Classifications

• • 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
  • H04N19/124—Quantisation
• • 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
  • H04N19/40—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using video transcoding, i.e. partial or full decoding of a coded input stream followed by re-encoding of the decoded output stream

Definitions

• the present invention pertains generally to the field of video/image compression, and in particular, the invention relates to systems and methods for reducing the quantization error introduced in cascaded compression of digital video and images.
• the videophone, digital television, teleconferencing, and the information highway are just a few of the elements of the emerging digital age.
• Developments in the processing of digital images and video have aided the progression into the digital age.
• digital image compression methods have played a key role in this evolution.
• Image compression reduces the amount of data required to represent a digital image. For example, color, grey scale, or binary images may be compressed and then decompressed to yield an accurate representation of the original image.
• Image compression is usually performed before storage or transmission of the data. This allows for vast amounts of information to be stored in an economical manner and/or transferred quickly.
• image compression is usually a two-way process involving compression and decompression. These processes may not be symmetrical, i.e. the time taken and/or computing power for one process may differ from the other given the type of compression algorithm used.
• Lossy compression techniques include sample subsampling, differential pulse coding modulation (DPCM), and quantization of discrete cosine transforms (DCT) coefficients.
• DPCM differential pulse coding modulation
• DCT discrete cosine transforms
• Lossless compression retains all of the data of the original image, i.e., essentially a completely reversible coding process.
• Lossless compression techniques include variable-length coding (VLC) and run-length coding (RLC).
• the compression ratio is typically defined as being the ratio between the data content to be compressed and the data that results after compression.
• the Lossy compression methods can provide compression ratios over 100:1.
• the Lossless compression methods generally achieving ratios of approximately 3 : 1.
• the trade-off being that, in general, as the lossy compression ratio increases, the degradation of the image also increases.
• the compression ratio may be achieved by one stage of compression or multiple cascaded stages of compression.
• An image or video source undergoes cascaded compression when the input source signal is subjected to multiple stages of compression in a serial manner.
• the source signal image or video
• this compressed data is then further subjected to a second or more level(s) of compression to achieve a higher compression ratio.
• the higher compression may be required for efficient storage or bandwidth restricted transmission.
• JPEG Joint Photographic Experts Group
• MPEG Motion Picture Experts Group
• JPEG Joint Photographic Experts Group
• MPEG Motion Picture Experts Group
• the JPEG standard is intended for compression of color or grey-scale images of natural real-world scenes. While the JPEG standard includes both a lossless and lossy mode, it is usually used in lossy mode to achieve a greater compression ratio.
• an image is transformed to the frequency domain using a discrete cosine transform (DCT).
• DCT discrete cosine transform
• DPCM differential pulse code modulation
• Huffman coded the adjustable nature of JPEG compression allows for variable compression ratios and fine-tuning the algorithm for a particular application's requirements.
• MPEG- 1 and MPEG-2 are typically used for low-resolution image sequences and higher-resolution sequences respectively.
• MPEG-4 focuses on unified audio-visual objects and scenes rather than frames.
• MPEG-7 aids in the location of audio-visual content.
• the above compression techniques utilize a DCT transformation followed by a quantization of the DCT coefficients and a variable length coding to achieve data compression.
• the quantization of the DCT coefficients makes these compression techniques lossy.
• a lossy compression scheme is one in which the uncompressed data is not an exact replica of the original data.
• each stage of compression in the cascaded compression is lossy in nature.
• performing the multiple cascaded compression also introduces additional loss.
• Fig. 1 shows two lossy compression scenarios (a) and (b).
• source data 10 is compressed to a ratio of 20:1 by compression system 11.
• scenario (b) the source data 10 first undergoes a first stage of compression of 10:1, followed by a second stage of 2:1 compression by cascaded compression system 12.
• scenario (b) the second compression stage does not have access to the original source data 10, but only the compressed signal obtained as the output of the first compression stage.
• both of the scenarios (a) and (b) achieve the same 20:1 compression.
• the mean square error (MSE) introduced in scenario (b) will be always greater than or equal to scenario (a) because of the cascaded compression. This additional error is due in part to the selection of quantizer values in the second or higher level compression stages.
• MSE mean square error
• One preferred embodiment relates to reducing the quantization error introduced within the framework of JPEG and MPEG compression schemes.
• One aspect of the invention relates to a method for a cascaded compression system including the steps of determining an expected quantization error introduced by a second or higher stage of the cascaded compression system and comparing the expected quantization error of at least two quantizers for the second or higher stage. The method also includes the step of selecting one of the quantizers in accordance with a result of the comparison to minimize the expected quantization error for the cascaded compression system.
