JP3989343B2 - Image processing apparatus, image processing method, and recording medium on which image processing method is recorded - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an application program, a device driver such as a printer driver, an image processing apparatus that performs compression encoding of color image data in a device that handles color images, an image processing method thereof, and a recording medium on which the image processing method is recorded.
[0002]
[Prior art]
  Conventionally, in the image encoding device described in Patent Document 1, by setting a plurality of maximum quantization step values for each frequency distribution information, a larger amount of code can be obtained in a block having a large influence of quantization processing. It can be assigned, and thereby excessive image quality deterioration can be suppressed.
[0003]
[Patent Document 1]
          JP-A-10-112860
[0004]
  Conventionally, the commonly used JPEG baseline system is image coding using orthogonal transform for a color image, a zigzag scan from the low frequency component side, and a quantization step by multiplying a quantization table by a scaling factor value The procedure is such that linear quantization is performed using the value, and the quantized value is encoded sequentially from the DC component.
[0005]
  In the case of a color image, the quantization table used for the above quantization can be switched for each component to use an appropriate quantization table. This quantization table can be changed by changing each coefficient itself in the quantization table or by changing a scaling factor value for performing quantization weighting, thereby changing the compression ratio. .
[0006]
  Conventionally, there is a method of simultaneously satisfying both the improvement of the compression efficiency and the reproduction of high image quality in all image quality modes by having a plurality of quantization tables according to the compression rate. However, this method has a problem that a large memory capacity is required to hold the quantization table.
[0007]
  Therefore, there is a need for a method that satisfies both the improvement of compression efficiency and the reproduction of high image quality by changing the scaling factor value using one of the initially set quantization tables or two of luminance and color difference. The
[0008]
  In general, in the quantization table, a small value is set for the low frequency component, considering that the human visual characteristics are sensitive to the low frequency component but insensitive to the high frequency component. Is set to a large value.
[0009]
  The scaling factor value is initially set according to the image quality mode (for example, low image quality, standard image quality, high image quality, etc.), or is changed based on the code amount by first scanning the image data.
[0010]
  However, with a single quantization table, it is not possible to simultaneously satisfy both the improvement of the compression efficiency and the reproduction of high image quality in all image quality modes only by changing the scaling factor value.
[0011]
  Therefore, when the image quality is set low (the compression rate is high), the image quality is excessively deteriorated. When the image quality is set high (the compression rate is low), the compression efficiency is poor.
[0012]
  Therefore, in the image encoding device described in Patent Document 1, by setting a plurality of maximum quantization step values in accordance with each frequency component region, more codes can be obtained in a block having a large influence by the quantization process. It is possible to allocate an amount, thereby suppressing excessive image quality deterioration while maintaining compression efficiency.
[0013]
  There are many methods for suppressing excessive image quality degradation caused by high compression, such as the method in the image encoding device described in Patent Document 1. However, a control method using a quantization step width for suppressing a decrease in compression efficiency that occurs when low compression is used has not been proposed.
[0014]
[Problems to be solved by the invention]
  The present invention has been made in view of the above problems, and an image processing apparatus capable of increasing compression efficiency while maintaining image quality by using an optimal quantization step width even when high image quality is set. Another object of the present invention is to provide an image processing method and a recording medium on which the image processing method is recorded.
[0015]
[Means for Solving the Problems]
  In order to achieve the above object, the invention according to claim 1A quantization table storing preset quantization step widths, a setting unit for setting an image quality mode, an orthogonal transform unit for performing orthogonal transform on image data composed of a luminance component and a color difference component, and orthogonal by an orthogonal transform unit Based on the image area separation means that performs image area separation processing on the converted data into character area and photo area, and the quantization table based on the quantization table and the scaling factor value set by the image quality mode, according to the image area separation result Limit value setting means for setting the quantization step width using the limit value set only for the frequency component of the color difference component, and orthogonal transform by the orthogonal transform means based on the quantization step width set by the limit value setting means An image processing apparatus having a quantization means for quantizing the quantized data and an encoding means for encoding the quantized image data, wherein the color difference component of the character area Lower limit, the image processing apparatus and greater than the lower limit value used for the chrominance components of the photographic area usedIt is about.
