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1. WO1996002895 - METHOD AND APPARATUS FOR COMPRESSING IMAGES

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

WHAT IS CLAIMED IS:
1. An encoder that compresses digital images, comprising:
a first data compressor that receives an input digital image comprising a plurality of pixels, each pixel having at least one component representing an intensity level, said pixels of said input digital image being represented by a first plurality of data bytes, said first data compressor outputting decimated data bytes, said decimated data bytes comprising a second plurality of data bytes, said second plurality of data bytes being less in number than said first plurality of data bytes, said second plurality of data bytes representing said plurality of pixels;
a second data compressor that receives data corresponding to said second plurality of data bytes, said second data compressor outputting further decimated data bytes, said further decimated data bytes comprising a third plurality of data bytes that represent a compressed coarse quality digital image;
a residual calculator that receives said first plurality of data bytes and said second plurality of data bytes, said residual calculator expanding said third plurality of data bytes to a fourth plurality of data bytes having a same number of data bytes as said second plurality bytes, said residual calculating a difference between said second plurality of data bytes and said fourth plurality of data bytes, said difference comprising a plurality of residual data bytes; and
a compressor that compresses said plurality of residual data bytes to generate compressed residual data bytes, said encoder outputting said third plurality of data bytes and said compressed residual* data bytes for storage as a compressed image, said third plurality of data bytes representing a coarse quality digital image, said compressed residual data bytes expandable to said plurality of residual data bytes and combinable with said third plurality of data bytes to convert said third plurality of data bytes to said second plurality of data bytes representing a higher quality image.

2. A decoder that receives compressed data input stream representing a digital image, said decoder outputting expanded data representing a reconstructed digital image, said decoder comprising:
a decompressor that receives a first portion of said compressed data input stream, said decompressor expanding said first portion of said compressed data stream to first expanded data and outputting said first expanded data as a first output data stream to be displayed as a coarse quality digital image;
a second compressor that receives a second portion of said compressed data input stream, said decompressor expanding said second portion of said data input stream to second expanded data; and
an adder that combines said second expanded data with said first expanded data to generate a second output data stream to be displayed as a higher quality digital image, said first output data stream and said second output data stream independently selectable for display as a digital image.

3. A system that compresses and decompresses data representing a digital image so that different quality levels of said digital image can be selectably displayed on a video monitor, said system comprising:
an encoder that compresses said data into compressed data layers that comprise data having a plurality of data formats, wherein at least a first one of said data layers comprises data representing a low quality image and at least a second one of said data layers comprises data to convert said low quality image to a higher quality image; and
a decoder that receives said compressed data layers, said decoder expanding said first one of said data layers to generate first expanded output data representing a low quality image, said decoder expanding said second one of said data layers to generate second expanded output data, said decoder combining said second expanded output data with said first expanded output data to generate output data representing a higher quality image.

4. A method of producing image information
indicative of an original image to be sent over a channel in a way to receive the image information in progressively-rendered stages, comprising:
forming a first compressed version of the image, the first compressed version being compressed relative to the original image by a first compression technique, and the first compressed version being of a smaller overall file size than the original image, the first compressed version including sufficient information such that, when displayed, a reduced-resolution version of the original image can be seen; and
forming a second compressed version of the image, said second compressed version including additional information about the image beyond that information produced by said first compressed version to produce a second image which has further resolution than said reduced resolution version, said second compressed version formed using a second compression technique which is different than said first compression technique; and
producing an output file indicative of said first compressed version and said second compressed version.

5. A method as in claim 4, wherein said forming a second compressed version includes analyzing at least a portion of information indicative of the image to determine an optimal compression scheme which will optimally compress said information from among a plurality of different compression schemes; and
forming said second compressed version using said optimal compression technique as part of said second compression technique.

6. A method as in claim 5, further comprising dividing said information into blocks, and classifying each block of the image according to a particular one of said plurality of compression schemes that optimize an amount of compression for each said block.

7. A method as in claim 4, further comprising producing a third compressed version of the image, said third compressed version comprising information providing a highest quality version of the image and including additional information beyond that information producing by said first and second composed versions.

8. A method as in claim 4, further comprising:
initially selecting a number of stages in which said image will be compressed;
and further comprising:
compressing said image in said number of stages, said first and second compressed versions being the first two stages of said number of stages.

