it compares with other techniques From left to nght, top to bottom the images are - 2 consecutive frames from a video clip of an aircraft landing at an airport, - a difference map for the two frames showing changed pixels in black, - a threshold difference map showing pixels that have changed by more than + /- 12 grey scales, - a ϋhreshold difference map (as per the previous image) where the frames have first been pre-filtered using a median convolution, - the results of applying a statistical profile based noise reduction scheme. Figure 2 shows output from a compression application that uses a statistical profile to filter the video and select blocks for compression witii a secondary CODEC Figure 3 illustrates the use of a statistical profile as part of a motion detection/object tracking system. The profile is generated using a series of signal analyses, effectively stages. Each stage is designed to further enhance the separation of vanous signal components within the profile The order of the stages does not necessaπly need to be as descnbed below, and one or more stages may be omitted In one initial stage (stage 1) a threshold filter examines the difference between corresponding pixels in consecutive frames (or neighbouπng pixels in the same frame) If the bnghtness or colour differs by more than a threshold amount then the difference is recorded in a pixel difference map. An alternative initial or further stage (stage 2) is designed to selectively suppress noise within the signal Within the difference domain, signal and noise components tend to have different characteπstic signatures In particular signal components tend to result m coherent and connected patterns of change. So, for example, a single pixel that changes in isolation from its neighbours is less likely to be part of the signal component and more likely to be part of the noise component. A modified pixel map is created for changed pixels (or non-zero pixels in the threshold difference map created in stage 1 where the value (or score)) of each pixel is a function of the amount by which it has changed and the number of changed neighbours or pattern of changed neighbours. For example a possible scoring system might be:- if (pixel value in difference map is non-zero) and (number of non-zero neighbours in difference map is = 3)) then Pixel score — (magnitude of change/ 16) + (number non-zero neighbours in difference map -3) In this example the score will have the following characteristics :- - Zero if a pixel has not changed by more than a threshold amount. - Zero if at least 3 of the pixel's immediate neighbours have not changed by more than the threshold amount. - Larger for larger changes. - Larger if a larger number of the pixel's neighbours have changed by more than the threshold amount. The output from this stage is a pixel map in which the score of any pixel is likely to be higher if it forms pan of the signal and lower for pixels that have not changed except as a result of noise. A third stage (stage 3) seeks to amplify the scores of those pixels that are likely to be important in the definition of object features and boundaries, in particular, changing boundaries/features or the boundaries/features of moving objects. Where an object moves relative to the field of view (either because it is moving or the field of view is changing) this will result in characteristic patterns of change within the difference domain. Specifically, movement will tend to result in lines of changed pixels peφendicular to the direction of movement at positions coπesponding to the boundaries of the object and high contrast features within it (both at the old positions and the new positions). Each pixel that has changed by more than a threshold value (stage 1) and/or achieved a score in a specific range during stage 2, is examined to see if it forms part of a line of pixels (for example horizontal, vertical, diagonal or a curve) meeting the same criteria. The score given to each pixel is a function of the length of the line of which it forms a part. The scores obtained will tend to be higher for pixels that are important to the accurate definition of object boundaries or features and lower for those that have changed only as a result of random noise or those that form part of an area of uniform change (for example when the light level in a scene changes). Stages 1 - 3 provide excellent separation for many categories of signal component. However, subtle changes in colour or luminance may not be adequately separated from noise components. For example, a small change in die lighting conditions that results in a luminance shift that is less than the threshold used in stage 1 may be treated as noise. Stage 4 seeks to separate such subtle changes from pure noise by taking advantage of the non-random nature of the change. If a region, block or group of neighbouring pixels have changed only as a result of random noise then it is to be expected that the number of pixels showing an increase in luminance (or the prominence of a particular colour component) will be roughly equal to the number of pixels that have changed in the opposite direction. This contrasts with the situation where there is a systematic shift in colour or luminance. Even if the change is quite subtle (i.e. smaller than the magnitude of the noise) the balance of change will shift one way or the other. For example, if, in a block of pixels, 90% have increased in luminance, 5 % are unchanged and 5% show decreased luminance, then it is unlikely tiiat the change is the result of noise alone. The further the balance shifts in a particular direction, the stronger the indication that a non-noise signal component is present. Examining the average change for a group of pixels can also be useful in some circumstances (this is a widely used technique). However, such averaging techniques can be unduly influenced by larger changes in just one or two pixels. Stage 4 is therefore employed to further modify the pixel scores, or the scores of groups of pixels, in order to refine the profile (classification) of the video signal. Thus, as stated, stages 1 to 3, possibly also including stage 4, provide a sound basis for the separation of various signal components. In many instances this will be adequate. However, a variety of additional techniques (stage 5) can be used to further refine the profile including:- - Searching for specific patterns of linked pixels that have some higher level meaning. - Application of conventional noise suppression or feature enhancing filters to highlight or suppress specific signal components. - The patterns of