[0016]
  Claim2The described invention is claimed.1In the described invention,SystemThe limit value setting means isstraightA different limit value is set for each conversion coefficient that is a coefficient of image data orthogonally converted by the alternating conversion means.
[0017]
  Claim3The described invention is claimed.1In the described invention,SystemThe limit value setting means isstraightThe coefficients of the image data orthogonally transformed by the alternating transform means are grouped for each identical frequency component, and a limit value is set according to the characteristics of the frequency component in the group.
[0018]
  Claim4The described invention is claimed.1In the described invention,PictureImage data consists of a plurality of components.SystemIt is characterized by setting a limit value.
[0019]
  Claim5The described invention is claimed.4In the invention described inDoubleThe number component consists of luminance and chrominance components,colorTo the difference componentSystemIt is characterized by setting a limit value.
[0020]
  Claim6The described inventionA storage process for storing a quantization step width preset for each region in the quantization table, a setting process for setting an image quality mode, and an orthogonal transform process for performing orthogonal transform on image data composed of a luminance component and a color difference component And an image area separation process for performing image area separation processing on the data orthogonally transformed in the orthogonal transformation process into a character area and a photographic area, and a quantization table and a scaling factor value set by an image quality mode. , Image area Based on the limit value setting step that sets the quantization step width using the limit value that is set only for the frequency component of the color difference component according to the separation result, and the orthogonal transformation based on the quantization step width that is set in the limit value setting step An image processing method having a quantization step for quantizing the data orthogonally transformed in the step and an encoding step for encoding the quantized image data, wherein the lower limit value used for the color difference component of the character region is An image processing method characterized by being larger than a lower limit value used for a color difference component of a photographic regionIt is about.
[0021]
  Claim7The described invention is claimed.6In the described invention,SystemIn the limit value setting process,straightA different limit value is set for each conversion coefficient which is a coefficient of image data orthogonally converted in the cross conversion process.
[0022]
  Claim8The described invention is claimed.6In the described invention,SystemIn the limit value setting process,straightThe coefficients of the image data orthogonally transformed in the alternating transformation process are grouped for each identical frequency component, and a limit value is set according to the characteristics of the frequency component in the group.
[0023]
  Claim9The described invention is claimed.6In the described invention,PictureImage data consists of a plurality of components.SystemIt is characterized by setting a limit value.
[0024]
  Claim10The described invention is claimed.9In the invention described inDoubleThe number component consists of luminance and chrominance components,colorTo the difference componentSystemIt is characterized by setting a limit value.
[0025]
  Claim11The described invention is claimed.6From10A program for executing the image processing method according to any one of the above.
[0026]
  Claim12The described invention is claimed.11Described inNoIt is a recording medium that records a program and can be read by a computer.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing a configuration of an embodiment of an image processing apparatus of the present invention. As shown in FIG. 1, an HDD 10, a RAM 20 (inside a PC), a CPU 30 (inside a PC), and a printer 40 are connected via a data bus. When the original image is printed out, the CPU 30 of the PC communicates with the CPU 50 of the printer 40 via a data path, and the image quality level (or compression rate) corresponding to the predetermined performance of the printer 40 is determined. Compression ratio). The image data is compressed by the PC according to the determined image quality level (compression rate), and the compressed data is transmitted to the printer 40. Since the amount of data transmitted to the printer 40 is reduced, the transmission time is shortened, and high-speed printing is possible even when the time required for compression / expansion is taken into account. Here, PC means a personal computer.
[0028]
  Next, the operation of the image processing apparatus will be described. First,(1)The original image recorded on the HDD 10 is read onto the RAM 20 by a command from the CPU 30.(2)The CPU 30 reads an image on the RAM 20 and compresses it.(3)The CPU 30 writes the compressed data in another area on the RAM 20.(4)The compressed data is recorded on the RAM 60 in the printer 40 in accordance with a command from the CPU 30.(5)The CPU 50 in the printer 40 reads the compressed data, obtains a decoded value, and decompresses the image.(6)The CPU 50 writes the decompressed data on the RAM 60.(7)Thereafter, the printer 40 prints out the decompressed data in a predetermined procedure [magnification is required]. here,bracketsThe numbers surrounded by the numbers indicate the order of operations in Fig. 1.Indicationis doing.
