Abstract:
When highly redundant information that expresses five tones using 4 bits is transferred to a printer in terms of a unit data length in data transfer, efficiency is very poor. Hence, multi-valued data is quantized to 5-valued data, which is output as a 4-bit code that can express five values. 4-bit codes for three bits are combined to be converted into an 8-bit code. The 8-bit codes are packed into data of a 16-bit unit, and the packed data is transferred to the printer.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an image processing apparatus and method, and a storage medium and, more particularly, to an image processing apparatus and method for quantizing image data. 
   BACKGROUND OF THE INVENTION 
   When a printer such as an ink-jet printer, the number of tones that can output is limited, is used, the number of tones of image data is reduced to that the printer can express by a quantization process by means of a printer driver on a host computer, and that image data is then transferred from the host computer to the printer. 
   The size of image data to be transferred increases and the time required for transferring image data from the host computer to the printer increases with increasing resolution of printer, resulting in a low print throughput. In such case, the following method may be used. That is, the printer driver sends only tone information of a density pattern using a density pattern method, and the printer converts the received tone information into dots. In this method, the data size can be smaller than that of binary data to be directly transferred from the host computer to the printer. For example, when the resolution of a printer is 600 dpi, and a unit density pattern is formed by collecting a total of four dots to be output from the printer, i.e., 2 vertical dots×2 horizontal dots, five tones can be expressed, as shown in  FIG. 1 . That is, when the printer driver executes a 5-valued quantization process for 300-dpi pixel information, and sends its tone information alone to the printer, it can make the printer output a pseudo continuous tone image. 
   When image data is transferred from the host computer to the printer by the aforementioned method, the aforementioned 5-valued quantization data is expressed by quantization codes each having a given bit length, and the quantization codes are packed to undergo data transfer. In terms of this packing process (since data transfer is done in units of 8 or 16 bits), the bit length of each quantization code is 2, 4, or 8 bits, and a 4-bit quantization code is used in case of the 5-valued quantization data. Therefore, since this quantization data has only tone information for five values with respect to 16 tones that 4 bits can express, it becomes information with very high redundancy. 
   Even such highly redundant information, which expresses five tones using 4 bits, can be used while the data transfer rate or the memory size of the printer has a large margin. However, as the printer requires higher resolution and higher speed, the data transfer rate and the data size that the printer can hold pose a problem. That is, when highly redundant information that expresses five tones using 4 bits is transferred to the printer, this results in very poor efficiency. 
   In order to combat this problem without changing the unit density pattern, when the number of tones is reduced from five values to four values, the quantization code can be expressed by 2 bits. However, a reduction of the number of tones leads to loss of tone information, production of false contours, an increase in granularity, and the like, thus deteriorating the image quality of an output image. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the aforementioned problems, and has as its object to generate quantization data with low redundancy by quantizing image data without deteriorating image quality. 
   In order to achieve the above object, a preferred embodiment of the present invention discloses an image processing apparatus comprising: quantization means for quantizing multi-valued image data into N-valued data (where N is a natural number), and outputting the N-valued data as a K-bit code (where K is a natural number) that can express the N values; conversion means for combining and converting K-bit codes for M pixels (where M is a natural number) into an L-bit code (where L&lt;M×K); and output means for packing and outputting data output from said conversion means into data of a predetermined bit unit. 
   Also, there is disclosed an image processing method comprising the steps of: quantizing multi-valued image data into N-valued data (where N is a natural number), and outputting the N-valued data as a K-bit code (where K is a natural number) that can express the N values; combining and converting K-bit codes for M pixels (where M is a natural number) into an L-bit code (where L&lt;M×K); and packing and outputting data output from the conversion step into data of a predetermined bit-unit. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing an example of density patterns; 
       FIG. 2  is a block diagram showing the arrangement of an image processing system according to the first embodiment of the present invention; 
       FIG. 3  is a block diagram showing the arrangement of an image processor shown in  FIG. 1 ; 
       FIG. 4  is a block diagram for explaining the functional arrangement of a data compression unit; 
       FIG. 5  is a view for explaining the process of the data compression unit; 
       FIG. 6  is a view showing an example of a conversion table of an LUT shown in  FIG. 4 ; 
       FIG. 7  is a block diagram showing another arrangement of a data compression unit shown in  FIG. 3 ; 
       FIG. 8  is a block diagram showing the arrangement of a decoder shown in  FIG. 2 ; 
       FIG. 9  is a block diagram showing the arrangement of an image processor according to the second embodiment of the present invention; 
       FIG. 10  is a graph for explaining the relationship among the image data size, required memory size, available memory size, and ON/OFF state of a compression process; 
       FIG. 11  is a block diagram showing the arrangement of an image processing system according to the third embodiment of the present invention; 
       FIG. 12  is a block diagram showing the arrangement of an image processor according to the fourth embodiment of the present invention; and 
       FIG. 13  is a view showing an example of density patterns in the fourth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An image processing apparatus according to an embodiment of the present invention will be described in detail hereinafter with reference to the accompanying drawings. 