change exhibited by a pixel or in a region of the picture over a longer period of time can be monitored and taken into account. - More detailed patterns of change among neighbouring pixels may be taken into account. (E.g. if a group of neighbouring pixels exhibits a similar change in luminance this has a different meaning from the situation where some neighbours have become darker whilst other have become brighter). - The absolute brightness/colour of a pixel or its relative brightness/colour compared to its neighbours may be taken into account. These or other enhancements may be appropriate under specific circumstances where further refinement of the profile is sufficiently important to outweigh the cost of the additional processing required. After combining the scores from at least some of stages 2 to 5 it is possible to categorise the pixels (stage 6). For example :- - High significance transition (e.g. an edge pixel for a moving object). - High contrast transition (e.g. a hard shadow or the body of a moving object) - Medium significance transition (e.g. subtle light level or colour changes) - Low significance transition or noise transition - Unchanged background The scores from groups of neighbouring pixels can be combined (stage 7) to give a composite score for a block or region. These regions can be categorised in much the same way as the individual pixels. Aggregation of individual pixel scores into regions in this way has the advantage of suppressing the effect of individual rogue pixels and generally gives a more reliable categorisation of the image However, if the regions are too large, there is a danger that significant but small structures may be classified A similar profile may also be generated for a still image by comparing the original image with copies of itself offset by a small distance (e g one pixel) in various different directions (e g up, down, left, right and along the diagonals) Once regions of the image (be they individual pixels or groups of pixels) have been classified according to their likely signal content or significance, this 'profile' can be used in a vanety of ways One example is noise filtration In this case, the profile is used to define regions (pixels or larger areas) that do not need to be updated because they have not changed or contain no significant 'non- noise' information Excluding these regions from the data stream has the effect of eliminating much of the noise from the signal The effectiveness of this technique is illustrated in Figure 1 of the accompanying drawings An alternative and complementary approach to the exclusion based filtering descnbed above, is to use the profile to identify regions (or pixels) that are likely to contain a high noise component and then selectively appl> other noise suppression or filtration techniques to those regions (e g averaging kernels or median filters) Another example is video compression Profiling, as previous!) described, benefits video compression systems in a number of different ways For example - - Elimination/filtration of noise from the video stream can gready reduce the amount of data that needs to be compressed (see Figure 2 of the accompanying drawings) - The profile can also be used to regulate the use of different compression algonthms to optimise the compression ratio/quality/processmg requirement equation (For example one might choose to use a less lossy or higher quality vanant of an algoπtfim to compress blocks containing object boundanes.) - The profile can be used as a basis for dynamically assigning different frame rates/update frequencies to different areas of the image Still another example is motion vector analysis The determination of motion vectors in digital video sequences is notonously computationally intensive A profile generated as previously descnbed can greatly speed this process Firstly, it enables much of the image (for example those parts that contain only noise) to be excluded from the motion vector search. Secondly, it provides a basis for pπoπtising the search In particular, motion vectors are most likely to link areas or pixels with similar scores In another example, video motion/alarm event detection, objects can be defined by searching for groups of linked pixels or blocks with charactenstic scores The objects can then be further sub-classified acordmg to properties such as size, shape or speed of movement This section provides a simplistic illustration of how the profile can be applied to the detection of motion or alarm events within a digital video sequence Traditionally, 'in video' motion/event detection has been plagued by problems ln particular 'false alarms' caused by for example - - Birds flying across the field of view - Changes in lighting - Trees rustling in the wind - Noise or flicker in the image However, a stistical profile produced as described above can provide an excellent starting point for discnminating between real events and false alarms such as those descnbed above. Figure 3 of the accompanying drawings shows a series of frames taken from a clip of motorway traffic (frames 1 , 5, 10 and 15) The white blocks coπespond to areas where motion has been detected and must satisfy all the following rules 1. They must have a composite score above a threshold value 'T\ This eliminates most of the noise and small fluctuations the image. 2. At least immediate neighbours must also satisfy rule 1 This provides a mechanism for further suppressing the effects of random noise and for discnminating against very small objects. 3 If the area of a single block is 'A' then the total area of a group of connected blocks satisfying rules 1 and 2 must be m the range (Y x A) to (Z x A) This rule allows objects with particular area and size characteristics to be selected or excluded 4 They must have satisfied rules 1 and 2 m die previous frame as well This eliminates some transient effects. The sample contains a number of possible causes of false alarms including significant noise/flicker (because the sample was digitised from a video tape rather than direct from a camera), moving vegetation, \ ibration (the shot was taken from a road bridge), and light level changes (e.g. between frames 10 and 15). Finally, just as the profile can be used to selectively remove die noise component of a signal, it can also be used to selectively target or modify other signal components. For example, in a time lapse video clip of the sun setting, the profile could be used to selectively filter out fast moving (relatively) foreground objects from the scene whilst allowing more subde and gradual changes to pass through unaltered.

