Abstract:
In an apparatus and method for image/video enhancement, pre-processing and post-processing techniques are employed to effectively modify the transforms used in a fixed, standardized data compression coder. In this manner, alternative transforms, for example overlapping-basis-type transforms, are made to be applicable to, and compatible with, various data compression standards, thereby improving system performance.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/070,689, filed Jan. 7, 1998, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods and apparatus for improving image quality in image and video compression systems. More particularly, the present invention relates to a technique that employs pre- and post-processing to allow alternative transforms (such as Wavelet Transform (WLT), Wavelet Packet Transform (WPT), Lapped Orthogonal Transform (LOT), Generalized Lapped Orthogonal Transform (GenLOT), and Generalized Lapped Biorthogonal Transform (GLBT)) to be used with standard-compliant video compression coders (for example JPEG/MPEG/H.26x standards). 
     2. Background of the Related Art 
     The trend of visual communication has evolved rapidly in recent years as a result of advances in hardware technology, as well as the proliferation of multimedia-based software applications, such as Internet browsers. Standards for transmitting and storing visual information also encourage rapid deployment of interchangeable multimedia consumer products. 
     Contemporary high-quality image/video compression techniques commonly employ some form of forward and inverse transforms. Widely-used image and video compression standards include the JPEG, MPEG, H.261, and H.263 compression techniques, which are based on the Discrete Cosine Transform (DCT). The well-known JPEG image encoding technique, developed by the Joint Photographic Expert Group, is widely used in image compression software and hardware. As illustrated in the block diagram of FIG. 1, the image is divided into a number of 8×8 blocks of data elements  40 , each of which is then transformed at a transform process  42  using a 2-dimensional DCT. The transform coefficients are next arranged into 64 sub-bands at spectrum estimator  44 , are scalar-quantized at quantizer  46 , adaptively baseline-coded and Huffman-coded at coder  48 , and stored in memory  50 . 
     The well-known MPEG video encoding technique, developed by the Motion Pictures Expert Group, achieves a high compression ratio, and a corresponding significant bit rate reduction, by taking advantage of the correlation between adjacent pixels in the spatial domain (using the DCT), and the correlation between image frames in the time domain (using motion estimation and prediction). 
     The JPEG technique yields good results for compression ratios of 10:1 and below (assuming 8-bit gray-scale images); however, at higher compression ratios, the underlying block nature of the transform begins to manifest itself in the compressed image. As compression ratios approach 24:1, only the DC (lowest frequency) coefficient has data bits allocated to it, and, at this ratio, the input image has been approximated by a set of 8×8 blocks. The reconstructed image therefore will exhibit blocking artifacts. 
     Several transforms with overlapping basis have been proposed for addressing the blocking artifacts. Among them are the Lapped Orthogonal Transform (LOT),  Signal Processing with Lapped Transforms,  H. S. Malvar, Norwood, Mass.: Artech House, 1992; the Generalized Lapped Orthogonal Transform (GenLOT),  The GenLOT: Generalized Linear-Phase Lapped Orthogonal Transform,  R. L. de Queiroz, T. Q. Nguyen and K. R. Rao, IEEE Transaction on Signal Processing, V44, N3, pp. 497-507, March 1996; the Wavelet Transform (WLT),  Wavelets and Filter Banks,  G. Strang and T. Nguyen, Wellesley-Cambrige Press, 1996; and the Generalized Lapped Biorthogonal Transform (GLBT),  The Generalized Lapped Biorthogonal Transform,  T. Tran, R. deQueiroz and T. Nguyen, Proceeding of the IEEE International Conference in Acoustics, Speech and Signal Processing, April 1998. 
     These overlapping-basis transforms reduce blocking artifacts by borrowing pixels from adjacent blocks to produce the transform coefficients of the current block. FIG. 2 depicts the aforementioned process for the case of the 8-channel LOT where the basis functions of the forward and inverse transforms have a length of 16. 
     Referring to FIG. 2, the transformed sequences X DCT (k)  54 A and X LOT (k)  54 B, both of length M, are computed as: 
     
       
           X   DCT (k)= T   DCT   ·x   DCT (n), DCT Processing  (1a) 
       
     
     