9. A method as in claim 4, wherein said forming a first compressed version comprises obtaining a thumbnail image of the original image, said thumbnail being a version of the image that is sized and intended to be displayed in a smaller scale than the original image, over a smaller of pixels than are contained in the original image; and
interpolating said thumbnail image into a full sized image as said first compressed version.

10. A method as in claim 9, further comprising fitting said thumbnail image to a function which increases its accuracy.

11. A method as in claim 9, further comprising
processing information indicative of said thumbnail image to determine which of a plurality of compressing schemes will optimize a compression ratio for said second compressed version, and said forming a second compressed version comprises
compressing said information using the compression scheme which will best compress said image.

12. A method as in claim 11, wherein said plurality of compression schemes include vector quantization, discrete cosine transform, pulse code modulation, and run length encoding.

13. A method as in claim 4, wherein said forming a first compressed version comprises decimating the original image by a predetermined factor along particular dimensions.

14. A method as in claim 13, wherein said decimating comprises decimating each of the color components of the image by a factor of 2 along vertical and horizontal dimensions.

15. An image compression apparatus, comprising:
a first element operating to receive a source image to be compressed;
a formatting element which changes said source image into a form which is susceptible of being processed;

a first compression device including a decimating element which decimates and compresses said source image using a first compression technique that produces information indicative of a reduced quality image;
a second image compressing device, which provides second image information about the source n addition to that contained in said first image information, said second image compressing device compressing using a second compression technique which is different than said first compression
technique; and
a message assembling element, assembling said first image information into a first part of a message to be transmitted and said second image information into a second part of the message, said first and second parts of the message being separately readable.

16. An apparatus as in claim 15, further comprising an image-classifying element which determines a most efficient compression technique to compress information indicative of said source image among a plurality of compression techniques and said second image compressing device compresses said image to form said second image information using said most efficient
technique.

17. An apparatus as in claim 15, wherein said first image information is a decimated and re-interpolated image.

18. A method of transferring a progressively-rendered, compressed image, over a finite bandwidth channel, comprising:
producing a coarse quality compressed image at a source and transmitting said coarse quality compressed image over a channel as a first part of a transmission to a destination end;
receiving the coarse quality compressed image at a receiver at the destination end at a first time and displaying an image based on said coarse quality compressed image on a display system of the receiver when received at said first time;
creating additional information about the image, at the source end, from which a standard quality image can be displayed, said standard quality image being of a higher quality than said coarse quality image, and sending compressed
information over said channel indicative of information for said standard quality image, said sending said standard quality image information occurring subsequent in time to said sending of all of said information for said coarse quality image;
receiving said standard quality image information at the receives at a second time, subsequent to the first time, and decompressing said standard quality image information, to improve the quality of the image displayed on said display system, and to display said standard quality image;

obtaining further information about the image beyond the information in said standard quality image, to provide an enhanced quality image, and compressing said information for said enhanced quality image, said enhanced quality image having more image details than said standard quality image;
transmitting said information for said enhanced quality image, at a time subsequent to transmitting said
information for said coarse quality image and said standard quality image; and
receiving said enhanced quality image information at said receiver, at a third time subsequent to said first and second times, and updating a display on said display system to display the additional enhanced quality image.

19. A method as in claim 18, wherein said producing the coarse quality image uses a different compression technique than said creating additional information indicative of the standard quality image.

20. A method as in claim 18, wherein said coarse quality image includes information indicative of a miniature version of an original image, and said displaying the coarse quality image comprises interpolating said miniature to a size of the original image and displaying said image.

21. A method as in claim 19, wherein said creating additional information comprises determining a characteristic of the image, determining which of a plurality of different
compression techniques will best compress the characteristic determined; and compressing said image using the determined technique.

22. A method as in claim 21, further comprising determining a plurality of areas in said image, and determining, for each area, which of the plurality of different compression techniques will optimize the compression ratio.

23. A method as in claim 22, further comprising interleaving and channel encoding different portions of the compressed image.

24. A method as in claim 22, wherein said compression techniques include vector quantization and discrete cosine transform.

