|Publication number||WO1994022108 A1|
|Publication date||29 Sep 1994|
|Filing date||17 Mar 1994|
|Priority date||23 Mar 1993|
|Publication number||PCT/1994/2914, PCT/US/1994/002914, PCT/US/1994/02914, PCT/US/94/002914, PCT/US/94/02914, PCT/US1994/002914, PCT/US1994/02914, PCT/US1994002914, PCT/US199402914, PCT/US94/002914, PCT/US94/02914, PCT/US94002914, PCT/US9402914, WO 1994/022108 A1, WO 1994022108 A1, WO 1994022108A1, WO 9422108 A1, WO 9422108A1, WO-A1-1994022108, WO-A1-9422108, WO1994/022108A1, WO1994022108 A1, WO1994022108A1, WO9422108 A1, WO9422108A1|
|Inventors||Rafael Retter, Isaac Shenberg|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Non-Patent Citations (2), Referenced by (40), Classifications (11), Legal Events (5)|
|External Links: Patentscope, Espacenet|
RAPID THUMBNAIL IMAGE RECONSTRUCTION OF DCT COMPRESSED IMAGE DATA
BACKGROUND OF THE INVENTION This invention relates generally to image compression, and more particularly the invention relates to discrete cosine transform (DCT) -based compression and coding of images and image reconstruction therefrom.
Image compression is used in reducing large volumes of data in digitized images for convenient and economical storage and for transmission across communication networks having limited band width. Image compression technology is important in digital still video cameras, color scanners, color printers, color fax machines, computers, and multimedia.
The Joint Photographic Experts Group (JPEG) has established a color image data compression standard for use in a variety of still image applications. Compression employs
DCT-based processes operating on discrete blocks of the image. The DCT coefficients are then quantized based on the measurements of the threshold for visibility. For coding, an 8x8 pixel array of DCT coefficients is reorganized into a one- dimensional list using a zigzag sequence which tends to concentrate coefficients expressing the lowest spatial frequencies at lower indices with the DC component being number 0 in the zigzag. Because the higher frequency components contain the fine details of the image and are generally less sensitive to human vision, they can be quantized more coarsely than the low frequency components and may be discarded with negligible effect on image quality. The quantized AC coefficients are then encoded using a Huffman coder. Finally, headers and markers are inserted in the codes for individual blocks along with bit and byte stuffings for JPEG data compatibility. Fig. 1 illustrates the JPEG compression algorithm, and Fig. 2 illustrates the zigzag order of coefficient sequence. The compressed data can then be stored (as in an electronic still camera) or transmitted efficiently over a limited band width communication network. Reconstruction of the image requires a reverse process in which the headers and markers are extracted, the Huffman code is decoded, coefficients are dequantized, and an inverse DCT (IDCT) operation is performed on the coefficients.
Image reconstruction out of a baseline JPEG- compatible file is a function of the number of pixels in the image and can be time consuming. Progressive construction of the image can be effected through spectral selection, successive approximation, and hierarchical coding. However, in baseline JPEG coded data, the user must decode all image data for image reconstruction. This process can be time consuming, especially if the user wants to quickly browse various images when seeking a specific image.
SUMMARY OF THE INVENTION In accordance with the present invention, a method is provided for accelerating the decoding process and reconstructing partial images to facilitate a quick survey of images based on baseline JPEG coded data. The process eliminates zero-quantized high-frequency coefficients and can use only DC coefficients which are dequantized to obtain unsigned numbers. The unsigned numbers are then used to generate an image which is equal to the original image size decimated by 8 in two dimensions (i.e., - - size) and which is built out of the biased DC values of each block.
The invention can be implemented in a JPEG decoder which first Huffman decodes the compressed image data stream.
The quantized DC coefficient is generated out of the differential DC value with the quantized DC value then dequantized and biased to generate an unsigned value which is used for displaying the image. The remainder of the block which contains AC values is not used and need not be dequantized. The unsigned bias DC values are produced each time a block of data completes the de-Huffman process without requiring DCT. The invention and objects and features thereof will be more fully apparent from the following detailed description and appended claims with taken with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a functional block diagram illustrating a compression/decompression algorithm.
Fig. 2 illustrates the zigzag order of coefficient sequence in an 8x8 array of DCT coefficients. Fig. 3 is a flow diagram illustrating the imaging process based on dequantized DC coefficient using a JPEG decoder in accordance with the invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENT Fig. 1 is a functional block diagram of the compression/decompression implemented with a JPEG coder/decoder such as the ZR36040 product of Zoran Corporation, assignee, and Fig. 2 illustrates the zigzag sequence of an 8x8 array of quantized DCT coefficients used for "coding." The DC coefficient is coefficient 0 and the higher frequency coefficients are the higher numbers. Because the higher frequency components contain the fine details of the image and are generally less sensitive to human vision, they can be quantized more forcefully than the lower frequency components and may be discarded with negligible effect on image quality. In coding, the coarser quantization of high frequency coefficients results in long strings of zeros in the coded image data.
In accordance with the present invention, thumbnail or reduced size images are generated based on the DC values of the DCT coded image data. The compressed DCT coded data must first be decoded with a Huffman decoder, where the non-zero quantized coefficients are generated out of the Huffman coded data. This step is proportional in processing time to the number of non-zero quantized coefficients. The quantized DC value is then generated out of the differential DC values and is dequantized and biased to generate the DCT coefficient' which is used for the thumbnail image. This step takes one clock period since only one coefficient (DC coefficient) is going through the stage.
The remainder of the process, including the dequantization of the quanitized AC coefficients and the IDCT, is skipped. Fig. 3 is a flow diagram illustrating the imaging process based on dequantized DC coefficients with a JPEG decoder in accordance with the invention.
The relative time it takes to decode the image and generate the thumbnail image is the number of non-zero quantized coefficients divided by the image size. The extreme case is a compressed file which contains only DC values with all the AC values being zero. In this case, since an end of block (EOB) is stored also, the generation of the thumbnail image takes about 3% of the time required to decompress the full image.
The invention has been implemented using the Zoran 36040 JPEG Image Coder/Decoder in which the compressed bit-stream is first Huffman decoded and then the quantized DC coefficients are generated out of the differential DC value. As noted above, the quantized DC value is then dequantized and biased to generate an unsigned value, which is then used for displaying the image. The rest of the block of data containing the AC values is skipped and there is no need to dequantize all of the AC values and execute the inverse DCT operation. The unsigned biased DC values are output every time a block completes its de-Huffman process, which facilitates the imaging of data for quick scanning. The process of generating the thumbnail is therefore part of the regular decoding process, requiring no special logic. The later part of the process is skipped.
While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.
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|International Classification||H04N7/30, G06T9/00, H04N7/26|
|Cooperative Classification||H04N19/30, H04N19/18, H04N19/60, H04N19/132|
|European Classification||H04N7/26A8C, H04N7/26E2, H04N7/30H, H04N7/26A4Z|
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|22 Dec 1994||DFPE||Request for preliminary examination filed prior to expiration of 19th month from priority date (pct application filed before 20040101)|
|17 Jul 1996||122||Ep: pct application non-entry in european phase|