       
           X   LOT (k)= T   LOT   ·x   LOT (n), LOT Processing  (1b) 
       
     
     where T DCT  and T LOT  represent matrices consisting of M-basis functions for the DCT and LOT forward transforms  53  respectively. The sizes of the T DCT  and T LOT  matrices are M by M and M by 2M, respectively. The vectors x DCT (n)  52 A and x LOT (n)  5 B, of sizes M and 2M respectively, contain appropriate samples from the input image. The reconstructed sequence {circumflex over (x)} DCT (n)  56 A and {circumflex over (x)} LOT (n)  56 B can be defined similarly by applying the inverse transform  55  to the transformed sequences  54 A,  54 B. Note that the above description can be extended to two-dimensional sequences (as images), to three-dimensional sequences (as group of images or video) and to multidimensional images. The sizes of the transform can also be arbitrary (not necessarily 2M as in the LOT processing case), as in GenLOT, GLBT and wavelet processing. 
     With application of the overlapping basis function transform, blocking artifacts are substantially reduced or eliminated. However, these overlapping transforms are not standard compliant and therefore are not compatible for use with compression standards such as JPEG, MPEG, H.261, H.263, etc., since the standards are fixed, and do not allow for a change of basis function. Furthermore, the transform operations are commonly embedded in the coder hardware or software in a manner that does not allow for user access. Once widely deployed in the form of an application specific integrated circuit (ASIC) or software, alteration of the transform is nearly impossible. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for image/video enhancement. More particularly, the present apparatus and method employ pre-processing and post-processing techniques to effectively modify the transforms used in a fixed, standardized coder. In this manner, alternative transforms, for example overlapping-basis-type transforms, are made to be applicable to, and compatible with, various data compression standards, thereby improving system performance. 
     In a first embodiment, the present invention is directed to an apparatus and method for pre-processing input data in a data compression system employing a fixed transform. The input data is modified by a cancellation transform which is the substantial inverse of the fixed transform to generate pre-processed data. The pre-processed data are applied to the fixed transform to generate compressed data, the compressed data being substantially unaffected by the fixed transform. 
     In a preferred embodiment, the present invention further comprises modifying the input data by an alternative transform, for example a Lapped Orthogonal Transform, a Generalized Lapped Orthogonal Transform, a Wavelet Transform, a Wavelet-Packed Transform, or other artifact-reduction transform. The compressed data may be decompressed by applying a decompression transform which is the substantial inverse of the fixed transform. The decompressed data may be further applied to the fixed transform, to generate transformed decompressed data which is substantially unaffected by the fixed transform. The fixed transform may comprise a Discrete Cosine-Based Transform (DCT). The transformed decompressed data may be applied to an inverse of the alternative transform to generate output data. 
     The input and output data may comprise a variety of signals, for example image data, video data, audio data, and multidimensional data. For multidimensional data, the pre- and post-processing techniques are preferably applied across some or all rows and columns, and across some or all dimensions. The fixed transform may be applied to a standard compression system, for example JPEG, MPEG-I, MPEG-II, H.261, H.263, H.263+, and H.324. The pre- and post-processing techniques of the present invention may be applied to all images of a video or audio sequence, or a subset thereof. The data are preferably intensity-scaled to match system dynamic range. 
     In this manner, alternative transforms, for example image-enhancing transforms may be employed in systems utilizing standardized data compression techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a block diagram of a conventional image transform-based coder. 
     FIG. 2 illustrates Discrete Cosine Transform (DCT) processing and Lapped Orthogonal Transform (LOT) processing on a one-dimensional sequence in the conventional image transform-based coder of FIG.  1 . 
     FIG. 3 is a block diagram illustrating the processing steps for application of an alternative transform T to a pre-processing technique employing a coder having a forward transform T coder  and an inverse transform U coder , in accordance with the present invention. 
     FIG. 4 is a block diagram of the processing steps employed for application of an alternative inverse transform U to a post processing technique employing a coder having a forward transform T coder  and an inverse transform U coder  in accordance with the present invention. 
     FIG. 5 is a block diagram of the processing steps for application of an alternative transform T to a pre-processing technique for a standard-compliant DCT-based coder having a forward transform T DCT  and an inverse transform U DCT  in accordance with the present invention. 
     FIG. 6 is a block diagram of the processing steps for application of an alternative inverse transform U to a post-processing technique for a standard-compliant DCT-based coder having a forward transform T DCT  and an inverse transform U DCT . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Given a coder C which employs a forward transform T coder  for data compression, and an inverse transform U coder  for data decompression, a pre-processing transform T pre  and a post-processing transform U post  can be generated: 
     
       
           T   pre   =U   coder   ·T,  Pre-processing  (2a) 
       
     
     