[0029]
  FIG. 2 is a block diagram showing the configuration of the first embodiment of the image processing apparatus of the present invention. The first embodiment will be described with reference to FIG. Here, the image data is composed of RGB primary color signals, luminance and two types of color difference signals.
[0030]
  First, the image data is divided into a plurality of preset blocks, and orthogonal transformation is performed on the block data.
[0031]
  Next, the orthogonally transformed image data is input to the quantizer. Here, as the orthogonal transformation, discrete cosine transformation (DCT) is generally used, but other transformations such as DWT and Hadamard transformation may be used.
[0032]
  Next, the quantization table storing the initial quantization step width is multiplied by the scaling factor value set according to the image quality mode, and the limit value is used for the multiplied quantization table, and the provisional quantization step width is set. Ask. Then, quantization is performed by dividing the transformed coefficient by the obtained quantization step width.
[0033]
  Here, the image data may be scanned twice to calculate the scaling factor value. In the first scan, a provisional code amount is calculated, a scaling factor value corresponding to the code amount is calculated, and in the second scan, actual encoding is performed using the calculated scaling factor value. There is also a method.
[0034]
  Finally, the quantized transform coefficient is converted into variable length data or fixed length data by a variable length coding method such as run length or Huffman code or a fixed length coding method, and a variable length code corresponding to the quantized transform coefficient value Word or fixed-length codeword is output as compressed data.
[0035]
  FIG. 3 is a block diagram showing the configuration of the second embodiment of the image processing apparatus of the present invention. The second embodiment will be described with reference to FIG. The image data is composed of RGB primary color signals, luminance and two types of color difference signals.
[0036]
  First, the image data is divided into a plurality of preset blocks, and orthogonal transformation is performed on the block data.
[0037]
  Next, as is well known, image area separation is performed using image data subjected to orthogonal transformation, and a quantization table set for each area is selected. Then, the image data is input to the quantizer. As an example of this known technique, there is an “image processing method and apparatus” described in JP-A-9-27904.
[0038]
  Here, as the orthogonal transform, discrete cosine transform (DCT) is generally used, but other transforms may be used. In the image area separation, the area may be determined by using block data.
[0039]
  Next, the quantization table storing the initial quantization step width is multiplied by the scaling factor value set according to the image quality mode, and the limit value obtained by determining the region is used for the multiplied quantization table. The provisional quantization step width is obtained. Then, quantization is performed by dividing the transformed coefficient by the obtained quantization step width.
[0040]
  Finally, the quantized transform coefficient is converted into variable length data or fixed length data by a variable length coding method such as run length or Huffman code or a fixed length coding method, and a variable length code corresponding to the quantized transform coefficient value Word or fixed-length codeword is output as compressed data.
[0041]
  FIG. 4 is a block diagram showing the configuration of the third embodiment of the image processing apparatus of the present invention. A third embodiment will be described with reference to FIG. The image data is composed of RGB primary color signals, luminance and two types of color difference signals.
[0042]
  First, the image data is divided into a plurality of preset blocks, and orthogonal transformation is performed on the block data. Here, as the orthogonal transform, discrete cosine transform (DCT) is generally used, but other transforms may be used.
[0043]
  Further, as is well known, the user designates the selected area information for designating the importance of each area from the outside, so that the area is designated as a high importance area and a low importance area. As an example of this known technique, there is "Image data encoding method and apparatus" disclosed in Japanese Patent Laid-Open No. 6-164941.
  Then, a quantization table set in advance for each region is selected.
[0044]
  Next, the quantization table storing the initial quantization step width is multiplied by the scaling factor value set according to the image quality mode, and the limit value obtained by determining the region is used for the multiplied quantization table. The provisional quantization step width is obtained. Then, quantization is performed by dividing the transformed coefficient by the obtained quantization step width.
