WO1999013646A2 - Procede et appareil de traitement du signal image - Google Patents

Procede et appareil de traitement du signal image Download PDF

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Publication number
WO1999013646A2
WO1999013646A2 PCT/SE1998/001597 SE9801597W WO9913646A2 WO 1999013646 A2 WO1999013646 A2 WO 1999013646A2 SE 9801597 W SE9801597 W SE 9801597W WO 9913646 A2 WO9913646 A2 WO 9913646A2
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coefficients
image signal
threshold function
image
predetermined
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PCT/SE1998/001597
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English (en)
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WO1999013646A3 (fr
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Lars Farm
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Limt Technology Ab
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Priority to AU91015/98A priority Critical patent/AU9101598A/en
Priority to EP98943163A priority patent/EP1013095A2/fr
Publication of WO1999013646A2 publication Critical patent/WO1999013646A2/fr
Publication of WO1999013646A3 publication Critical patent/WO1999013646A3/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/80—Details of filtering operations specially adapted for video compression, e.g. for pixel interpolation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
    • H04N19/117—Filters, e.g. for pre-processing or post-processing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/136—Incoming video signal characteristics or properties
    • H04N19/137—Motion inside a coding unit, e.g. average field, frame or block difference
    • H04N19/139—Analysis of motion vectors, e.g. their magnitude, direction, variance or reliability
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/136—Incoming video signal characteristics or properties
    • H04N19/14—Coding unit complexity, e.g. amount of activity or edge presence estimation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
    • H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/18—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a set of transform coefficients
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/186—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a colour or a chrominance component
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/48—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using compressed domain processing techniques other than decoding, e.g. modification of transform coefficients, variable length coding [VLC] data or run-length data
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/146—Data rate or code amount at the encoder output
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/146—Data rate or code amount at the encoder output
    • H04N19/152—Data rate or code amount at the encoder output by measuring the fullness of the transmission buffer

