Fast preview processing for JPEG compressed images
AssigneeXerox Corporation
InventorsRicardo L. de Queiroz, Reiner Eschbach
Life of this patent
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Abstract
A method of fast decompressing a document image compressed using transform coding for scaling and previewing purposes. A fast algorithm is derived by utilizing a fraction of all available transform coefficients representing the image. The method is particularly efficient using the discrete cosine transform which is used in the JPEG ADCT algorithm. In JPEG ADCT, a very fast and efficient implementation is derived for a resolution reduction factor of 16 to 1 (4 to 1 in each direction) without needing any floating point arithmetic operations.
Claims6
A method of decompressing an image, compressed with a frequency space transform operation, for preview or scaling purposes, while approximating an accurate reproduction thereof, including the steps of:
- receiving from a transmission line or retrieving from memory storage devices a set of frequency space transform coefficients, representing a compressed image block of image signals;
- selecting a subset of said coefficients including fewer coefficients than said set thereof, said subset corresponding to coefficients for a predetermined group of low frequency components of said image;
- recovering an image approximation with a linear transform relating said subset of coefficients to pixels of the said image approximation.
A method of decompressing an image, compressed with a frequency space transform operation, for preview purposes, while approximating an accurate reproduction thereof, including the steps of:
- receiving from a transmission line or retrieving from a memory storage device, a M.times.M set of frequency space transform coefficients, representing a compressed M.times.M image block of image signals;
- selecting a N.times.N subset of said coefficients, said subset corresponding to coefficients for a predetermined group of low frequency components of said image;
- recovering an image approximation from said subset of coefficients, in accordance with the function:
- ##EQU20## where each z value is a pixel value in the described relative spatial relationship in an set of output image signals;
- each y value is a received coefficient value;
- and D.sub.N is an N.times.N inverse transform matrix.
A method of decompressing an image, compressed with a frequency space transform operation, for preview purposes, while approximating an accurate reproduction thereof, including the steps of:
- receiving from a transmission line or retrieving from a memory storage device, an 8.times.8 set of frequency space transform coefficients, representing a compressed 8.times.8 image block of image signals;
- selecting a 2.times.2 subset of said coefficients, said subset corresponding to coefficients for a predetermined group of low frequency components of said image;
- recovering an image approximation from said subset of coefficients, in accordance with the function:
- ##EQU21## where each z value is a pixel value in the described relative spatial relationship in an set of output image signals;
- and each y value is a received coefficient value.
Dependencies are inferred from the claim text.