[0045]
  Finally, the quantized transform coefficient is converted into variable length data or fixed length data by a variable length coding method such as run length or Huffman code or a fixed length coding method, and a variable length code corresponding to the quantized transform coefficient value Word or fixed-length codeword is output as compressed data.
[0046]
  FIG. 18 is a flowchart showing an encoding operation in the first embodiment of the image processing apparatus of the present invention. The operation in the first embodiment of the image processing apparatus of the present invention will be described with reference to FIG. The mode in which the image data is compressed is selected from preset modes (for example, low image quality, standard image quality, high image quality, etc.). These modes are selected by the user from a menu on the PC screen or set according to the performance of the printer. When the image data is encoded as the selected mode, the image data is DCT-converted in predetermined block units. Then, quantization is performed using a different quantization table set in advance from the converted data.
[0047]
  First, when the image data is composed of a luminance Y component and color difference Cb and Cr components, each component is divided into blocks of 8 × 8 pixels (step S1801), and the divided 8 × 8 pixel blocks are divided. DCT conversion is performed every time (step S1802).
[0048]
  Next, a preset quantization table is selected depending on in which image quality mode the converted data is encoded. That is, the quantization table is multiplied by the scaling factor, and quantization is performed using the quantization table in which the lower limit value is set in the low frequency region (step S1803).
[0049]
  Here, for example, when initializing the quantization table of the luminance Y component and the color differences Cb and Cr components as shown in FIGS. 5 and 6 and encoding the scaling factor value in the high image quality mode (low compression), 1 / 2. Assume that 1 / 1.5 is set when encoding in the low image quality mode (high compression). Note that the scaling factor value depends on the performance of the output device regardless of the target compression, such as 1/2 when the output device performance is good and 1 / 1.5 when the output device performance is poor. Setting is also possible.
[0050]
  Now, the quantization table of the luminance Y component in the high image quality mode, the color difference Cb and Cr components, and the provisional quantization table obtained by multiplying the scaling factor values are as shown in FIGS. Also, the quantization table of the luminance Y component, the color difference Cb and Cr components in the low image quality mode, and the provisional quantization table obtained by multiplying the scaling factor values are as shown in FIGS.
[0051]
  Further, the frequency components are divided into two groups as shown in FIGS. 11 and 12, for example, FIG. 11 is based on the low frequency components and FIG. 12 is based on the high frequency components. A lower limit value is set for the frequency component shown in FIG. For example, when the lower limit value 9 is set for the color differences Cb and Cr, the provisional quantization table is as shown in FIG.
[0052]
  Therefore, in the high image quality mode, the luminance Y is based on the quantization table in FIG. 7, the color differences Cb and Cr are in FIG. 13, and in the low image quality mode, the luminance Y is in FIG. 9, and the color differences Cb and Cr are quantized based on the quantization table in FIG. To do. Quantization is realized by dividing the transformed data by the coefficient of the quantization table.
[0053]
  Finally, the quantized transform coefficient is converted into variable-length data or fixed-length data by a variable-length encoding method such as run length or Huffman code or a fixed-length encoding method in units of blocks (step S1804), and the entire screen ends ( In step S1805), a variable length codeword or a fixed length codeword corresponding to the quantized transform coefficient value is output as compressed data.
[0054]
  FIG. 19 is a flowchart showing an operation of setting and encoding a lower limit value for an area depending on the types of different image areas such as a character area and a photographic area in the second embodiment of the image processing apparatus of the present invention. The operation in the second embodiment of the image processing apparatus of the present invention will be described with reference to FIG. In this embodiment, image data is encoded as a high image quality (low compression) mode. At that time, the image data is subjected to DCT conversion in units of predetermined blocks, image area separation is performed from the converted data, and quantization is performed using different quantization tables set in advance for each area.
[0055]
  First, when the image data is composed of a luminance Y component and color difference Cb and Cr components, each component is divided into blocks of 8 × 8 pixels (step S1901), and the divided 8 × 8 pixel blocks are divided. DCT conversion is performed every time (step S1902).
[0056]
  Next, the image area is separated into a known character area and photographic area by the converted data. As an example of this known technique, there is an “image processing method and apparatus” described in JP-A-9-27904 described above. Then, a quantization table set in advance for each region is selected.