Definitions

  • the present invention relates generally to a processing method and apparatus for image signal compression, and more particularly to an adaptive filter for use in image signal compression, for example in an image signal encoding system.
  • a moving image signal or a video signal involves data in four dimensions, viz. the magnitude of a sample, the horizontal and vertical spatial positions and the time. Compression can be undertaken in any combination of the four dimensions, and in fact hybrid-coding techniques combining spatial and temporal compression are known to be the most efficient ones.
  • the so called compression factor defined as the ratio between the source data rate and the output data rate, is a commonly used measure of the efficiency of a performed compression.
  • the underlying aim of image compression is to remove redundancy from an image signal in order to represent the image signal with a minimum of data.
  • Processing of an image signal is normally carried out in order to express the image information in the best way for identifying and removing redundant data in the actual compression stage.
  • Examples of compression methods known in state of the art are the standard compression schemes ISO JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group). JPEG and MPEG are examples of methods that comprise what is called block based image compression, where the image signal is processed block by block.
  • a block is in this context a part of an image consisting of e.g. 8x8 pixels.
  • the blocks are transformed from the spatial domain to a frequency domain, in most cases by means of discrete cosine transform (DCT), but other frequency domain transforms are also conceivable.
  • DCT discrete cosine transform
  • frequency components are usually quantized and scaled to small numbers, and coefficients are then stored or transmitted as small numbers together with a separate scaling factor.
  • This kind of encoding technique further employ what is known as run-length encoding. In run-length encoding, information about the number of subsequent coefficients having the same value is stored or transmitted instead of the coefficient values themselves.
  • typical image material often give rise to a number of signal transform coefficients having a zero amplitude, and therefore subsequent or trailing zeroes are particularly favourable for these compression schemes.
  • the decoder receiving as an input the compressed image signal in the form of transformed and quantized blocks, applies the scaling factor to the small transform coefficients and reproduces the indicated number of zero coefficients, and then transforms each block back to the spatial domain in order to recreate the non- compressed image signal.
  • the scaling of image signal transform coefficients is one of the major contributors to data compression in this kind of methods.
  • the quantization scale factor is a very important instrument for controlling the compression factor, which in particular is required in for instance fixed bit rate coding schemes.
  • the scale factor can also be varied in order to adapt to the actual compression need, whereby lowering the scale factor improves the image quality and can be allowed when there is sufficient space for data. Raising the scale factor entails a better compression gain, however, to the price of decreased image quality.
  • the scaling of transform coefficients can cause visible artefacts, for instance in the form of quantization noise such as blocking. The latter appearing when the edges of neighbouring transform blocks do not match and the transform block boundaries become visible as a mosaic structure. The result is a recreated image signal representing images of unsatisfactory poor quality.
  • the US Patent No. 5,309,231 to Takahiro Hamada et al shows an example of a method for adaptive quantization of a block of coefficients for use in a moving or still- picture encoder.
  • This quantization method operates on two levels, on one hand on picture level and on the other hand on a block level.
  • the adaptive algorithm contains the following measures: A. For each picture L: al- pre-quantizing by conventional means; a2- computing a complexity measure for the picture L, a provisional code length M' per block and an average provisional code length L' per block; a3- determining a pre-designed number of coefficients to transmit per block dependent on L' as a function of M' such that L' is small when M' is large and vice versa.
  • the threshold is fixed, not adaptive, and is devised for computing a complexity measure by counting the number of significant coefficients. It is, furthermore, a fairly complex method with implementation and processing requirements, which are more severe than e.g. MPEG. The method also requires much general control data to be transmitted for each block or macro-block. This gives inefficient compression and excludes the method for use in MPEG and similar schemes.
  • the US patent No. 5,301,242 to Gonzales and Viscito shows in its turn a method for adaptive scaling of quantized coefficients in an MPEG or similar moving or still- picture encoders.
  • the scaling is controlled by a scale factor that is part of the MPEG standard, which requires scaling or quantized coefficients without prescribing or even describing how to compute such a scale factor.
  • the adaptive quantization according to this piece of prior art can be summed up in the following steps; A. Choosing two quantizer matrices that can be used through the entire stream in accordance with MPEG-rules; B. For each macro-block, i.e.
  • this piece of prior art is directed to adaptive scaling of quantized data.
  • the scaling adapts to the "energy" of a block and the filling rate of an output buffer. It is also suitable for MPEG, however, some of the previously mentioned problems related to quantization remain.
  • the object of the invention and the problem to be solved is thus to provide a processing method and an apparatus for use in an image signal coding system capable of improving the balance between required compression factor and image quality.
  • a particular aspect of the problem to be solved is the capability of reducing or eliminating the detrimental effects of quantization and scaling of image signal transform coefficients normally found in compression processes involving image data represented in a frequency domain.
