JP3910192B2 - Image compression apparatus and image expansion apparatus, and computer-readable recording medium recording a program for causing a computer to execute the image compression method and the image expansion method - Google Patents

Description

  The present invention relates to an image compression apparatus and an image expansion apparatus, and a computer-readable recording medium that records a program for causing a computer to execute the image compression method and the image expansion method. In particular, an image compression apparatus and an image expansion apparatus that can efficiently encode an image without causing color misregistration, and a program for causing a computer to execute the image compression method and the image expansion method, respectively. The present invention relates to a recorded computer-readable recording medium.

  Japanese Laid-Open Patent Publication No. 7-221993 discloses a thinning method for thinning out data of color difference components by taking advantage of the characteristics of human eyes that are relatively insensitive to color changes in image compression. In this thinning method, the rate of change of the color difference component in the small block is obtained. When the rate of change is large, the color difference component is thinned out. When the rate of change is small, the image data is compressed.

  However, in general, when the color difference component is thinned out, a phenomenon that a false color called a color shift phenomenon may occur. Such a color misregistration phenomenon is particularly noticeable at the edge of a character.

  In the conventional thinning method, the thinning interval is determined based only on the change rate of the color difference component. For this reason, the thinning process is performed at the same thinning interval between a small block made up of single color characters as shown in FIG. 20A and a small block made up of figures of multiple colors as shown in FIG. Will be done. However, in the case where many colors are mixed as shown in FIG. 20B, since a person cannot recognize a figure in a small block as an object such as a character, it is difficult to notice even if there is a slight color shift. . Therefore, in such a case, it should be possible to further increase the thinning interval.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an image compression apparatus that can efficiently encode an image without causing color shift visually. That is.

  Another object of the present invention is to provide an image expansion device that expands an encoded image without causing color shift to the eye.

  Still another object of the present invention is to provide a computer-readable recording recording a program for causing a computer to execute an image compression method capable of efficiently encoding an image without causing color misregistration. To provide a medium.

  Still another object of the present invention is to provide a computer-readable recording medium on which is recorded a program for causing a computer to execute an image expansion method for expanding an encoded image without causing color misregistration. It is.

  According to an aspect of the present invention, an image compression device is an image compression device that compresses color image data composed of a plurality of blocks, and each of the component image data of the color image data has a resolution at the time of reduction. Is determined adaptively according to a predetermined standard for each block, and resolution determination processing means for generating resolution information, and for each of the component image data, data is generated by reducing each block as necessary based on the resolution information Data generation processing means for encoding and encoding processing means for encoding the data generated by the data generation processing means, and the resolution determination processing means receives each component image data from the first color space of the color image data. Resolution is determined for each block according to the degree of influence when converting to a second color space different from the first color space, and resolution information To create.

  According to the present invention, the color image data may be converted from the first color space to the second color space at the time of decoding. At this time, each component image data of the color image data not important for the conversion is used. The resolution is determined so as to reduce the block. As a result, the image data can be compressed without causing a color shift to human eyes.

  According to still another aspect of the present invention, an image compression apparatus is an image compression apparatus that compresses luminance component image data and color difference component image data each composed of a plurality of blocks, the luminance component image data and the color difference component For each of the image data, the resolution at the time of reduction is adaptively determined according to a predetermined standard for each block, and resolution determination processing means for creating resolution information, and is connected to the resolution determination processing means, luminance component image data and color difference component images For each piece of data, based on the resolution information, a data creation processing unit that creates data in which each block is reduced as necessary, and a data creation processing unit that is connected to the data creation processing unit and encodes the data created by the data creation processing unit And a resolution determination processing unit for each of the plurality of blocks. Minute image data and color difference component image data according to the degree of influence when converting the red component, the image data of the green component and blue component, set the resolution for each block, creating resolution information.

  When decoding, luminance component image data and two types of color difference component image data may be converted into red component, green component, and blue component image data. At this time, luminance component image data or The resolution is determined so as to reduce the block of the color difference component image data. As a result, the image data can be compressed without causing a color shift to human eyes.

  According to another aspect of the present invention, an image expansion device is an image expansion device that decodes encoded data encoded by the above-described image compression device and reproduces image data. A data decoding processing means for decoding the data, and an image creation processing means for connecting the data decoding processing means and enlarging the encoded data to the resolution before reduction based on the resolution information to create image data Including.

  An encoded image can be decoded without causing a color shift to the appearance.

  According to still another aspect of the present invention, the image compression method is an image compression method for compressing color image data composed of a plurality of blocks, wherein each component image data of the color image data is reduced. The resolution is created adaptively according to a predetermined standard for each block, and the resolution information is created for each block, and for each of the component image data, data is generated by reducing each block as necessary based on the resolution information And a step of encoding the generated data. The step of generating the resolution information includes converting each component image data from the first color space of the color image data to the first color space. Determines the resolution for each block in accordance with the degree of influence when converting to a different second color space, and creates resolution information.

  According to the present invention, the color image data may be converted from the first color space to the second color space at the time of decoding. At this time, each component image data of the color image data not important for the conversion is used. The resolution is determined so as to reduce the block. As a result, the image data can be compressed without causing a color shift to human eyes.

