Patent Publication Number: US-7218786-B2

Title: Method of compressing and decompressing images

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a method of image compression and decompression and, in particular, to a method of image compression and decompression in a compression and decompression chip with the JBIG standard. 
   2. Related Art 
   After sampling and digitization, an original picture will have a very large size of data. Such a large data size is very inconvenient in image processing, storage and transmissions. The image compression technique is to represent the same picture using less data, reducing the data transmission time. 
   The basic idea in image compression is to remove redundant data from the picture. The data redundancies in pictures have the following sources: 
   (1) Coding redundancy: Data with a greater probability to appear in a picture file can be represented by a shorter code. 
   (2) Interpixel redundancy: Since neighboring pixels have very high correlations, therefore, the data of a particular pixel can be derived from another. 
   JBIG (Joint Bi-level Image Group) is the black-and-white (BW) image compression standard set by CCITT in 1993. The method employed by the standard is to represent each pixel in a bi-level image by one bit. The complete image compression job can be achieved using a single CODEC (compression and depression) chip with the JBIG standard. As the JBIG technique is fairly mature, it has been widely used in printing and faxing. 
   The flowchart of the image data CODEC method in the JBIG standard is shown in  FIG. 1 . When the CODEC chip processes the data, all steps shown in the drawing have to be performed in each cycle T to compress or decompress each bit in the data. 
   First, the system performs memory access (step  10 ) to read data from memory. This step has to be done for either compressing or decompressing data. The accessed data are combined to form a context (CX), which is then used to look up a table and get a corresponding state (ST). 
   Step  20  performs look-up-table (LUT) to find the LSZ value corresponding to ST obtained in step  10 . Afterwards, subtraction (A′=A−LSZ) is performed in step  30  to subtract the LSZ value obtained in step  20  from a default value A, obtaining an A′ value. 
   Step  40  performs a selection (A=A′ or A=LSZ) to determine which one to continue the subsequent operations. Afterwards, another subtraction (C′=C−A+LSZ) is performed (step  50 ). However, this time uses another default value C to find a new C′ value by subtracting A from C, added by the LSZ value. 
   Afterwards, step  60  performs a C value selection (C=C′ or C=C) to determine which one to use in subsequent operations. Step  70  is the renormalization to renormalize A between 0.5 and 1. Step  80  is the renormalization of C to scale it up. 
   Finally, step  90  performs byteout/bytein to output compressed or decompressed data to memory. This then concludes the flow of image data CODEC. 
   For a 100 MHz central processing unit (CPU), its work cycle is 10 ns. However, it takes about 10.5 ns to encode a single bit of the image data, from memory access (step  10 ) to byteout/bytein (step  90 ). For decoding, the circuit is more complicated and therefore the whole process takes about 11.5 ns. 
   In summary, we see that the time it takes for the CPU to complete either a compressing or decompressing task is longer than its work cycle. Thus, it will cause delays in image data processing and lower the printing speed of the printer. 
   If there is no way to reduce the time it takes to compress and decompress image data in each cycle, the printing speed will not be able to increased even when a faster CPU is used. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an objective of the invention is to provide a method of image data compression and decompression (CODEC) for CODEC chips with the JBIG standard. 
   This method divides the computation method for compressing and decompressing image data into three parts: memory access, numerical calculation, renormalization and byteout/bytein. Each step takes one work cycle. 
   In a work cycle, the data compression and decompression of three bits can be simultaneously performed. Therefore, the CODEC speed of the whole image can be increased by at least a factor of 2. The disclosed method can be implemented in a high-speed CPU to increase the data processing speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a flowchart of the computation method for image data compression and decompression with the JBIG standard in the prior art; 
       FIG. 2  is a flowchart of the disclosed method; 
       FIG. 3  is a schematic view of compressing or decompressing image data; 
       FIG. 4  is a schematic view of decompressing the tenth bit of data in an image; and 
       FIG. 5  is a schematic view of parallel processing three pixels at the same time in a work cycle. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The disclosed image compression and decompression (CODEC) method is implemented on a CODEC chip with the JBIG standard. Its flowchart is shown in  FIG. 2 . The drawing shows the steps that are required to carry out when compressing or decompressing a single bit of the image data. 
   First, the first cycle (n−1) performs memory access (step  100 ). For either compressing or decompressing data, the image data have to read from memory to the CODEC chip, converting reference pixels around the pixel to be decompressed from context (CX) into state (ST). 
   As shown in  FIG. 3 , when compressing or decompressing the image data in each pixel (white square), the surrounding pixels are taken as the reference pixels CX (gray squares). 
   In the second cycle n, the ST data obtained in the first cycle undergoes certain numerical operations for subsequent steps. Such numerical operations include look-up-tables (LUT&#39;s), subtractions, and selections. The ST data and more probable symbol (MPS) obtained from step  100  are used to find a corresponding LSZ value by looking up the table (step  110 ). 
   Subsequently, the system performs subtractions for two different parameters, a first default parameter A and a second default parameter C (steps  121 ,  122 ). The first default parameter A is subtracted by the LSZ value to give a first subtracted value A′=A−LSZ. The second default parameter C is subtracted with the first default parameter A and added with the LSZ value obtained by LUT (step  110 ) to give a second subtracted value C′=C−A+LSZ. 
   Afterwards, the system performs selections of two different parameters, a new first default parameter A and a new second default parameter C (steps  131 ,  132 ). The new first default parameter A is selected between the LSZ value and the first subtracted value A′. The new second default parameter C is selected between the second default parameter C and the second subtracted value C′. The new first default parameter A and new second default parameter C selected in steps  131 ,  132  are used to be the first default parameter A and the second default parameter C needed in the numerical operation for the next bit. 
   Finally, in the next cycle, the third cycle (n+1), the new first default parameter A and the new second default parameter C are renormalized (steps  141 ,  142 ). The image data processed with numerical computations and renormalization are output (step  143 ). 
   Steps  141 ,  142  renormalize the first default parameter A and the second default parameter C. Byteout/bytein in step  143  output compressed or decompressed image data via the JBIG-standard CODEC chip to other memory for storage. 
   Please refer to  FIG. 4  for the application of the invention to the decompression of the tenth bit of image data. In the (n−1) work cycle, the ninth bit of data is not completely decompressed; therefore, the decompression of the tenth bit cannot be performed. 
   In this case, the ninth bit can be temporarily set as 0 or 1, with CX=0 or 1. The system can then decompress the tenth bit. In the work cycle n, the ninth bit is decompressed. Thus, the CX value for the ninth bit is known and is used to obtain the correct decompressed image data. 
   With reference to  FIG. 5 , each work cycle simultaneously processes three pixels. In comparison, the work cycle of the prior art without pipeline improvement is longer. This method can effectively shorten the work cycle of each image data process, increasing the speed of compressing and decompressing image data. 
   From the data obtained from the 0.35 procedure simulation, we see that the disclosed CODEC method can effectively increase the compression and decompression speed of image data by at least a factor of 2. This method is thus applicable to high-speed CPU structures. 
   Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.