  • a further aspect of the problem to be solved is to increase the compression factor with maintained subjective image quality or, conversely, to improve the image quality for a given compression factor in other words, to increase image quality per coded bit ratio.
  • Yet other aspects of the problems to be solved is to achieve the objects in existing decoders or encoders without significant changes to these existing devices, in particular without increased running time.
  • a method and an apparatus which in essence is an adaptive filer operating to pre-process an image signal in order to enhance the effect of a subsequent coding step by increasing the number of zeroes in pixel values or length of zero clusters without impairing image quality.
  • the adaptive filter in accordance with the invention is based on the occurrence of different conditions or cases during compression of a stream of image data, and measures are taken in order to utilize or meet the requirements of the specific conditions in each case. So, in a first predetermined condition priority may be given to letting through pixel coefficients contributing to image quality, whereas in a second pre-determined condition priority instead may be given to filter out pixel coefficients giving a significant contribution to the output bit rate.
  • a first filtering step is selected in a first case, i.e. in a first pre-determined condition, wherein the compression factor and the resulting image quality is smoothly balanced or the image quality even given priority, preferably when there is a moderate requirement on the instantaneous or the overall compression factor. Normally, an extra margin is thereby created for compression needs later in the image information signal.
  • a second filtering step is selected in a second case, i.e.
  • the image quality is momentarily sacrificed or disregarded, for example when the image quality has gone below a predetermined minimum quality level due to a strained bit budget or when image details are undetectable by the human vision system.
  • the bit budget balance is recovered faster than with prior art bit budget control and image quality can thereafter be given priority again.
  • the threshold coefficients may depend on a selection of, for example the following criteria: - output buffer status, i.e. currently available buffer space desired output bit rate; and image properties such as: local image complexity motion - brightness colour edges inter/intra block coding, etc.
  • the inventive processing is carried out by selectively filtering the image signal before it is introduced into the actual compression or coding stage and the normal bit rate control mechanisms of an image signal coding system.
  • the mentioned steps can be carried out separately, i.e. in different cases, or in sequence, depending on the predetermined criteria. Different filtering control parameters are used in different embodiments, and for example the resulting image quality can be indirectly controlled by means of a compression control parameter upon which image quality depends.
  • the invention seeks to make it possible to avoid unnecessary quantization and scaling by selectively filtering a frequency domain representation of an image block in order to reduce the amount of information needed to code or compress.
  • a coding system may at most produce a predetermined maximum allowed bit rate for an output compressed image signal and the system has to work to stay within a given bit rate budget.
  • image processing coding systems has to take account not only of the possible compression rate but also of the resulting image quality and, as explained above, there is generally a trade off between image quality and the bit rate of a compressed image signal. The invention is thus based on the inventors realisation that the image quality can be given priority under certain conditions, e.g.
  • bit rate budget when there is a sufficient margin in the bit rate budget, and conversely temporarily disregarded under other conditions, e.g. when the bit rate budget is strained.
  • bit rate budget is strained.
  • block based coding systems occasional blocks of poor quality are hard to detect due to the fact that they are spatially and temporally limited. The image quality of occasional blocks can therefore be sacrificed in order to balance the bit rate budget.
  • An exemplifying embodiment of the invention provides a processing method for use in an image signal coding system, wherein an image comprises digitized image data in a plurality of pixels, wherein the image data is transformed into a frequency domain and wherein the system optionally includes quantization means for quantizing image data in accordance with a predetermined required compression factor.
  • a set of pixels, commonly called a block, which constitutes a part of an image, is received as an input.
  • the image data of the pixels are preferably in the form of coefficients in a frequency domain, where the pixels for example may be transformed by means of a discrete cosine transform.
  • the bit rate budget is controlled by means of a bit rate control parameter, preferably the quantization scale factor.
  • the current quantization scale factor is an indirect measure of the compression rate of a current block, of the compression rate of precedent blocks, of deviation from a desired compression rate and image quality of a current block.
  • Many compression apparatuses can in fact be looked upon as feedback control loops, where the bit rate is controlled e.g. as in state of the art by varying the quantization scale factor and the resulting bit rate is fed back to the compression apparatus itself.
  • the invention is applied in the bit rate control loop and operates to reduce the degree of quantization needed in subsequent blocks, whereby quantizing in subsequent blocks can be carried out with a smaller quantizing step size and hence subsequent data can be coded with better precision.
  • the block of pixels, or rather the frequency transform coefficients of the pixels are selectively filtered in order to reduce the amount of information to be coded.
  • the filtering is carried out in the following manner.
  • the coefficients are filtered in accordance with a first coefficient threshold function.
  • the first coefficient threshold function is preferably dependent on coefficient frequency and the state of the encoder.