  According to still another aspect of the present invention, an image compression method is an image compression method for compressing luminance component image data and color difference component image data each composed of a plurality of blocks. For each of the component image data, the resolution at the time of reduction is adaptively determined in accordance with a predetermined standard for each block, and the resolution information is generated, and for each of the luminance component image data and the color difference component image data, based on the resolution information A step of creating data in which each block is reduced as necessary, and a step of encoding the created data, wherein the step of creating resolution information comprises: And color difference component image data given when converting to red component, green component and blue component image data Depending on the degree of the sound, it sets the resolution for each block, comprising the step of creating the resolution information.

  At the time of decoding, luminance component image data and color difference component image data may be converted into red component, green component, and blue component image data. The resolution is determined to reduce the block of data. As a result, the image data can be compressed without causing a color shift to human eyes.

  According to another aspect of the present invention, an image expansion method is an image expansion method that decodes encoded data encoded by the above-described image compression method and reproduces the image data. The image decompression method includes a step of decoding the encoded data that has been encoded, and a step of enlarging the encoded data to have a resolution before reduction based on the resolution information, and creating image data.

  An encoded image can be decoded without causing a color shift to the appearance.

  According to another aspect of the present invention, a computer-readable recording medium on which an image compression program is recorded records an image compression program for causing a computer to execute the above-described image compression method.

  According to another aspect of the present invention, a computer-readable recording medium on which an image expansion program is recorded records an image expansion program for causing a computer to execute the above-described image expansion method.

[Embodiment 1]
Referring to FIG. 1, a multi-resolution image compression apparatus 20 according to an embodiment of the present invention is connected to an image input apparatus 22 for inputting a color image, and the image input apparatus 22, and receives color input image data. An image compression device 24 that compresses each block at a different resolution, and a data output device 26 that is connected to the image compression device 24 and outputs final encoded data are included.

  The image compression device 24 is connected to the image input device 22 and is connected to the input image buffer 32 for temporarily storing the input image data, and to the input image buffer 32, and R (Red), G (Green), B (Blue). ) Three input image data into a color space (YUV space) image data composed of a luminance signal (Y) and color difference signals (U, V), and a color space conversion processing unit 34. Connected to the color space conversion image buffer 36 for temporarily storing the three pieces of image data in the color space, and the color space conversion image buffer 36, and the resolution of the image data in the color space is determined for each block of each image data. A resolution determination processing unit 38 that determines each of them according to a predetermined standard described later, and a resolution information buffer 40 that is connected to the resolution determination processing unit 38 and temporarily stores the determined resolution information Is connected to the resolution information buffer 40, and a resolution information encoding unit 42 for encoding by any reversible coding such as reversible coding JBIG scheme resolution information.

  The image compression device 24 is further connected to the resolution information buffer 40 and the color space conversion image buffer 36, and each block of the color space image data is halved based on the resolution information stored in the resolution information buffer 40. A data creation processing unit 44 that creates reduced data (hereinafter referred to as “reduced data”) and data that is the same as each block of image data in the color space (hereinafter referred to as “same size data”), and a data creation processing unit 44 Connected to a reduced data buffer 46A for temporarily storing reduced data, connected to a data creation processing unit 44, connected to an equal-sized data buffer 46B for temporarily storing equal-sized data, and a reduced data buffer 46A, The reduced data encoding processing unit 48A for JPEG encoding the reduced data and the same size data buffer 46B are connected. PEG-encoded resolution data encoding processing unit 48B, resolution information encoding processing unit 42, reduced data encoding processing unit 48A and equal-magnification data encoding processing unit 48B connected to JPEG encoded resolution information, An encoded data joint processing unit 50 that joins the reduced data and the same-size data to one data, and the joined encoded data (hereinafter referred to as “joined encoded data”) connected to the encoded data joint processing unit 50. And a joint encoded data buffer 52 that temporarily stores and outputs the data to the data output device 26.

  Referring to FIG. 2, resolution determination processing unit 38 is connected to color space conversion image buffer 36 and uses the same method as the reduction method in data creation processing unit 44 based on a plurality of predetermined resolutions. An image reduction processing unit 64 to be reduced, a reduced image data buffer 66 that is connected to the image reduction processing unit 64 and temporarily stores the reduced image data, and is connected to the reduced image data buffer 66 so that the reduced image data is converted into the original image data. An image enlargement processing unit 68 that enlarges to the same size as the image data, an enlarged image data buffer 70 that is connected to the image enlargement processing unit 68 and temporarily stores the enlarged image data, and a color space conversion image buffer 36 And the enlarged image data buffer 70 and the image data held in the color space conversion image buffer 36 and the enlarged image data buffer 70. A region error calculation processing unit 72 that calculates a region error between the image data for each block and a region error calculation processing unit 72 are connected to each block based on the output of the region error calculation processing unit 72. And an area resolution creation processing unit 74 that calculates the best resolution information and writes the resolution information into the resolution information buffer 40.

  The resolution determination processing unit 38 is further connected to the color space conversion image buffer 36, and color difference component quantization processing for quantizing image data of color difference components (U and V components) out of color space image data to 16 gradations. Is connected to the color difference component quantization processing unit 82 and the resolution information buffer 40, and the number of colors included in the block is calculated for each block according to the output of the color difference component quantization processing unit 82. Accordingly, a resolution information change processing unit 84 that changes the resolution information stored in the resolution information buffer 40 is included.