       
           U   post   =U·T   coder , Post-processing  (2b) 
       
     
     such that a new, alternative forward transform T, of size M by N, and its corresponding inverse transform U, of size N by M, can be compatibly employed with the coder C. Assume the dimensions of T coder  and U coder  to be M by L and L by M, respectively. Accordingly, the pre-processing and post-processing transforms T pre  and U post  have dimensions L by N and N by L, respectively, where L, M, and N are integers. 
     FIG. 3A is a block diagram of a pre-processing technique in accordance with the present invention. Input data x(n) are applied to an alternative forward transform T  60  which may comprise an overlapping-basis transform as described above, The transformed data is next applied to an inverse of the fixed transform U coder    62 , which, as explained above, is the inverse of the fixed transform T coder  performed in coder C. The combination of the alternative forward transform T  60  and the inverse of the fixed transform U coder    62  is referred to herein as the pre-processing transform T pre    68  as shown in FIG. 3B, and expressed above in Equation  2   a.  In this manner, the input data x(n) are processed by a pre-processing transform T pre    68  to compress the data according to the newly-introduced transform T  60 , and further to cancel the effects of the fixed transformed T coder  included with the data compression encoder. 
     The pre-processed data is applied to an intensity scaling process  64  in order to scale the dynamic range of the data to match that of the coder  66 . The scaled data is then compressed at coder  66  employing the fixed transform T coder  in accordance with standard data compression techniques. 
     As shown in FIGS. 4A and 4B, the compressed data are received and decompressed at a decoder  70  which employs a fixed inverse transform U coder , which is the inverse transform of the fixed transform T coder . The decoded data are intensity-scaled at a scaling process  72 , and the scaled data are applied to a forward transform T coder    74  which operates to substantially cancel the effects of the inverse transform U coder . The transformed data are next applied to an inverse transform U  76  which is the inverse of alternative forward transform T  60  of FIG. 3A, thereby generating output data {circumflex over (x)}(n). The combination of the forward transform T coder  and inverse transform U are represented in FIG. 4B, and expressed above in equation ( 2   b ), as post-processing transform U post    78 . 
     In this manner, data are compressed and decompressed according to an enhanced alternative transform T and alternative inverse transform U in a manner consistent with an otherwise non-compatible standard data compression coder C having fixed forward and inverse data transforms, without modifying the structure and operation of the coder C. 
     In alternative embodiments, the pre-processing transform T pre    68  may be precalculated as a single matrix and therefore operate on the input data in a single step, or may be applied as individual serial processes  60 ,  62 . The same applies to the post-processing transform U post . 
     In one embodiment, as shown in FIGS. 5A,  5 B,  6 A and  6 B, the present invention employs a standard-compliant image/video data compression coder  82  and decoder  84  (JPEG/MPEG/H.26X) which utilizes a fixed Discrete Cosine Transform matrix T coder =T DCT   86  and a fixed Inverse Discrete Cosine Transform matrix U coder =U DCT    80 , where (as in the above examples) U DCT ·T DCT =Identity matrix I. 
     Specifically for DCT-based standard-compliant image and video coders, the pre-processing  68  and post-processing  78  transforms are represented by: 
     
       
           T   pre   =U   DCT   ·T,  Pre-processing  (3a) 
       
     
     
       
           U   post   =U·T   DCT , Post-processing  (3b) 
       
     
     where the sizes of T DCT  and U DCT  are 8 by 8 and 8 by 8, respectively. This implies that the alternative forward transform T  60  and its inverse transform U  76  are of size 8 by N and N by 8, respectively. 
     Assuming that the Lapped Orthogonal Transform (LOT) is employed as the alternative transform T, then: 
       T   pre   =U   DCT   ·T   LOT , Pre-processing  (4a) 
     
       
           U   post   =U   LOT   ·T   DCT , Post-processing  (4b) 
       
     
     where T LOT  and U LOT  are of size 8 by 16 and 16 by 8, respectively. 
     The invention can be applied to all images in a video sequence or to a subset thereof. For a multidimensional signal the pre- and post-processing steps can be applied over all data rows and columns, over all dimensions, or over a subset of rows, columns, or dimensions. One embodiment of the invention in a standard-compliant MPEG (or H.26x) video coder applies the pre-processing and the post-processing steps on the intra frame of the video sequence. Another embodiment of the invention in the standard-compliant MPEG (or H.26x) video coder/encoder applies the pre-processing and the post-processing steps on all images in the video sequence and at any of the various processing stages of the video coder/encoder. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.