  • the coefficient frequency dependency is for the first threshold function devised such that coefficients contributing significantly to image quality are prioritised to have an intact amplitude value and the rest of the coefficients are replaced with a predetermined run-length amplitude value, preferably amounting to zero. In principle, this is carried out such that transform coefficients having a low amplitude are removed and replaced by e.g.
  • the compression factor is increased because of an increased number of zeroes in the coefficients, a correspondingly decreased number of non-zero coefficients and the fact that adjacent zeroes will be run-length encoded in this embodiment.
  • Large transform coefficients i.e. coefficients of any frequency having large amplitude, are preferably kept intact since they contribute most significantly to the image quality.
  • low frequency transform coefficients are kept intact since their absence contribute significantly to the visibility of certain coding artefacts, particularly the phenomenon known as blocking.
  • the first step can be used preventively to create a margin for the bit rate budget without compromising with image quality, and is mostly applied when the bit rate is balanced.
  • the above mentioned fact that the first step entails remaining data to be coded with better precision, in its turn often even results in an improved image quality.
  • the condition parameter e.g. the instantaneous or current bit rate is outside said predetermined deviation range
  • the coefficients are instead filtered in accordance with a coefficient threshold function dependent e.g. on the quantization scale factor or on another parameter or criterion such that the bit rate deviation is decreased. This is in one embodiment in principle carried out by replacing a larger number of coefficients with the predetermined run- length amplitude value.
  • the coefficient threshold function is in this case dynamically adapted to the state of the encoder and to the characteristics of each block or group of blocks.
  • the second step is primarily carried out as a reactive measure in cases when image quality can be relinquished, for example when image quality already has deteriorated due to far driven compression. This second step can also be performed preventively, i.e. to prepare for future compression needs, for example when image quality cannot be appreciated by the human vision system.
  • the above described conditional steps are performed such that the first step is always carried out as a minimum measure for each block regardless of the condition parameter, e.g. the current bit rate.
  • the second step is thereafter carried out if the condition for that said second step is fulfilled.
  • the status of the condition parameter e.g. the current bit rate or a parameter reflecting or indicating the image quality, is first checked and thereafter the appropriate step is carried out.
  • a third embodiment is devised such that the first step or the second steps, or a sequence of said first and second steps are performed dependent on selected condition parameters.
  • the luminance value can be used to trig the prioritising of compression over image quality, since very dark and very bright image areas conceal details and colour to the human eye. Motion within the block also conceals detail and such blocks can also be further compressed. When temporal or spatial masking is used, image quality for masked blocks may be less critical.
  • the characteristics of the coefficient threshold function may also be dependent on whether a processed block is an intra-coded block having coefficients in absolute values, or a non-intra-coded or predicted block involving motion estimation and therefore having coefficients expressing a difference between blocks. To sum-up, a mild filtering is applied e.g.
  • the mild filtering allows comparatively much information to be coded.
  • a harsher filtering is applied e.g. for blocks where the bit rate budget is strained or exceeded, and/or for blocks where the harsher treatment is judged to pass unnoticed regarding the image quality.
  • the harsher treatment of coefficients can be applied preventively, when the treatment is expected to be harmless to image quality, and/or reactively when the compression has been found to be insufficient and image is already lost and therefore momentarily unimportant.
  • the inventive procedure is advantageously applied in addition to and before quantization in encoding processes involving a frequency domain transform and quantization. A particular advantage is that a decoder needs no information about the inventive processing.
  • Fig 1 shows a block diagram of an inventive processing apparatus employed in an image signal encoding system
  • FIG 2 shows a more detailed block diagram of an embodiment of the processing apparatus according to the invention employed in one variety of an image signal encoding system
  • FIG 3 shows an example of a coefficient threshold function used in the invention.
  • FIG. 1 there is shown an embodiment of an apparatus according to the invention in the shape of a selective filtering means 2 integrated in an image signal coding system 1, comprising an image signal transforming means 4 and an output coding stage 6.
  • An input of the filtering means 2 is communicatively coupled to the transformer 4, which is devised for transforming an image signal from one domain, e.g. a spatial domain, to a frequency domain.
  • An output of said filtering means 2 is coupled to the output coding stage 6 where the actual coding is carried out.
  • the transformer takes as an input an image signal 3a comprising digitized image data represented as pixel values and outputs a frequency domain image signal 8 wherein the pixel values are represented in the form of coefficients in the current frequency domain.
  • the output coding stage 6 produces and outputs a coded or compressed image signal 12a, which thereafter e.g. can be stored in compressed form or transmitted as a compressed bit stream in a data transmission channel (not shown).
  • the selective filtering means preferably takes as an input one or a number of control parameters or condition parameters, in FIG. 1 exemplified by a feedback coded image signal 12b, a coder status signal 14 and/or the input spatial domain image signal 3b.