• a probability distribution function is used to determine the expected quantization error.
• Another aspect of the invention relates to a memory medium and an apparatus for carrying out the above method.
• Fig. 1 is a schematic of a non-cascaded compression system and a cascaded compression system.
• Fig. 2 is a diagram of quantizers in a cascaded compression system.
• Fig. 3 is a diagram shown quantization error in a cascaded compression system.
• Fig. 4 is a block diagram of an exemplary computer system in accordance with one aspect of the invention.
• Figure 2(a) shows the reconstruction points and decision boundaries for a uniform quantizer with step size of Q .
• the reconstruction points are indicated by dark circles and the decision boundaries by short vertical lines.
• the nth reconstruction point is located at Q" ⁇ (not shown) and the decision boundaries on either side of Q n ⁇ are located at D ⁇ ⁇ and ⁇ n (not shown).
• the decision boundaries lie approximately halfway between two successive reconstruction points.
• the reconstruction points lie at multiples of Q . Any value of an input signal source falling in between two decision boundaries is quantized to the reconstruction point lying in between the two decision boundaries.
• the quantizer Q is used in the first stage (a) of cascaded compression and quantizer Q 2 is used in the second stage (b) of compression.
• the quantizer Q 2 is used for a single stage of compression.
• the step size of quantizer Q 2 is shown to be greater than quantizer Q ⁇ It is noted that the larger the quantizer step size, the higher the compression ratio that can be achieved, albeit at the expense of introducing more loss.
• uniform quantizers are used such as those used in MPEG I-frames and JPEG compression schemes. However, other step sizes and non-uniform quantizers may be used.
• x denotes the value of the input source signal that is to be quantized. If x falls in the range of [Q° 2 , D° 2 ), then with the single stage quantizer (in this case using only stage (b)), this value will be quantized to Q° 2 . It is noted that the symbol "[" indicates the value is included in the range and the symbol ")" indicates that the value is not included in the range. In the case of two stage quantizer (both (a) and (b)), the output of the first stage quantizer is Q° ⁇ if x falls in the range of [0,D° ⁇ ), and Q 1 1 if it falls in the range of [D° 1 ,D° 2 ).
• stage (a) The output from stage (a) is then passed through the second quantizer stage (b), which quantizers Q° ⁇ to Q° 2 , and Q ⁇ to Q' 2 . Therefore, with cascaded compression, x values in the range of [0, D° ⁇ ) are quantized to Q° 2 and x values in the range [D° ⁇ , D° 2 ) are quantized to Q' 2 Thus the values of x in the range [D° ⁇ , D° 2 ) are (incorrectly) quantized with larger mean square error using cascaded compression as compared to a single stage quantizer.
• the value ranges of x that would be incorrectly quantized must first be determined, given the quantizers Qi and Q 2 .
• a particular decision boundary on the Q 2 quantizer at D n 2 is considered.
• the two closest decision boundaries of the quantizer O one being larger (D m ⁇ ) and one being smaller (D m" ⁇ ) than D n 2 are located as shown in Figs. 3(a) and 3(b).
• the following cases are considered:
• the single stage Q 2 quantizer will quantizer x to Q n 2 .
• the Qi quantizer will quantize x to Q" ⁇ and the following Q 2 quantizer will quantize it to Q (n+ thereby introducing additional quantization error.
• the probability distribution of black and white pixels can be different. For example, in an all white context, the probability of coding a white pixel will be much greater than that of coding a black pixel.
• the expected quantization error introduced by the cascaded quantization using the quantizers Qj and Q can be computed as follows:
• the symbol ⁇ represents the set containing all ranges of values of x that are incorrectly quantized as determined above.
• quantization error in the above equation is used to select of appropriate quantizers in the second stage of cascaded computing. For example, if the quantizer Qi is used in the first stage and assuming that there are two possible quantizers Q 2 and Q 2 ' for the second stage, then the quantization errors can be computed for both these quantizers. These two possible quantizers are test quantizers. The minimum quantization error value is used to decide the most appropriate selection of the quantizer, i.e., Q 2 or Q 2 '.
• the quantizers Q 2 is expected to offer a bit rate r (the larger the rate, the lower the compression) with quantization error E(Q 2 )+E(Q 1> Q 2 ) and the quantizers Q 2 ' is expected to offer a rate of r' with a quantization error of E(Q 2 ') + E(Q ⁇ ,Q 2 ') then the ratio of the rate to the quantization error can be used as a measure in the selection of the quantizer.
• E(Q 2 ) and E(Q 2 ') are the quantization error inherently generated by the quantizer and is not related to additional error caused by the cascaded quantization.
• quantizers other than Q or Q 2 ' discussed immediately above may be used as a starting point.
• quantizers are based in part on the overall compression ratio that is desired. Also some trial and error may be used in selecting the initial quantizers. The quantization error for several quantizers may be computed, as discussed above, and the most appropriate quantizer is then selected.