  The resolution determination processing unit 38 is further connected to the color space conversion image buffer 36, and a luminance component edge detection processing unit 78 that detects an edge component from the luminance component (Y component) image data out of the color space image data. And a resolution information change processing unit 80 that is connected to the luminance component edge detection processing unit 78 and changes the resolution information stored in the resolution information buffer 40 based on the output of the luminance component edge detection processing unit 78.

  Referring to FIGS. 3 to 5, each part of multiresolution image compression apparatus 20 operates as follows. The image input device 22 inputs the three input image data of R, G, and B, and stores them in the input image buffer 32 (S2). The color space conversion processing unit 34 reads the three pieces of input image data stored in the input image buffer 32 and performs a process of converting them into three pieces of image data in the color space (YUV space) (S4). The color space conversion processing unit 34 stores the converted three pieces of image data in the color space conversion image buffer 36 (S6).

  The image reduction processing unit 64 reads the three pieces of image data stored in the color space conversion image buffer 36, and reduces each image data so as to have a predetermined resolution (S8). Here, it is assumed that the resolution is determined so that the image data after the reduction is ½ times the image data before the reduction. The image reduction processing unit 64 stores the reduced image data in the reduced image data buffer 66 (S10). The image enlargement processing unit 68 reads the reduced image data from the reduced image data buffer 66 and enlarges the reduced image data so as to have the same size as the image data before reduction (S12). The image enlargement processing unit 68 stores the enlarged image data in the enlarged image data buffer 70 (S14).

  Referring to FIG. 6, the three pieces of image data stored in color space conversion image buffer 36 and the three pieces of enlarged image data stored in enlarged image data buffer 70 are each divided into nl × ml blocks. It shall be divided. The region resolution creation processing unit 74 extracts one block from the nl × ml blocks (S16). Let the position of the extracted block be (n, m). The extraction order starts from (n, m) = (0,0), and is performed in the raster scan order until (n, m) = (nl-1, ml-1).

  The area error calculation processing unit 72 obtains a difference-square cumulative sum SN_Y of pixel values to be described later between the image data before reduction and after reduction and enlargement for the (n, m) block of the Y component image. The creation processing unit 74 determines whether or not the difference square cumulative sum SN_Y is greater than a predetermined threshold value SNth_Y (S18).

  Referring to FIG. 7, each block has a size of 16 × 16, and the pixel value of Y component image data before reduction is set to f_Y (x, y) (x = 0 to 15, y) for each block. = 0 to 15), and the pixel value of the Y component image data after being reduced and enlarged is represented by g_Y (x, y) (x = 0 to 15, y = 0 to 15). The sum of squared differences SN_Y is calculated according to equation (1). Similarly, the pixel values of the U component image data and the V component image data before the reduction are represented as f_U (x, y) and f_V (x, y), respectively, and the pixels of the U component image data and the V component image data after the reduction are displayed. The values are expressed as g_U (x, y) and g_V (x, y), respectively. The difference square cumulative sums SN_U and SN_V of the U component and the V component are calculated according to equations (2) and (3), respectively.

  In parallel with the processing of S18, the region resolution creation processing unit 74 determines whether or not the difference square cumulative sums SN_U and SN_V of the U component and the V component are larger than predetermined threshold values SNth_U and SNth_V, respectively ( S24, S30).

  When the difference square accumulated sum SN_Y of the Y component is larger than the threshold value SNth_Y (YES in S18), the resolution information F_Y (n, m) for the block (n, m) of the Y component is referred to with reference to FIG. 1 is substituted (S22). If the difference square accumulated sum SN_Y is equal to or smaller than the threshold value SNth_Y (NO in S18), 0 is substituted into the resolution information F_Y (n, m) (S20). If the resolution information is 0, it means that there is little error with the original image even if the reduced image is enlarged and restored. For this reason, it represents that reduction is possible. Further, when the resolution information is 1, it indicates that the image cannot be reduced.

  Similarly, when the difference square accumulated sum SN_U of the U component is larger than the threshold value SNth_U (YES in S24), 1 is substituted into the resolution information F_U (n, m) for the block (n, m) of the U component. (S28). When the sum of squared differences SN_U is equal to or less than the threshold value SNth_U (NO in S24), 0 is substituted into the resolution information F_U (n, m) (S26).

  If the difference square cumulative sum SN_V of the V component is larger than the threshold value SNth_V (YES in S30), 1 is substituted into the resolution information F_V (n, m) for the block (n, m) of the V component (S34). . When the sum of squared differences SN_V is equal to or smaller than the threshold value SNth_V (NO in S30), 0 is substituted into the resolution information F_V (n, m) (S32).

  The region resolution creation processing unit 74 determines whether or not the currently processed block is the last block (nl-1, ml-1) (S36). If it is not the last block (NO in S36), the processes after S16 are repeated.

  If it is the last block (YES in S36), the region resolution creation processing unit 74 determines each block (n, m) (n = 0 to nl-1, m = 0 to 0) obtained in the processing from S18 to S34. ml-1) resolution information F_Y (n, m), F_U (n, m) and F_V (n, m) are written into the resolution information buffer 40 (S38).