25. A method as in claim 20, wherein said obtaining a miniature comprises decimating along vertical and horizontal axes.

26. A layered progressively-compressed image
compression system, comprising:
a first image compression element obtaining a source image to be compressed and compressing said source image using a first image compression scheme to produce a first image layer;
a second image compression element, said second image compression element compressing information indicative of said source image using a different compression technique than said first image compression element to produce a second image layer; and
an output message assembling element, said output message assembling element receiving said first and second image layers from said first and second image compressing elements, respectively, a first image layer stored in a first area which will be output first, said first image layer including
information from which a coarse image can be reconstructed; and said second image layer including information from which a finer image, having more detail than said coarse image, can be
reconstructed, and said second layer being stored in a location where it will be transmitted after said first layer is
transmitted.

27. A system as in claim 26, wherein said first layer indicative of a coarse image is produced by obtaining a thumbnail miniature of the original information by decimating the source image, and interpolating said thumbnail miniature to a size of a full image display.

28. A system as in claim 26, wherein there are a plurality of layers, each layer including a complete set of information to be displayed at a decoding end, each layer progressively including more information than a previous layer.

29. A method of transmitting and displaying a
compressed image comprising:
first obtaining and sending a first layer of information indicative of a compressed miniature image at a first time;
first receiving said first layer at said decoder end and decompressing and displaying a first coarse image indicative thereof;
second obtaining and sending information
indicative of a compressed improved resolution image having more details than said first coarse image, and transmitting said information at a second time subsequent to said first time; and
second receiving and decompressing said improved resolution image information to provide an updated display which improves the resolution of said first coarse image.

30. A method as in claim 29, wherein said obtaining coarse information comprises:
transmitting information indicative of a compressed miniature of the image;
receiving the compressed miniature of the image;
interpolating the compressed miniature of the image into a full sized image; and
displaying the full sized image.

31. A method as in claim 30, wherein the first coarse image is compressed using a first compression technique and the second image is compressed using a second compression technique which is different from the first compression technique.

32. A method as in claim 31, further comprising determining which of a plurality of different image compression techniques will most efficiently code information indicative of said image.

33. A method as in claim 32, wherein said determining uses fuzzy logic techniques.

34. Image encoding system, comprising: a first element, operating to receive a source image and to format the source image in a way to allow its coding;
a first compression coder, which filters the formatted source image, to form a first compressed and coarse quality image;
an image classifier, operating to classify the information contained in the image according to a characteristic thereof that is related to an amount by which the image can be compressed;
a compression encoder, determining one of a plurality of compression methods that will optimize the amount of compression based on a result of said image classifier, and encoding said information using the optimized compression method to produce a second compressed image;
a message assembling element interleaving
information indicative of the first image and the second image into a desired form in message transmitting format, and
transmitting said message to a channel.

35. A system as in claim 34, wherein said first element includes a decimating stage, and said compression encoder uses a different kind of compression than said decimating.

36. An image decoder system, comprising: a first element, connected to a transmission channel to receive transmitted, compressed data indicative of an image therefrom, said compressed data received in layers;
a display interface which receives information to be displayed;
a first layer detector and decompression element, detecting a complete first layer, and decompressing said first layer when complete, to produce first information indicative of a reduced quality image, based on said first layer after decoding said first layer using a decompression technique and sending said first information to said display interface; and
a second layer detector and decompression element, receiving a second layer of image information, compressed using a different compression technique than said first layer, and detecting that at least a unit of said second layer has been completely received, and decompressing said second layer to produce additional information which is coupled to said display interface to improve a displayed image resolution.

37. A system as in claim 36, further comprising a third layer detector, receiving and decompressing a third layer of information, forming a final display.

38. A system as in claim 37, wherein said units of said second layer are display panels, each panel displayed when completed, to form the second layer of the image in panels.

39. A method as in claim 31, wherein said first obtaining comprises decimating data on the image to form a reduced quality image, fitting the decimated data to a first model which partially restores source image detail lost by decimation, and calculating reconstruction values from the fitting.

40. A method as in claim 39, further comprising using said reconstruction weights to interpolate the decimated data into a full sized image while minimizing a mean squared error between original image components and interpolated image
components.

41. A method as in claim 31, wherein said first step comprises forming miniature versions of the original source image for each of a plurality of primary colors.

42. A system as in claim 36, further comprising an image classifying module, that determines a characteristic of the image indicative of a best technique of compression, to output a measure indicative of said best technique.