   First Embodiment 
   (Arrangement) 
     FIG. 2  is a block diagram showing the arrangement of an image processing system according to this embodiment. 
   Application software  102 , which runs on a host computer  101  and is used to create and edit an image, outputs image data of the created and/or edited image to an image processor  103 . Image data output from the application software  102  is 8-bit multi-valued data per color of R, G, and B or C, M, Y, and K if an image is a continuous tone image. 
   The image processor like a printer driver which runs on the host computer  101  executes a quantization process, compression process, and the like of the input image data, thus generating image data to be transferred to a printer  104  such as an ink-jet printer. 
   The image data input to the printer  104  is stored in a RAM  105 . Since the image data stored in the RAM  105  has been compressed by the image processor  103 , it is expanded to image data to be printed by a decoder  106 . The expanded image data is sent to an engine  107 , thus forming and outputting an image based on the image data. 
   (Image Processor) 
     FIG. 3  is a block diagram showing the arrangement of the image processor  103 . 
   A quantizer  201  converts input multi-valued (e.g., 8 bits, 256 tones per color) image data into N-valued image data per C, M, or Y, or C, M, Y, or K. In this embodiment, a case will be explained wherein N=5, i.e., 5-valued quantization is done. Also, since pseudo halftoning is done to correct quantization errors produced upon quantization, the image finally output has continuous tone. As pseudo halftoning, known error diffusion, dithering, or the like is used. 
   A data compression unit  202  inputs the quantized data in units of pixels. In this embodiment, since image data is quantized to 5-valued data, quantized data has 4 bits per pixel. This data of 4 bits per pixel is compressed to data of 8 bits per three pixels by a compression process (to be described later) of the data compression unit  202 , and the compressed data is sent to a packing unit  203 . 
   The packing unit  203  packs the compressed data input from the data compression unit  202  into a transfer unit from the host computer  101  to the printer  104 . For example, when data transfer from the host computer  101  to the printer  104  is done in units of 16 bits, two 8-bit compressed data are packed into 16-bit data. 
   (Data Compression Unit) 
     FIG. 4  is a block diagram for explaining the functional arrangement of the data compression unit  202 . 
   A switch  301  receives 4-bit pixel data, which has been quantized to 5-valued data, in units of pixels, and separately outputs the received pixel data at three pixel cycles 3n, 3n+1, and 3n+2, as shown in  FIG. 5 . In the example shown in  FIG. 5 , pixels a and d are distributed and output as (3n)-th pixels; pixels b and e as (3n+1)-th pixels; and pixels c and f as (3n+2)-th pixels. Although each pixel data is 4-bit data, since five values can be expressed by, e.g., “0000”, “0001”, “0010”, “0011”, and “0100”, upper 1 bit is not necessary. For this reason, the bits to be output from the switch  301  can be three bits. 
   The (3n)-th and (3n+1)-th pixel data of those distributed to three pixel cycles are input to a look-up table (LUT)  302  and are converted into 5-bit data in accordance with a table example shown in  FIG. 6 . As a result, the number of bits of data is reduced by one, but no information is omitted. This is because since 3-bit data for one pixel has only information for five values, there are only 5×5=25 different pieces of information even when data for two pixels are combined. Furthermore, this 5-bit data and 3-bit data as the (3n+2)-th pixel data are combined, and the combined data is output from the data compression unit  202  as 8-bit information. 