  The luminance component edge detection processing unit 78 applies a Sobel filter to the Y component image data stored in the color space conversion image buffer 36 to detect an edge component (S40). The resolution information change processing unit 80 extracts one block from the nl × ml block as in the process of S16 (S42).

  The resolution information change processing unit 80 determines whether or not the edge density ED_Y included in the Y component block (n, m) is larger than a predetermined threshold EDth_Y (S44). The edge density ED_Y refers to the number of edge components having a value exceeding a predetermined threshold among edge components included in a certain block. If the edge density ED_Y of the Y component is larger than the threshold value EDth_Y (YES in S44), 0 is substituted into the resolution information F_Y (n, m) (S46). That is, it can be reduced.

  In parallel with the processing of S44 to S46, the color difference component quantization processing unit 82 obtains the number of colors CN included in the block (n, m) according to a method described later, and the obtained number of colors CN is predetermined. It is determined whether or not it is larger than the threshold value CNth (S48). If the obtained number of colors is larger than a predetermined threshold value, 0 is substituted for resolution information F_U (n, m) and F_V (n, m), respectively (S50, S52). That is, it can be reduced.

  Here, a method of calculating the number of colors included in one block will be described. It is assumed that the pixel values f_U (x, y) and f_V (x, y) of the U component and the V component can take values in the range of 0 to 255, respectively. At this time, the function convert () is defined as shown in FIG. That is, the function convert () is a function that quantizes a 256 level value to a 16 level value. The color number color (x, y) of each pixel (x, y) in the block (n, m) is determined as in the following equation (4).

color (x, y) = convert (f_U (x, y)) + [convert (f_V (x, y)) + 1] × 16 (4)
The number of colors CN included in the block (n, m) is defined as the number of types of color (x, y) included in the block (n, m), and the number of colors CN is obtained.

  The resolution information change processing unit 84 determines whether or not the currently processed block is the last block (nl-1, ml-1) (S53). If it is not the last block (NO in S53), the processes after S42 are repeated.

  If it is the last block (YES in S53), the data creation processing unit 44 receives the resolution information F_Y, F_U and F_V and the image data of the Y component, U component and V component held in the color space conversion image buffer 36. The reduced data and the same size data are created for each component (S54).

  With reference to FIG. 10, a method of creating reduced data and same-size data will be described. Of the Y component resolution information F_Y, a block whose resolution information is 0 is extracted, the corresponding block is reduced to 1/2 from the Y component image data, and reduced data is created by arranging the blocks. Further, from the resolution information F_Y, a block with the resolution information of 1 is extracted, a corresponding block is selected from the Y component image data, and is arranged to create the same size data. Similarly, the same-size data and reduced data are created for each of the U component and the V component.

  The data creation processing unit 44 stores the reduced data in the reduced data buffer 46A and the same size data in the same size data buffer 46B (S56).

  The reduced data encoding processing unit 48A and the same size data encoding processing unit 48B read the reduced data and the same size data from the reduced data buffer 46A and the same size data buffer 46B, respectively, and perform JPEG encoding. Also, the resolution information encoding processing unit 42 reads the resolution information from the resolution information buffer 40 and JBIG encodes it in a lossless format (S58). The encoded data concatenation processing unit 50 includes JBIG-encoded resolution information obtained from the resolution information encoding processing unit 42, the reduced data encoding processing unit 48A, and the equal-size data encoding processing unit 48B, and JPEG-encoded. By joining the reduced data and the same size data, the joint encoded data as shown in FIG. 11 is obtained and written into the joint encoded data buffer 52. Thereafter, the data output device 26 reads the joint encoded data from the joint encoded data buffer 52 and outputs it (S60).

  Referring to FIG. 12, multi-resolution image expansion apparatus 100 according to the present embodiment is connected to data input apparatus 102 for inputting the joint encoded data created in multi-resolution image compression apparatus 20, and to data input apparatus 102. The input data buffer 104 that temporarily stores the input joint encoded data, and is connected to the input data buffer 104, reads the joint encoded data stored in the input data buffer 104, and from the joint encoded data, And an encoded data separation processing unit 106 that separates the encoded resolution information, reduced data, and equal-size data.

  The multi-resolution image expansion apparatus 100 is further connected to an encoded data separation processing unit 106, and includes a resolution information encoded data buffer 108 for temporarily storing the encoded resolution information, and a resolution information encoded data buffer 108. A resolution information data decoding unit 110 connected to decode the encoded resolution information; a resolution information data buffer 112 connected to the resolution information data decoding unit 110 to temporarily store the decoded resolution information; An encoded reduced data buffer 114A that is connected to the data separation processing unit 106 and temporarily stores the encoded reduced data, and a reduced data that is connected to the encoded reduced data buffer 114A and decodes the encoded reduced data. It is connected to the decoding processing unit 116A and the reduced data decoding processing unit 116A, and the decoded reduced data Is connected to the reduced data buffer 118A, the reduced data buffer 118A, and the data expanding unit 119 that doubles the reduced data to the original size data, and the data expanding unit 119. An enlarged data buffer 121 that temporarily stores the enlarged data, an encoded data buffer 114B that is connected to the encoded data separation processing unit 106 and temporarily stores the encoded equal data. , Connected to the encoded equal-size data buffer 114B, connected to the equal-size data decoding processing unit 116B that decodes the encoded equal-size data and the equal-size data decoding processing unit 116B, and temporarily stores the decoded equal-size data And a normal-size data buffer 118B for storing the same.