43. A system as in claim 42, wherein said image classifying module uses fuzzy logic techniques.

44. A system as in claim 42, wherein said image types include gray scale, graphics, text, photographs, high activity and low activity images.

45. A method as in claim 29, wherein said first obtaining comprises obtaining a miniature image, and further comprising analyzing the miniature image to classify the image into one of a plurality of classes indicative of which of a plurality of compression techniques will best compress said image.

46. A method of encoding a source image, comprising:
obtaining a first compressed version of the image, said first compressed version of the image corresponding to a coarse version of the image indicative of coarse details only, said first compressed version of the image obtained using a first compression technique;
analyzing said coarse version of the image to determine which of a plurality of different compression
techniques will best further compress said image; and
further compressing said image to obtain further information indicative of a better rendering of said image, than said coarse version of said image, using the compression
technique determined by said analyzing.

47. A method as in claim 46, wherein said first compression technique includes decimation of image components followed by interpolation.

48. A method as in claim 46, further comprising:
dividing information indicative of the image into a plurality of block units;
classifying each block unit according to a characteristic which will most efficiently compress said each block unit; and
outputting a control script that specifies an optimized compression method for each said block unit.

49. A method as in claim 48, further comprising compressing, in a third stage, according to the control script.

50. A method as in claim 49, further comprising channel encoding according to the control script.

51. A method as in claim 46, further comprising evaluating information indicative of the coarse image, and determining discrete cosine transform coefficients of said information.

52. A method as in claim 51, further comprising obtaining a reconstructed coarse image from the discrete cosine transform coefficients, determining a residual between the reconstructed image and the coarse image and compressing the residual.

53. An image encoding device, comprising:
a first stage, operating to produce first
information indicating a reduced quality compressed version of the original image;
a second stage which analyzes the first
information to determine an image classification thereof, and outputs a control script indicative of an efficient compression method based on said image classification;
a third stage, responsive to said control script, to select a compression method from among a plurality of
compression methods based on said control script and compressing image information using said compression method to produce second information; and
a fourth stage which assembles the first
information and the second information into a message to be sent.

54. A method as in claim 53, wherein said second stage comprises a discrete cosine transform device, operating to obtain discrete cosine transform coefficients indicative of the image and to determine quantization step sizes therefrom.

55. A device as in claim 53, wherein said first stage comprises a component separator, separating chrominance
components from luminance components; wherein said first stage decimates said chrominance components; and said third stage compresses said luminance components using a discrete cosine transform technique.

56. A device as in claim 55, wherein said second stage comprises a discrete cosine transform coefficient determining device, determining optimal quantization step sizes, and
quantizing discrete cosine transform coefficients using said optimal step sizes.

57. A device as in claim 56, further comprising a dequantizer for dequantizing the discrete cosine transform quantized values to determine an error between the dequantized values and the original image to form a residual, and a fifth stage operating for compressing said residual.

58. A device as in claim 57 wherein said fifth stage comprises an adaptive vector quantizer operating to compress the residual by matching the residual against a group of commonly-occurring block patterns in a codebook.

59. A device as in claim 55, wherein said chrominance is compressed by decimating the color, and fitting the decimated data to a spline function to determine optimal reconstruction weights to minimize a mean squared error.

60. An apparatus as in claim 15, wherein said first and second parts of the message are totally separate.

61. A method of encoding an image, comprising:
processing the image according to a first
technique to produce a processed image;
separating color components of the processed image from intensity components of the processed image;
compressing said color components of the image by compressing using a color component compression technique; and
further compressing the intensity components of the image using a intensity component compression technique different than the color component compression technique.

62. A method as in claim 61 wherein said first
technique is a compression technique which includes a decimation technique, said color component compression technique includes a discrete cosine transform compression technique and said
intensity component compression technique includes a differential pulse code modulation technique.

63. A method as in claim 61 wherein said first
technique includes a decimation technique followed by a technique of reconstructing information from the decimated data obtained from the decimation technique.

64. A method as in claim 61 further comprising
determining optimal compression techniques, and producing a control script indicative thereof, at least one of said
compression techniques being chosen based on said control script.

65. A method as in claim 61 wherein said color
component compression technique is an optimized discrete cosine transform.

66. A method as in claim 64 wherein said color
component compression technique is an optimized discrete cosine transform.