   The arrangement of the data compression unit  202  is not limited to that shown in  FIG. 4 , but the arrangement shown in  FIG. 7  may be used. That is, all 4-bit data for three pixels may be input to an LUT  801  and converted into 8-bit data. When a process is done by software such as a printer driver, the arrangement shown in  FIG. 6  can make the processing load lighter. 
   (Decoder) 
   The compressed image data is transferred to the printer  104  and is stored in the RAM  105 . The decoder  106  decodes (expands) image data stored in the RAM  105  in synchronism with the image formation timing of the engine  107 . 
     FIG. 8  is a block diagram showing the arrangement of the decoder  106 . 
   The decoder  106  basically executes a process opposite to that of the data compression unit  202  shown in  FIG. 4 . That is, 5-bit data extracted from the input 8-bit data is input to an LUT  701  that makes inverse conversion to that of the LUT  302  of the data compression unit  202  to be converted into 3-bit pixel data for two pixels. The pixel data for two pixels output from the LUT  701 , and the remaining 3-bit data of the 8-bit data are input together to a switch  702  to restore pixel data for three successive pixels. Finally, the pixel data output from the switch  702  is supplied to a pattern table  703  to generate five different dot patterns shown in  FIG. 1 . 
   As described above, according to the first embodiment, 4-bit information per pixel is compressed to 8-bit data per three pixels, and the compressed data is sent to the printer  104  and stored in the RAM  105 . Hence, image data to be transferred and stored in the RAM  105  is 8/3=2.67 bits per pixel, and efficient data transfer and storage can be realized. According to the compression method of this embodiment, since image data undergoes lossless compression, it is free from any omission of information resulting from lossy compression such as JPEG or the like, and is also free from any deterioration of image due to compression. 
   Second Embodiment 
   An image processing apparatus according to the second embodiment of the present invention will be described below. Note that the same reference numerals in this embodiment denote the same parts as in the first embodiment, and a detailed description thereof will be omitted. 
   In the second embodiment, the compression process of the data compression unit  202  described in the first embodiment is ON/OFF-controlled depending on image data.  FIG. 9  is a block diagram showing the arrangement of the image processor  103  of the second embodiment. In  FIG. 9 , a data compression controller  204  is added to the arrangement of the first embodiment shown in  FIG. 3 . The data compression controller  204  computes the memory size that the printer  104  requires for processing on the basis of, e.g., the size of image data input to the image processor  103 . When the memory size that the printer  104  can use is smaller than the required memory size, the data compression controller  204  controls image data to pass through the data compression unit  202  without any compression process. 
     FIG. 10  is a graph for explaining the relationship among the image data size, required memory size, available memory size, and ON/OFF state of the compression process. Note that the border line of ON/OFF of the compression process may be fixed in accordance with the memory size that the printer  104  can use or may dynamically change on the basis of information obtained from the printer  104 . 
   The reason why such process is required will be briefly explained. In a serial printer such as an ink-jet printer, the print speed changes largely depending on the image data size, and the processing speed required for the host computer  101  also changes. Hence, when the compression process is kept ON irrespective of the image data size, the load on the compression process is large when the image data size is small, and data transfer from the host computer  101  cannot often catch up with the print speed of the printer  104 . If the image data size is originally small, since such data need not be compressed in consideration of the memory size of the RAM  105  of the printer  104 , the compression process of the data compression unit  202  is preferably turned off so as not to increase the processing load on the image processor  103 . 
   As described above, according to the second embodiment, since the compression process of the image processor  103  is ON/OFF-controlled depending on the image data size, efficient data transfer and storage can be realized in case of a relatively large data size, and an increase in processing load due to the compression process can be suppressed in case of a relatively small data size. 
   Third Embodiment 
   An image processing apparatus according to the third embodiment of the present invention will be explained below. Note that the same reference numerals in this embodiment denote the same parts as in the first embodiment, and a detailed description thereof will be omitted. 
   The data compression process in the first embodiment is done on the host computer  101  side. By contrast, the data compression process in the third embodiment is done on the printer  104  side.  FIG. 11  is a block diagram showing the arrangement of an image processing system according to the third embodiment. 