  The multi-resolution image decompression apparatus 100 is further connected to the resolution information data buffer 112, the enlarged data buffer 121, and the same size data buffer 118B, and joins the enlarged data and the same size data based on the resolution information. A resolution data joining processing unit 120 that creates image data of Y, U, and V components, and is connected to the resolution data joining processing unit 120, and temporarily stores image data of the Y component, U component, and V component An YUV image data buffer 122; an image conversion processing unit 124 connected to the YUV image data buffer 122, for converting Y component, U component, and V component image data into R component, G component, and B component image data; Connected to the conversion processing unit 124 and temporarily stores R component, G component, and B component image data. It includes a force image data buffer 126 is connected to the output image data buffer 126, and an image output unit 128 for outputting the image data stored in the output image data buffer 126.

  Referring to FIG. 13, each part of multi-resolution image decompression apparatus 100 operates as follows. The data input device 102 inputs the joint encoded data created by the multi-resolution image compression device 20, and stores the input joint encoded data in the input data buffer 104 (S72). The encoded data separation processing unit 106 separates the joint encoded data stored in the input data buffer 104 into resolution information, reduced data, and equal-magnification data (S74). The encoded data separation processing unit 106 stores the resolution information, the reduced data, and the same size data in the resolution information encoded data buffer 108, the encoded reduced data buffer 114A, and the encoded equal size data buffer 114B (S76).

  The resolution information data decoding unit 110 decodes the resolution information stored in the resolution information encoded data buffer 108 (S78). The resolution information data decoding unit 110 stores the decoded resolution information in the resolution information data buffer 112 (S80). The reduced data decoding processing unit 116A decodes the reduced data stored in the encoded reduced data buffer 114A (S82). The reduced data decoding processing unit 116A stores the decoded reduced data in the reduced data buffer 118A (S84). The data enlargement unit 119 enlarges the reduced data stored in the reduced data buffer 118A by a factor of 2, and creates data of the original size (S86). The data expansion unit 119 stores the original size data in the expansion data buffer 121 (S88).

  The equal-size data decoding processing unit 116B decodes the equal-size data stored in the encoded equal-size data buffer 114B (S90). The equal magnification data decoding processing unit 116B stores the decoded equal magnification data in the equal magnification data buffer 118B (S92). Referring to FIG. 14, the resolution data joining processing unit 120 joins the enlarged data and the same size data based on the resolution information stored in the resolution information data buffer 112, so that the Y component, U component, and V-component image data is created and stored in the YUV image data buffer 122 (S94). The image conversion processing unit 124 converts the Y component, U component, and V component image data stored in the YUV image data buffer 122 into R component, G component, and B component image data, and creates output image data. The output image data is stored in the output image data buffer 126. Thereafter, the image output device 128 outputs the output image stored in the output image data buffer 126 (S96).

  As described above, the multi-resolution image compression apparatus 20 according to the present embodiment reduces the size of image data for each block, and when an error from the original image data is small when the image data is enlarged, The image data is reduced. In addition, when many colors are mixed in a block, a person cannot recognize a graphic in the block as an object such as a character. In general, a character is often expressed in one color, and when many colors are mixed, there is a high possibility that a figure in the block is not a character. For this reason, even if there is a slight color shift in a portion where many colors are mixed, it is difficult to notice it. Therefore, image data is reduced in such a block. Furthermore, a character contains many edges. For this reason, a block whose luminance component edges are not dense is highly likely not to include characters, and therefore image data in the block is reduced. Therefore, it is possible to compress an image without causing a color shift visually. The image compression method according to the present embodiment is particularly effective for image data including characters.

  The edge of the color difference component (U component, V component) is detected for each block, the density of the edges in the block (the sum of the absolute values of the edge components) is calculated for each color difference component, and the edge You may make it reduce about a block whose density is below a predetermined threshold value. If the edge position of the color difference component is shifted by reducing the block, this causes color shift. For this reason, the color misregistration is prevented without reducing the block including many edges. The determination of whether or not to reduce based on the density of the edge of the color difference component may be used in combination with the above-described three determination methods, or may be used independently.

  In the above-described embodiment, the same resolution as the original image and the resolution when the original image is reduced to ½ have been described as the resolution, but it goes without saying that other resolutions may be used. Further, more resolutions may be determined in advance, and it may be determined whether the image data can be reduced based on each resolution using the above-described determination method. When it is determined that the image can be reduced based on a plurality of resolutions, each block may be reduced based on a resolution that minimizes the image data size.

  The resolution information is encoded by the JBIG method, but any method may be used as long as it is a lossless compression method such as Huffman encoding. Further, any compression method may be used for compressing the reduced data and the same size data.

  Further, it goes without saying that the edge detection method using other filters such as a Canny filter may be used other than the edge detection method using the Sobel filter.