[0057]
  Here, for example, as shown in FIGS. 5 and 6, it is assumed that the quantization table of the luminance Y component and the color differences Cb and Cr components is initialized. In addition, since the character region that is prominent in image degradation is compressed to a low level and the photo region that is not prominent in image degradation is compressed to a high level, the scaling factor value is 1/2 for the character region and 1/1 for the photo region. .5 is set.
[0058]
  In that case, the provisional quantization table obtained by multiplying the luminance Y component, the color difference Cb, Cr component quantization table, and the scaling factor value in the character area is as shown in FIGS. Also, the temporary quantization table obtained by multiplying the luminance Y component, the color difference Cb, Cr component quantization table, and the scaling factor value in the photographic area is as shown in FIGS.
[0059]
  Then, the frequency component is divided into two groups as shown in FIGS. 11 and 12 (FIG. 11 is a low frequency component group and FIG. 12 is a high frequency component group), and only the lower limit value is applied to the frequency components of the color difference Cb and Cr components. Set. Further, different lower limit values are set according to the types of different image areas such as a character area and a photo area (step S1903) (steps S1904 and S1905).
[0060]
  This is because the amount of information of low frequency components in the character region becomes considerably large by compressing the character region low and compressing the photo region high.
[0061]
  For example, the lower limit value used for the color difference Cb and Cr component of the character area is 9 and the lower limit value used for the color difference Cb and Cr component of the photo area is 8, and the lower limit value used is larger in the photo area than the character area. It is assumed that it is set in advance. In that case, the provisional tables in which the lower limit value is set are as shown in FIGS.
[0062]
  Therefore, luminance Y is quantized based on the quantization table in FIG. 7 for the character area, color differences Cb and Cr are based on the quantization table in FIG. 14, and luminance Y is in FIG. 9 and color differences Cb and Cr are in the photo area in FIG. (Step S1906). Quantization is realized by dividing the transformed data by the coefficient of the quantization table.
[0063]
  Finally, the quantized transform coefficient is converted into variable-length data or fixed-length data by a variable-length encoding method such as run length or Huffman code or a fixed-length encoding method in units of blocks (step S1907), and the entire screen ends ( In the case of step S1908), the variable length codeword or fixed length codeword corresponding to the quantized transform coefficient value is output as compressed data.
[0064]
  FIG. 20 is a flowchart showing an operation of performing area designation and encoding in the third embodiment of the image processing apparatus of the present invention. The operation in the third embodiment of the image processing apparatus of the present invention will be described with reference to FIG. In this embodiment, image data is encoded as a high image quality (low compression) mode. At this time, the image data is subjected to DCT conversion in a predetermined block unit, and the converted data is designated by the user from the outside, and is quantized using a different quantization table set in advance for each area.
[0065]
  First, when the image data is composed of a luminance Y component and color difference Cb and Cr components, each component is divided into blocks of 8 × 8 pixels (step S2001), and the divided 8 × 8 pixel blocks are divided. DCT conversion is performed every time (step S2002).
[0066]
  As is well known, the user designates the importance of the area from the outside as a high importance area and a low importance area (step S2003). As an example of this known technique, there is an “image data encoding method and apparatus” described in JP-A-6-164941 described above. Then, a quantization table set in advance for each region is selected.
[0067]
  Here, for example, as shown in FIGS. 5 and 6, it is assumed that the quantization table of the luminance Y component and the color differences Cb and Cr components is initialized. Also, because the high importance area has a low compression ratio and the low importance area has a high compression ratio, the quantization step width is reduced when the compression ratio is low, and the quantization step width when the compression ratio is high. Increase For example, it is assumed that the scaling factor value is set to 1/2 when encoding a region with high importance and 1 / 1.5 when encoding a region with low importance.
[0068]
  In this case, the provisional quantization tables obtained by multiplying the luminance Y component, the color difference Cb and Cr component quantization tables, and the scaling factor values in the region of high importance are as shown in FIGS. In addition, the provisional quantization tables obtained by multiplying the luminance Y component, the color difference Cb and Cr component quantization tables, and the scaling factor values in the area of low importance are as shown in FIGS.