67. A method as in claim 66 further comprising
determining optimal quantization step sizes based on said control script.

68. A method as in claim 66 further comprising reverse discrete cosine transforming information obtained by said color component compression technique using the discrete cosine
transform-compressed signal, and determining a difference between the reverse-discrete-cosine transformed signal and the original signal to determine an error signal there between.

69. A method as in claim 68 further comprising
comparing said error to a codebook, and choosing a codebook entry which matches most closely with said error.

70. A method as in claim 61 wherein said further compression is by a differential pulse code modulation.

71. A compression device, comprising:
a first element receiving an image to be
compressed;
a second element carrying out an initial
compression on said image received by said first element to produce an output initially-compressed image indicative thereof;

a third element which separates said output initially-compressed image into intensity components and color components;
a fourth element which compresses said color components using a first compression technique; and
a fifth element which compresses said intensity components using a second compression technique, different than said first compression technique.

72. An encoding apparatus as in claim 71 wherein said first compression technique is a discrete cosine transform technique and said fifth compression technique is a differential PCM technique.

73. A system as in claim 72 wherein said second element is a decimating and curve fitting compressor.

74. A method of compressing data, comprising:
first compressing said data using a discrete cosine transform technique to produce an output signal indicative of a discrete cosine transform-compressed data;
reviewing said discrete cosine transform-compressed data, and re-converting said discrete cosine
transform-compressed data to reconstructed data of the same form as the starting data, and determining differences between said starting data and said reconstructed data;
comparing said differences to a plurality of quantized differences from a codebook and choosing a closest match; and
forming an output message that includes
coefficients of said discrete cosine transform and an index associated with said codebook, as compressed data indicative of the data.

75. A method as in claim 74 wherein said data to be compressed is an original image which has been pre-compressed using a technique which is different than said discrete cosine transform technique and said codebook technique.

76. A method as in claim 74 wherein said first technique is a decimate and curve fitting technique.

77. A method of selectively coding an image,
comprising:
dividing said image into a plurality of areas, each area representing a portion of the image;
comparing each said area with a value indicating whether said area should or should not be rendered in an enhanced mode;

adding a prioritized value to an enhancement list for each of said areas that will be rendered in the enhanced mode;
compressing values which are on said enhancement list using a high resolution compression technique; and
compressing values which are not on said
enhancement list using a different compression technique.

78. A method as in claim 77 wherein said high
resolution compression technique is a high resolution residual calculator.

79. A method as in claim 74 wherein said areas are blocks of a pre-compressed image.

80. A method as in claim 74 further comprising compressing said image using a discrete cosine transform, wherein said high resolution compression technique is a high resolution residual calculator, and said different compression technique less high resolution residual calculators.

81. An element as in claim (control script) further comprising a channel encoder, obtaining a plurality of data segments each of which indicates data from one of said compression techniques, said encoding being done in accordance with the control script.

82. An image compression system, comprising:
a first compression element which pre-compresses an image to produce a pre-compressed image;
a color converter device which separates said first pre-compressed image into intensity components and color components;
an image classifier, which classifies said image to determine at least one compression technique which will most efficiently compress said image, and produces a control script indicative thereof;
a compression element, including elements for operating according to one of a plurality of different
compression techniques, receiving said control script and compressing based on one of said plurality of compression techniques based on said control script; and
a channel coder which interleaves and channel-codes information from said compression element, said channel coder being responsive to said control script, and channel coding in accordance therewith.

83. A system as in claim 82 wherein said channel coder includes a plurality of different kinds of encoding techniques, which are selected by said control script.

84. A system as in claim 82 wherein said compressing element includes a discrete cosine transform compressing element and a differential pulse code modulation compressing element.

85. An adaptive vector quantizing compression device, comprising:
a first element for receiving data to be
processed;
an image subdivider, operating to subdivide the image data into a set of predetermined length of pixel blocks;
a codebook, which includes a plurality of vectors that correspond to common patterns found in the population of data;
a processing element, operating to determine a best match between said image element and said codebook by determining a minimum squared error summed over all elements in the block and to produce an index indicative thereof; and
transmitting the codebook value in place of the original image data.

86. A compressed file format, comprising:
a header segment which describes overhead information about the system and an object being compressed;
a plurality of segments of image information, each segment separate from each other segment, and each segment including separate information therefrom;
each of said separate information being separately
displayable information, and at least one segment of said information including an initial low resolution image.