[Embodiment 2]
Referring to FIG. 15, multi-resolution image compression apparatus 140 according to the present embodiment has a hardware configuration substantially similar to multi-resolution image compression apparatus 20 described with reference to FIG. The difference is that instead of the resolution determination processing unit 38, a resolution determination processing unit 142 that determines the resolution so as not to cause a color shift when converted into image data in the RGB space is used. The other parts are the same as those of the multi-resolution image compression apparatus 20, and therefore the detailed description thereof will not be repeated here.

  Referring to FIG. 16, the resolution determination processing unit 142 is connected to the color space conversion image buffer 36, and multiplies the pixel value of each pixel of the U component image data by a predetermined coefficient to weight the U component. A weighting unit 144, connected to the color space conversion image buffer 36, a V component weighting unit 146 that multiplies the pixel value of each pixel of the V component image data by a predetermined coefficient and performs weighting, and a color space conversion image Connected to the buffer 36, the U component weighting unit 144, and the V component weighting unit 146, according to the method described later, the Y component and the weighted U and V components are compared, and the best resolution is calculated for each block. , And a magnitude relation comparison unit 148 that writes to the resolution information buffer 40.

  When the resolution determination processing unit 142 according to the present embodiment returns each block of the image data in the YUV space to the image data in the RGB space, the resolution determination processing unit 142 determines that the image data block of the unimportant component can be reduced. Then, it is determined that the image data block of the important component cannot be reduced. The pixel values of the coordinates (x, y) of the RGB space image data are respectively set as f_R (x, y), f_G (x, y), and f_B (x, y), and the coordinates (x , Y) are set to f_Y (x, y), f_U (x, y), and f_V (x, y), respectively, the relationships shown in Expressions (5) to (7) are established.

  From Expressions (5) to (7), the maximum absolute value of the coefficient of the pixel value f_Y (x, y) is 1. Further, the maximum absolute value of the coefficient of the pixel value f_U (x, y) is 1.772. Further, the maximum absolute value of the coefficient of the pixel value f_V (x, y) is 1.402. Therefore, if Expression (8) is satisfied, it is determined that the pixel value f_V (x, y) is not important, and it is determined that the V component block can be reduced. If Expression (9) is satisfied, it is determined that the pixel value f_U (x, y) is not important, and it is determined that the U component block can be reduced. If Expression (10) holds, it is determined that the pixel value f_Y (x, y) is not important, and it is determined that the Y component block can be reduced.

  Actually, whether the expressions (8) to (10) are satisfied is determined based on whether or not the expressions (11) to (13) are satisfied. TH11, TH12, and TH13 represent threshold values obtained experimentally.

  Referring to FIG. 17, each part of multi-resolution image compression apparatus 140 operates as follows. The processing of S102 to S106 is the same as the processing of S2 to S6 in FIG. Therefore, detailed description thereof will not be repeated here. Assume that the three pieces of image data stored in the color space conversion image buffer 36 are divided into nl × ml blocks as in the first embodiment. The U component weighting unit 144, the V component weighting unit 146, and the magnitude relation comparison unit 148 each extract one block from the nl × ml blocks (S108). The blocks selected by the U component weighting unit 144, the V component weighting unit 146, and the magnitude relation comparison unit 148 are the same.

  The magnitude relation comparison unit 148 stores the pixel value of the U component image data and the pixel value of the V component image data weighted by the U component weighting unit 144 and the V component weighting unit 146, and the color space conversion image buffer 36, respectively. Using the pixel value of the Y component image data, the number of pixels for which Expression (8) is established in the block is counted. It is determined whether or not the number of pixels for which Expression (8) is satisfied is greater than a predetermined threshold value TH8 (S110). If the number of pixels for which Expression (8) is satisfied is larger than the threshold value TH8 (YES in S110), the target block of the Y component image data can be reduced, so that the resolution information F_V (n, 0 is substituted into m) (S112). If the number of pixels for which Expression (8) is satisfied is equal to or less than the threshold value TH8 (NO in S110), it is impossible to reduce the target block of the Y component image data, and therefore the resolution information F_V (n , M) is substituted for 1 (S114).

  In parallel with the processing of S110 to S114, the magnitude relation comparison unit 148 similarly counts the number of pixels in which the equation (9) is established in the block, and the number of pixels in which the equation (9) is established is predetermined. It is determined whether or not the threshold value is greater than TH9 (S116). If the number of pixels for which Expression (9) is satisfied is larger than the threshold value TH9 (YES in S116), the target block of the U component image data can be reduced, so that the resolution information F_U (n, 0 is substituted into m) (S118). If the number of pixels for which Expression (9) is satisfied is equal to or smaller than the threshold value TH9 (NO in S116), it is impossible to reduce the target block of the U component image data, and therefore the resolution information F_U (n , M), 1 is substituted (S120).

  In parallel with the processing of S110 to S114 and the processing of S116 to S120, the magnitude relation comparison unit 148 similarly counts the number of pixels in which the expression (10) is established in the block, and the expression (10) It is determined whether the number of established pixels is larger than a predetermined threshold value TH10 (S122). If the number of pixels for which Expression (10) is satisfied is larger than the threshold value TH10 (YES in S122), the target block of the Y component image data can be reduced, so that the resolution information F_Y (n, 0 is substituted into m) (S124). If the number of pixels for which Expression (10) is satisfied is equal to or less than the threshold value TH10 (NO in S122), it is impossible to reduce the target block of the Y component image data, and therefore the resolution information F_Y (n , M), 1 is substituted (S126).