Claims: Claims 1. A mediod of analysing a digital video signal to generate a statistical profile thereof, according lo which coπesponding pixels in consecutive frames and/or neighbouring pixels in the same frame are examined for relative brightness and/or relative colour and stored as a function of the amount of change and the number of and/or pattern of changed neighbouring pixels, thereby to produce a pixel map, and the mapped pixels are categorised into classes ranging from zero score for zero change to the highest score for the greatest change, enabling the classification, i.e. statistical profile, to be utilised during further processing of the digital video signal. 2. A method as claimed in claim 1, according to which, during an initial stage of analysis, coπesponding pixels in consecutive frames and/or neighbouring pixels in the same frame are examined by a threshold filter and differences in brightness and/or relative colour exceeding a threshold amount are recorded as a pixel map. 3. A method as claimed in claim 1 or claim 2, according to which, during an initial or further stage of analysis, in order to contribute to suppression of noise, pixel changes in brightness and/or relative colour are scored according to the number of and/or pattern of changed neighbouring pixels in order to produce a pixel map. 4. A met-hod as claimed in claims 2 and 3, according to which the pixel map resulting from tiie analysis stage of claim 2 is modified by the analysis stage of claim 3. 5. A method as claimed in claim 2 or claim 3 or claim 4, according to which, during a successive stage of analysis, changed pixels are examined to determine if any pixel forms part of a line (straight or curved) of changing pixels and the existing pixel map is further modified by scoring pixels as a function of the length of the line of which any changed pixel forms part. 6. A method as claimed in any of claims 2 to 5, according to which regions, blocks or groups of nighbouring pixels are examined for change in relative brightness and/or relative colour predominant in the overall region, block or group, and the pixel map is further modified accordingly. 7. A method as claimed in claim 6, according to which the number of pixels changing in relative brightness and/or relative colour, in both senses, are counted, and the number changing in one sense is compared with the number changing in the opposite sense. 8. A method as claimed in any of claims 2 to 7, according to which the pixel map is further modified by examining the average change in relative brightness or relative colour in a region, block or group of pixels. 9. A method as claimed in any of claims 2 to 8, according to which the pixels are examined for specific patterns of changed pixels other than lines and the pixel map is further modified accordingly. 10. A method as claimed in any of claims 2 to 9, according to which pixels are monitored over three or more successive frames to detect patterns of change and the pixel map is further modified accordingly. 11. A method as claimed in any of claims 2 to 10, according to which the digital video signal is passed through noise suppression or feature enhancing filters to highlight or suppress specific signal components prior to generation of the pixel map. 12. A method as claimed in any of claims 2 to 1 1 , according to which the absolute brightness or colour of pixels is taken into account in construction of the pixel map 13. A method as claimed in any of claims 1 to 12, according to which the pixel map is used to categorise pixels into at least two or more of the following classes - high significance transition, high contrast transition, medium significance transition, low significance or noise transition, and unchanged background. 14. A method as claimed in any of claims 2 to 13, according to which individual pixel scores for at least some of the pixels are aggregated into regions and the resulting pixel regions are categorised. 15. A method as claimed in any of claims 1 to 14, applied to the analysis of an original still image with offset copies of the original image. 16. A method according to any of claims 2 to 15, according to which the pixel classification is applied to noise filtration 17. A metiiod as claimed in any of claims 2 to 15, according to which the pixel classification is applied to video compression. 18 A method as claimed in any of claims 2 to 15. according to which the pixel classification is applied to motion vector analysis 19. A method as claimed in any of claims 2 to 15, according to which the pixel classification is applied to video motion or alarm event detection. 20. A method as claimed in any of claims 2 to 19, according to which the pixel classification is applied to obtain video special effects 21. Apparatus for analysing a digital video signal by die method of any of claims 1 to 20 22. Apparatus according to claim 21 , wherem the apparatus is a computerised apparatus programmed to perform the method by means of software. 23. Apparatus according to claim 21 , wherein the apparatus comprises hardware means for performing the mediod.

Inventors: WILKINS PAUL GORDON, WILKINS, PAUL, GORDON
Applicants: WILKINS PAUL GORDON, WILKINS, PAUL, GORDON
Application year: 1996
Main classification: G06T7
IPC codes: G06T7, G08B13