87. A data format as in claim 86, wherein said header segment includes information about a following data stream, including an indication of whether the data stream includes image information or includes resource information.

88. The system as in claim 87, wherein said resource information includes look-up tables and vector quantization tables.

89. The system as in claim 86, wherein said header include information indicative of a version of coding used for the image information.

90. A decoder system which decodes a compressed file into an uncompressed file, comprising:
an element which receives the compressed file including a plurality of panels;

a decompression element which serially expands each panel to produce file information therefrom; and
a file memory, storing information indicative of the file, and receiving more information from each panel to provide
progressively more file information than that present in a previous panel.

91. A decoder as in claim 90, wherein said file is an image and a first panel of image data is decompressed to provide a coarse representation of the image.

92. A decoder as in claim 91, wherein said first, coarse layer of the image comprises a miniature version of the image, having a smaller size than an original image, and which is interpolated to a full size image.

93. A decoder as in claim 92, comprising a first decoding element which decompresses at least one panel comprising a thumbnail image, a second decoding element which decompresses at least one panel comprising a splash image, a third decoding element which decompresses at least one panel comprising
information for a standard image and a fourth step which
decompresses at least one panel to provide information to provide a high detail image.

94. A decoder as in claim 92, wherein said interpolation includes an interpolator, controlled according to an interpolation factor, said interpolation factor controlling an amount of interpolation.

95. A decoder as in claim 91, further comprising an element for decompressing a discrete cosine transform ("DCT") data segment.

96. A decoder as in claim 95, further comprising an element for decompressing a vector quantitized residual from the DCT data segment.

97. A method of compressing an image, comprising:
decomposing an initial image to be compressed into a plurality of sub-images, each sub-image having a content which is homogeneous in content of a particular feature;
analyzing each of said sub-images and determining which of said sub-images are visually important;
optimizing compression methods for each of said sub-images in a way such that visually important sub-images have more information associated therewith.

98. A method as in claim 97, wherein one of said
compression methods is a discrete cosine transform ("DCT") and said optimizing includes setting a control script indicating a quantization step size of said DCT.

99. A method as in claim 97, wherein said compression technique is a vector quantization, and said optimizing includes setting a feature of a codebook used in said vector quantization.

100. A method as in claim 97, wherein said classification uses fuzzy logic to determine, among a plurality of classes, whether said image content is more like one class or more like another class.

101. A method as in claim 100, wherein said compression further comprises mapping input sets to corresponding output sets, said output sets indicating which of a plurality of
compression methods to apply, and blending said output steps to provide a control script.

102. A method of classifying an image, comprising:
dividing said image into a plurality of sub-images, each said sub-image having a characteristic which is uniform within the subimage by at least a predetermined amount;
carrying out a first kind of image compression on the subimage;
obtaining a combinational overview on the results of the first kind of image compression to determine a profile of the image component; and
comparing said combinational overview with a plurality of rules, using a plurality of fuzzy input sets having an input rule base, said input sets indicating a plurality of image types, to determine a type of said sub-image.

103. A method as in claim 102, wherein said first image compression is a DCT compression, and said combinational overview is a combination of each DCT component, histogrammed to provide a frequency domain profile.

104. A method as in claim 103, further comprising
determining of plurality of spacial domain blocks, and matching the spacial domain blocks with a special pattern list.

105. A method of identifying a optimal compression technique for a portion of an image, comprising:
determining a histogram of coefficients of at least a first compression technique;and
matching said histogram, using fuzzy logic, to a closest match to determine an ideal compression technique; and
determining components of said image; .

106. A method of determining enhancement values for an image, comprising;
dividing said image into a plurality of sub-images, each said sub-image having a predetermined characteristic;
testing a parameter of each said sub-image against a
threshold value, said threshold value being one which indicates that said sub-image has a lot of changes from a previous subimage; and
determining those parts of the image which compare with said normalized threshold as being enhanced portion images.

107. Method as in claim 106, wherein said value is values of color components, said color components being compared against a normalized threshold value for said color components.

108. Method as in claim 107, further comprising compressing said image using a discrete cosine transform technique and analyzing coefficients of the discrete cosine transform to determine regions where the compression ratio can be adjusted without effecting its quality.