Fast preview processing for JPEG compressed images

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.

We claim: 
     
       1. 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.   
     
     
       2. A method as described in claim 1, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform. 
     
     
       3. 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×M set of frequency space transform coefficients, representing a compressed M×M image block of image signals;   selecting a N×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 N  is an N×N inverse transform matrix.   
     
     
       4. A method as described in claim 3, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform of M channels and the inverse transform operation is the discrete cosine transform of N channels. 
     
     
       5. 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 8×8 set of frequency space transform coefficients, representing a compressed 8×8 image block of image signals;   selecting a 2×2 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.   
     
     
       6. A method as described in claim 5, wherein the forward transform coding operation using the frequency space transform operation is a discrete cosine transform.