  After the processing of S110 to S126, the magnitude relation comparison unit 148 determines whether or not the currently processed block is the last block (nl-1, ml-1) (S128). If it is not the last block (NO in S128), the processing after S108 is repeated.

  If it is the last block (YES in S128), the magnitude relation comparison unit 148 determines each block (n, m) (n = 0 to nl-1, m = 0 to ml) obtained in the processing from S110 to S126. -1) resolution information F_Y (n, m), F_U (n, m) and F_V (n, m) are written into the resolution information buffer 40 (S130).

  Then, the process of S132-S138 is performed and joint coding data is output. The processing of S132 to S138 is the same as the processing of S54 to S60 in FIG. Therefore, detailed description thereof will not be repeated here.

  The joint encoded data obtained as described above is decompressed by the multi-resolution image decompression apparatus 100 similar to that of the first embodiment, so that the three input images of R, G, and B can be restored.

  The multi-resolution image compressing apparatus 140 according to the present embodiment reduces the Y component, U component, or V component image data, which is determined not to be important for restoring the RGB space image data at the time of expansion, for each block. For this reason, the multi-resolution image compression apparatus 140 can compress an image without causing color shift visually.

  Also in the present embodiment, as in the first embodiment, a plurality of reduction ratios may be determined in advance, and it may be determined whether or not image data can be reduced for each reduction ratio.

  The multi-resolution image compression apparatuses 20 and 140 and the multi-resolution image expansion apparatus 100 described above can be realized by a computer. Referring to FIG. 18, multiresolution image compression apparatus 20 or 140 or multiresolution image expansion apparatus 100 is calculated by computer 151, keyboard 155 and mouse 156 for giving instructions to computer 151, and computer 151. A display 152 for displaying results and the like, a magnetic tape device 153 for reading programs executed by the computer 151, a CD-ROM (Compact Disc-Read Only Memory) device 157, and a communication modem 159 are included.

  A program of the image compression method described with reference to FIGS. 3 to 5 and 17 and the image expansion method described with reference to FIG. 13 is a magnetic tape 154 or a CD-ROM 158 which is a recording medium readable by the computer 151. And read by the magnetic tape device 153 and the CD-ROM device 157, respectively. Alternatively, the data is read by the communication modem 159 via the communication line.

  Referring to FIG. 19, computer 151 includes a CPU (Central Processing Unit) 160 for executing a program read via magnetic tape device 153, CD-ROM device 157 or communication modem 159, and the operation of computer 151. ROM (Read Only Memory) 161 for storing other programs and data necessary for the program, RAM (Random Access Memory) 162 for storing programs, parameters at the time of program execution, calculation results, etc., programs and data And a magnetic disk 163 for storing the above.

  The program read by the magnetic tape device 153, the CD-ROM device 157 or the communication modem 159 is executed by the CPU 160.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a block diagram which shows the structure of the multi-resolution image compression apparatus which concerns on Embodiment 1 of this invention. 3 is a block diagram illustrating a configuration of a resolution determination processing unit according to Embodiment 1. FIG. 3 is a flowchart of multi-resolution image compression processing according to the first embodiment. 3 is a flowchart of multi-resolution image compression processing according to the first embodiment. 3 is a flowchart of multi-resolution image compression processing according to the first embodiment. It is the figure which divided | segmented image data into several blocks. It is a figure explaining the relationship between the block before reduction and the block after reducing and enlarging. It is a figure which shows resolution information. It is a figure for demonstrating function convert (). It is a figure for demonstrating the production method of reduction data and equal magnification data. It is a figure for demonstrating joint encoding data. It is a block diagram which shows the structure of a multi-resolution image expansion | extension apparatus. It is a flowchart of a multi-resolution expansion process. It is a figure for demonstrating the method of producing the image data of a Y component, a U component, and a V component by joining enlarged data and equal magnification data based on resolution information. 6 is a diagram illustrating a configuration of a multi-resolution image compression apparatus according to Embodiment 2. FIG. 6 is a diagram illustrating a configuration of a resolution determination processing unit according to Embodiment 2. FIG. 10 is a flowchart of multi-resolution image compression processing according to the second embodiment. 1 is an external view of a computer that implements a multi-resolution image compression apparatus or multi-resolution image expansion apparatus. It is a block diagram which shows the internal structure of a computer. It is a figure for demonstrating the problem of the conventional thinning-out method.

Explanation of symbols

  20 multi-resolution image compression device, 22 image input device, 24 image compression device, 26 data output device, 34 color space conversion processing unit, 38, 142 resolution determination processing unit, 42 resolution information encoding processing unit, 44 data creation processing unit 48A reduced data encoding processing unit, 48B same size data encoding processing unit, 50 encoded data splicing processing unit, 64 image reduction processing unit, 68 image enlargement processing unit, 72 region error calculation processing unit, 74 region resolution creation processing 78, luminance component edge detection processing unit, 80 resolution information change processing unit, 82 color difference component quantization processing unit, 84 resolution information change processing unit, 100 multi-resolution image expansion device, 102 data input device, 106 encoded data separation processing 110, resolution information data decoding unit, 116A reduced data decoding processing unit, 116B equal size data No. processor, 119 data enlargement unit, 120 resolution data bonding process section, 124 image conversion processing section, 128 an image output device, 144 U component weighting unit, 146 V component weighting unit, 148 size relationship comparator unit.