   In the third embodiment, since the host computer  101  does not perform any compression, the quantized image data is directly transferred from the image processor  103  to the printer  104 . Hence, in the example explained in the first embodiment, 4-bit image data that has been quantized to 5-valued data is directly transferred to the printer  104 . The image data input to the printer  104  is compressed by a compression unit  108  by the same method as that described in the first embodiment, and the compressed data is stored in the RAM  105 . 
   According to the third embodiment, since both the compression and expansion processes of image data are done on the printer  104  side, the compression process required for the image processor  103 , and the memory size require for data storage at that time can be reduced. Hence, the processing load can be prevented from increasing due to the compression process in the host computer  101 , and hence, low print throughput can be avoided. 
   Fourth Embodiment 
   An image processing apparatus according to the fourth embodiment of the present invention will be explained below. Note that the same reference numerals in this embodiment denote the same parts as in the first embodiment, and a detailed description thereof will be omitted. 
   In the fourth embodiment, in particular, to reduce the data size when a color image is output, coarse quantization is done for a color in which quantization errors hardly stand out, and data compression is done for a color in which quantization errors readily stand out. 
   When a color image is formed by an image output apparatus represented by an ink-jet printer, a color image is formed by mixing four different color inks such as cyan, magenta, yellow, and black. For this reason, the use ratios of inks are determined in correspondence with input image data in a color conversion process in the image process, and image data is quantized in units of colors. 
     FIG. 12  is a block diagram showing the arrangement of an image processor of the fourth embodiment. For example, RGB 24-bit color image data output from the application software  102  is input to a color processor  205 , and is color-separated into multi-valued (e.g., 8 bits) data of four colors, i.e., cyan, magenta, yellow, and black (to be abbreviated as C, M, Y, and K hereinafter). Each color data is input to a corresponding quantizer  201 C,  201 M,  201 K, or  201 Y, and is independently quantized. 
   Of four, C, M, Y, and K colors, Y dots are very hard to see for the human eye. Hence, even when coarse quantization is done for Y image data, quantization errors of a Y component image formed hardly stand out. Hence, exploiting this nature, five tones expressed by 2×2 dots are used for three, C, M, and K colors, and Y is expressed by four tones by decreasing one gray level, as shown in  FIG. 13 . Hence, C, M, and K image data are quantized to 5-valued data by the quantizers  201 C,  201 M, and  201 K, the quantized data undergo the same data compression process as in the first embodiment by data compression units  202 C,  202 M, and  202 K, and the compressed data are input to the packing unit  203 . On the other hand, Y image data is quantized to 4-valued data by the quantizer  201 Y. Since 4-valued data can be expressed by 2 bits, the Y image data is sent to the packing unit  203  without being compressed. 
   According to the fourth embodiment, exploiting the nature that respective color components have different influences on image quality, data compression is selectively done for some color components. Hence, the processing load of the overall image process can be reduced, and the influence on image quality can be minimized. 
   Note that the fourth embodiment can be combined with not only the arrangement of the first embodiment, but also that of the second embodiment. 
   The present invention can be applied to a system constituted by a plurality of devices (e.g., host computer, interface, reader, printer) or to an apparatus comprising a single device (e.g., copy machine, facsimile). 
   Further, the object of the present invention can be also achieved by providing a storage medium storing program codes for performing the aforesaid processes to a system or an apparatus, reading the program codes with a computer (e.g., CPU, MPU) of the system or apparatus from the storage medium, then executing the program. 
   In this case, the program codes read from the storage medium realize the functions according to the embodiments, and the storage medium storing the program codes constitutes the invention. 
   Further, the storage medium, such as a floppy disk, a hard disk, an optical disk, a magneto-optical disk, CD-ROM, CD-R, a magnetic tape, a non-volatile type memory card, and ROM can be used for providing the program codes. 
   Furthermore, besides aforesaid functions according to the above embodiments are realized by executing the program codes which are read by a computer, the present invention includes a case where an OS (operating system) or the like working on the computer performs a part or entire processes in accordance with designations of the program codes and realizes functions according to the above embodiments. 
   Furthermore, the present invention also includes a case where, after the program codes read from the storage medium are written in a function expansion card which is inserted into the computer or in a memory provided in a function expansion unit which is connected to the computer, CPU or the like contained in the function expansion card or unit performs a part or entire process in accordance with designations of the program codes and realizes functions of the above embodiments. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.