• one embodiment of the present invention relates to applications using the JPEG and MPEG compression schemes. Both of these compression schemes divide the input source data spatially into contiguous blocks of size 8x8 which are subjected to a DCT transformation resulting in 64 DCT coefficients. This is followed by a quantization of the DCT coefficients. The DC coefficient is differentially coded. The 63 remaining AC coefficients are coded by specifying the run length of zero coefficients followed by the coding of the following non-zero coefficient's value.
• the entries of a quantization table determine the quantizer used for different DCT coefficients. Different quantization tables can be used for different bands (e.g., luminance and chrominance), but the quantization tables are fixed for a single band.
• knowledge of the probability distribution f(x) for each DCT coefficient is necessary.
• the distribution of the AC DCT coefficients follow a Laplacian distribution. It is noted that the parameter associated with the Laplacian distribution being different for different DCT coefficients. This parameter may be estimated or a different distribution, such as Rayleigh or Gaussian, can be obtained from the available compressed data.
• a Laplacian distribution may be used for both the luminance and chrominance channels of the DCT encoded image and MPEG error terms.
• the MPEG compression scheme uses DCT to encode error terms as well as picture information.
• the error terms are obtained from the MPEG motion compensation algorithm.
• An error term is obtained by subtracting an image block from a block on another picture in the sequence and applying a DCT to the difference. The allows the picture to be encoded using fewer bits if there are only a small number of changes in the images.
• a preferred embodiment focuses on I- (intracoded) frames in the MPEG format. I-frames are composed of intrablocks only without reference to other pictures. These frames can serve as random access points in the sequence.
• MPEG frame types such as P- (predictive coded) and B- (bidirectionally interpolated) frame may also be used.
• a quantizer value is decided by a quantizer_scale and a quantization table. Different quantization tables can be used for chrominance and luminance. The quantization tables are fixed for each frame, but the quantizer_scale can be changed for each macroblock.
• the quantizer selection methods discussed above are used to select the quantizer_scale for each MPEG frame macroblock. It is noted, however, that the quantizer_scale cannot be changed for each DCT coefficient. This value is fixed for the whole macroblock.
• FIG. 4 shows a video/image processing system 20 in which the present invention may be implemented.
• the system 20 may represent a television, a set-top box, a desktop, laptop or palmtop computer, a personal digital assistant (PDA), a video/image storage device such as a video cassette recorder (VCR), a digital video recorder (DVR). a TiVO device, etc., as well as portions or combinations of these and other devices.
• the system 20 includes one or more video/image sources 22, one or more input/output devices 24, a processor 25 and a memory 26.
• the video/image source(s) 22 may represent, e.g., a television receiver, a VCR or other video/image storage device.
• the source(s) 22 may alternatively represent one or more network connections for receiving video/images from a server or servers over, e.g., a global computer communications network such as the Internet, a wide area network, a metropolitan area network, a local area network, a terrestrial broadcast system, a cable network, a satellite network, a wireless network, or a telephone network, as well as portions or combinations of these and other types of networks.
• the input/output devices 24, processor 25 and memory 26 communicate over a communication medium 27.
• the communication medium 27 may represent, e.g., a bus, a communication network, one or more internal connections of a circuit, circuit card or other device, as well as portions and combinations of these and other communication media.
• Input video/images from the source(s) 22 is processed in accordance with one or more software programs stored in memory 26 and executed by processor 25 in order to generate output video/images which is supplied to a display device 28 such as a television display, a computer monitor, etc.
• the computation of the expected quantization error due to the cascaded compression and selection of appropriate quantizers is implemented by computer readable code executed by the system 20.
• the code may be stored in the memory 26 or read/downloaded from a memory medium such as a CD-ROM or floppy disk.
• hardware circuitry may be used in place of, or in combination with, software instructions to implement the invention.
• system 20 as shown in FIG. 4 is by way of example only. Those skilled in the art will recognize that the invention can be implemented using a wide variety of alternative system configurations. While the present invention has been described above in terms of specific embodiments, it is to be understood that the invention is not intended to be confined or limited to the embodiments disclosed herein. For example, the invention is not limited to any specific compression scheme, frame type or probability distribution. On the contrary, the present invention is intended to cover various structures and modifications thereof included within the spirit and scope of the appended claims.

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• Signal Processing (AREA)
• Compression, Expansion, Code Conversion, And Decoders (AREA)
• Compression Or Coding Systems Of Tv Signals (AREA)
• Complex Calculations (AREA)

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• 2000
  • 2000-10-13 JP JP2001534882A patent/JP2003513563A/ja not_active Withdrawn
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  • 2000-10-13 KR KR1020017008507A patent/KR100744442B1/ko not_active IP Right Cessation
  • 2000-10-13 WO PCT/EP2000/010158 patent/WO2001033860A1/en active Application Filing
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