  • the control parameters are used to select filtering rules dependent on the state of the encoder, the frequency content of the transformed image signal 8 and perhaps also the image data content of the input image signal 3b or other selected parameters.
  • the filtering rules are for example expressed as one or more coefficient threshold functions.
  • Fig 2 shows a more detailed embodiment of the invention applied in a per se known image signal coding system, for example an MPEG encoder.
  • An input image signal 3 a is received by a coding system input stage 14, which may comprise means for motion estimation, noise reduction means and the like.
  • the image signal 3a is transformed into the frequency domain by means of a discrete cosine transformer (DCT) 16 and the resulting frequency domain image signal 22 is thereafter input into an adaptable filter 34 comprised in the selective filtering means 2.
  • DCT discrete cosine transformer
  • a filtered signal 24 is then transmitted to a quantizer 18 for quantizing or scaling before the proper output coding stage 20, wherein the coding is performed.
  • the filter applies selectable filtering rules, e.g.
  • a filtering rule selecting means 32 in the form of a threshold function, under the control of a filtering rule selecting means 32, here in the shape of threshold function selector 32.
  • the filtering rule selector 32 is in a preferred embodiment devised to select a stored threshold function or to dynamically compute an appropriate threshold function dependent on predetermined parameters.
  • the filtering rule selector is in its turn controlled by means of an evaluating means 26 devised to evaluate filtering conditions with the aid of one or more condition parameters.
  • a condition parameter indicating the coder system status e.g. the quantization scale factor
  • a preferred status parameter to monitor is the output buffer status, i.e.
  • the quantization scale factor is a conveniently available parameter giving an indirect measure of the buffer status.
  • a DCT signal 30, the input image signal 3b and/or a control or status signal 5 from the input stage may as well be communicated to the evaluator 26.
  • the filtering rule may also be selected by means of a control signal or command input actuated by a user and received by the evaluation means or the filtering rule selecting means.
  • One embodiment of the inventive method as employed in the apparatus of FIG 2 comprises the steps of: -Receiving as input a set of pixels preferably constitutes a part of an image, the image data of the pixels being in the form of coefficients in a frequency domain.
  • the set of pixels is a block of 8x8 pixels.
  • the normal quantization and coding procedure comprising bit rate control etc. is then performed on the filtered image signal.
  • the manner of determining suitable criteria for the selection of threshold surfaces as well as determining actual threshold values is investigative and experimental. Today, there is no known way to optimize the selection criteria nor the threshold values in a computational manner. The man skilled in the art implementing the invention instead has to evaluate subjectively the consequences of different criteria and threshold values on a trial and error basis.
  • the important framework of the invention is that subjectively experienced image quality in certain situations may be sacrificed in order to avoid overflow, and conversely a high degree of available output buffer space may be utilized for perhaps bulky, but image quality contributing coefficients.
  • heuristic rules about an output buffer status and heuristic rules primarily related to subjective visual image content have been used to adapt a threshold matrix.
  • the adaptation as described below is set out in terms of level and slope of a threshold surface defined by the threshold matrix.
  • the individual values of the threshold matrix can also be altered dynamically in accordance with selected rules.
  • Image content properties such as smoothness, texture, edges, brightness, motion, colour, etc are detected by other means than the filter.
  • an MPEG encoder performs motion estimation and thereby detects the motion of a block, it detects the luminance and the chrominance of a block and yet other properties can be deducted from visual inspection of the raw image.
  • the deductible procedure is not a central feature of the invention itself, however the results of it are used in order to set criteria and threshold values.
  • the following table is a numerical example of a threshold matrix suitable for appliance in intrablock coding in a stable condition.
  • the threshold level can be varied by means of a parameter devised for the whole matrix.
  • the filter threshold is perceived as a surface having threshold coefficients Tij.
  • the filter threshold is perceived as a surface having threshold coefficients Tij.
  • Criteria a2, a3, a4 and a7 are examples of image properties entailing barely detectable image details, where a harsher filtering may be applied.
  • FIG 3 shows in a 3D-perspective an example of a coefficient threshold function for a block of 8x8 pixels in the frequency domain.
  • the coefficient positions 0-7 in the block are indicated in the horizontal area, the absolute value of the coefficient amplitude is indicated along the vertical axis and the coefficient threshold value is indicated as a function of the coefficient frequency and coefficient amplitude.
  • the lowest frequency is situated in the 0-0 position, whereas the highest frequency is situated in the 7-7 position.
  • the coefficient threshold function is preferably a three-dimensional surface and coefficients having an amplitude below the threshold surface are filtered out in accordance with the invention.
  • the level characteristics of the threshold surface are devised in accordance with selected criteria as discussed above.
  • inventive method can be realised by means of hardware as well as by means of a computer program executed on a computer comprising a processor, storage means and input/output devices. Any realisation comprises functional means devised to carry out the different steps and functions of the invention as described herein.
  • a computer program can further be embodied in a computer program product comprising a recording medium, and means, recorded on the recorded medium, for directing a computer to perform the functions and steps of the invention.