Claims (6)

An image compression apparatus for compressing luminance component image data and color difference component image data each composed of a plurality of blocks,
For each of the luminance component image data and the chrominance component image data, resolution determination processing means that adaptively determines the resolution after reduction according to a predetermined standard for each block, and creates resolution information;
Data creation processing means connected to the resolution determination processing means for creating data obtained by reducing the respective blocks as necessary for each of the luminance component image data and the color difference component image data based on the resolution information; ,
An encoding processing unit that is connected to the data generation processing unit and encodes the data generated by the data generation processing unit;
The resolution determination processing means includes:
For each of a plurality of blocks, the reduced resolution is blocked according to the degree of influence when the luminance component image data and the color difference component image data are converted into red component, green component, and blue component image data. And create the resolution information ,
The luminance component image data is Y component image data, the color difference component image data is U component image data and V component image data,
The pixel value of coordinates (x, y) in each block of the V component image data is f_V (x, y), and the pixel value of coordinates (x, y) in each block of the U component image data is f_U (. x, y) and the pixel value of coordinates (x, y) in each block of the Y component image data is f_Y (x, y),
The resolution determination processing means includes:
When the number of pixels satisfying the following expression (A1) among the pixels of each block of the V component image data is greater than a predetermined first number, the reduced resolution of the block of the V component image data Is defined as the resolution before the reduction is reduced at a predetermined reduction rate,
When the number of pixels satisfying the following expression (A2) among the pixels of each block of the U component image data is greater than a predetermined second number, the reduced resolution of the block of the U component image data The resolution before reduction is defined as the resolution reduced at the predetermined reduction rate,
When the number of pixels satisfying the following expression (A3) among the pixels of each block of the Y component image data is larger than a predetermined third number, the reduced resolution of the block of the Y component image data The resolution before reduction is defined as the resolution reduced at the predetermined reduction rate.

However, a, b, TH_V, TH_U, and TH_Y are predetermined values,
The predetermined reduction ratio is 1/2,
The predetermined value a is 1.772.
The image compression apparatus, wherein the predetermined value b is 1.402 . An image expansion device that decodes encoded data encoded by the image compression device according to claim 1 and reproduces the image data,
Data decoding processing means for decoding the encoded data that has been encoded;
An image decompression apparatus, comprising: an image creation processing means connected to the data decoding processing means and enlarging the encoded data to have a resolution before reduction based on resolution information to create image data. An image compression method for compressing luminance component image data and color difference component image data each composed of a plurality of blocks,
For each of the luminance component image data and the color difference component image data, adaptively determining a reduced resolution for each block according to a predetermined standard, and creating resolution information;
For each of the luminance component image data and the color difference component image data, creating data obtained by reducing each block as necessary based on the resolution information;
Encoding the generated data,
The step of creating resolution information includes:
For each of a plurality of blocks, the reduced resolution is blocked according to the degree of influence when the luminance component image data and the color difference component image data are converted into red component, green component, and blue component image data. Defining the resolution information and creating the resolution information ,
The luminance component image data is Y component image data, the color difference component image data is U component image data and V component image data,
The pixel value of coordinates (x, y) in each block of the V component image data is f_V (x, y), and the pixel value of coordinates (x, y) in each block of the U component image data is f_U (. x, y) and the pixel value of coordinates (x, y) in each block of the Y component image data is f_Y (x, y),
The step of creating the resolution information comprises:
When the number of pixels satisfying the following expression (A1) among the pixels of each block of the V component image data is greater than a predetermined first number, the reduced resolution of the block of the V component image data Is defined as the resolution before the reduction is reduced at a predetermined reduction rate,
When the number of pixels satisfying the following expression (A2) among the pixels of each block of the U component image data is greater than a predetermined second number, the reduced resolution of the block of the U component image data The resolution before reduction is defined as the resolution reduced at the predetermined reduction rate,
When the number of pixels satisfying the following expression (A3) among the pixels of each block of the Y component image data is larger than a predetermined third number, the reduced resolution of the block of the Y component image data The resolution before reduction is defined as the resolution reduced at the predetermined reduction rate.

However, a, b, TH_V, TH_U, and TH_Y are predetermined values,
The predetermined reduction ratio is 1/2,
The predetermined value a is 1.772.
The image compression method , wherein the predetermined value b is 1.402 . An image expansion method for decoding encoded data encoded by the image compression method according to claim 3 and reproducing the image data,
Decoding the encoded data encoded;
And enlarging the encoded data to have a resolution before reduction based on the resolution information, and creating image data. A computer-readable recording medium storing an image compression program for causing a computer to execute the image compression method according to claim 3 . A computer-readable recording medium on which an image expansion program for causing a computer to execute the image expansion method according to claim 4 is recorded.

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