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Abstract

L'invention concerne un procédé et un appareil de traitement destinés à être utilisés dans un système de codage du signal image, ce système permettant de coder un signal image d'entrée pour le transformer en un signal image de sortie comprimé, ce signal image comprenant des données images numériques correspondant à une multiplicité de pixels. On filtre de manière sélective ces données images, qui sont de préférence dans le domaine fréquentiel, de façon à attribuer une priorité de qualité d'image applicable audit signal de sortie comprimé. Dans un premier cas, on se sert des coefficients de pixels participant de manière notable à la qualité d'image, et on élimine par filtrage le reste de ces coefficients de pixels. Dans d'autres cas, on filtre les données images de façon à attribuer à un facteur de compression maximum réalisable applicable à l'ensemble desdits coefficients de pixels une priorité de qualité d'image applicable au signal de sortie comprimé. On élimine également par filtrage les coefficients de pixels participant de manière notable à la qualité d'image.
PCT/SE1998/001597 1997-09-08 1998-09-08 Procede et appareil de traitement du signal image WO1999013646A2 (fr)

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Application Number Priority Date Filing Date Title
AU91015/98A AU9101598A (en) 1997-09-08 1998-09-08 Image signal processing method and apparatus
EP98943163A EP1013095A2 (fr) 1997-09-08 1998-09-08 Procede et appareil de traitement du signal image

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SE9703233A SE512832C2 (sv) 1997-09-08 1997-09-08 Anordning och förfarande för bearbetning av en bildsignal
SE9703233-8 1997-09-08

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WO2008004521A1 (fr) 2006-07-03 2008-01-10 Nippon Telegraph And Telephone Corporation Procédé et dispositif de traitement d'image, programme de traitement d'image et support d'enregistrement contenant le programme
EP2096869A1 (fr) * 2006-12-28 2009-09-02 Nippon Telegraph and Telephone Corporation Procédé de traitement vidéo et dispositif, programme de traitement vidéo et support de stockage contenant le programme

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Cited By (7)

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Publication number Priority date Publication date Assignee Title
WO2008004521A1 (fr) 2006-07-03 2008-01-10 Nippon Telegraph And Telephone Corporation Procédé et dispositif de traitement d'image, programme de traitement d'image et support d'enregistrement contenant le programme
EP2037406A1 (fr) * 2006-07-03 2009-03-18 Nippon Telegraph and Telephone Corporation Procédé et dispositif de traitement d'image, programme de traitement d'image et support d'enregistrement contenant le programme
EP2037406A4 (fr) * 2006-07-03 2009-12-02 Nippon Telegraph & Telephone Procédé et dispositif de traitement d'image, programme de traitement d'image et support d'enregistrement contenant le programme
US8611434B2 (en) 2006-07-03 2013-12-17 Nippon Telegraph And Telephone Corporation Image processing method and apparatus, image processing program, and storage medium which stores the program
EP2096869A1 (fr) * 2006-12-28 2009-09-02 Nippon Telegraph and Telephone Corporation Procédé de traitement vidéo et dispositif, programme de traitement vidéo et support de stockage contenant le programme
EP2096869A4 (fr) * 2006-12-28 2012-02-29 Nippon Telegraph & Telephone Procédé de traitement vidéo et dispositif, programme de traitement vidéo et support de stockage contenant le programme
US8467460B2 (en) 2006-12-28 2013-06-18 Nippon Telegraph And Telephone Corporation Video processing method and apparatus, video processing program, and storage medium which stores the program

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SE9703233L (sv) 1999-03-09
SE9703233D0 (sv) 1997-09-08
SE512832C2 (sv) 2000-05-22
AU9101598A (en) 1999-03-29
WO1999013646A3 (fr) 1999-05-27
EP1013095A2 (fr) 2000-06-28

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