[0069]
  Then, for example, the frequency component is divided into two groups as shown in FIGS. 11 and 12 (FIG. 11 is based on the low frequency component and FIG. 12 is based on the high frequency component), and the color difference Cb and Cr component frequency components have lower limit values. Set. Further, different lower limit values are set according to different importance areas such as a higher importance area and a lower importance area (steps S2004 and S2005).
[0070]
  This is because a region with high importance has a low compression rate and a region with low importance has a high compression rate, so that a region with high importance has a larger amount of information of low frequency components.
[0071]
  For example, the lower limit value used for the color difference Cb and Cr component in the high importance area is 9 and the lower limit value used for the color difference Cb and Cr component in the low importance area is 8; Assume that the lower importance area is set larger than the higher area. In that case, the provisional tables in which the lower limit value is set are as shown in FIGS.
[0072]
  Accordingly, the luminance Y is in the quantization table of FIG. 7, the color differences Cb and Cr are in FIG. 16, and the luminance Y is in FIG. 9 and the color differences Cb and Cr are in the quantization table of FIG. Based on this, quantization is performed (step S2006). Quantization is realized by dividing the transformed data by the coefficient of the quantization table.
[0073]
  Finally, the quantized transform coefficient is converted into variable-length data or fixed-length data by a variable-length encoding method such as run length or Huffman code or a fixed-length encoding method in units of blocks (step S2007), and the entire screen ends ( In step S2008), a variable length codeword or a fixed length codeword corresponding to the quantized transform coefficient value is output as compressed data.
[0074]
  As described above, the present invention is an image processing apparatus that compresses and encodes image data, and includes a conversion unit that performs frequency conversion, a quantization unit that performs quantization processing, and a change that changes a quantization step. Means and limit value setting means for setting a limit value for the quantization step width. Here, by setting a limit value, both image quality and compression can be achieved.
[0075]
  In general, if the amount of code that can be encoded is the same, the information of the high frequency component is more subject to discarding (more roughening the quantization step width), and the target to leave the information of the low frequency component (more quantization step width) ). This is because human vision is a low-pass filter, and information on low-frequency components greatly contributes to image quality.
[0076]
  However, if the perceivable range is exceeded, even if a lot of low frequency component information is left, it may hardly contribute to the image quality. Furthermore, when orthogonal transformation such as DCT transformation is performed, numerical values are concentrated on low-frequency components in many images, and small values appear on high-frequency components. There arises a problem that the compression efficiency is lowered. Therefore, at the time of encoding, an optimum quantization step width can be obtained by suppressing the quantization step width of low frequency components to some extent at an image quality level that cannot be perceived by humans.
[0077]
  Therefore, in the present invention, the limit value is the lower limit value. By using the lower limit value, it is possible to control the quantization step width and to increase the compression efficiency at an image quality level that cannot be perceived.
[0078]
  The question is what is the “range that humans cannot perceive” described above. For example, there is PSNR as an evaluation index for objectively evaluating image quality. Generally, a PSNR value of up to about 40 dB is a perceivable range of human image quality degradation, and a PSNR value higher than that is hardly perceivable.
[0079]
  On the other hand, each output device has an image quality level, and in the case of an output device that outputs with high image quality, it is in a range where image quality deterioration cannot be perceived. Therefore, when encoding, the compression efficiency can be improved while maintaining the image quality by setting the lower limit value according to the performance of the output device.
[0080]
  Therefore, in the present invention, the lower limit value is changed according to the performance of the output device that outputs the image data. In addition, depending on the performance of the output device, an optimal quantization step width can be obtained, and compression efficiency can be increased while maintaining image quality.
[0081]
  In addition, even when the target compression rate is considerably low, it is in a range in which image quality degradation can hardly be perceived. Therefore, when encoding, an optimal quantization step width can be obtained by setting a lower limit value in accordance with a target compression rate, and compression efficiency can be increased while maintaining image quality.
[0082]
  Therefore, in the present invention, the lower limit value is changed according to the target compression rate. An object of the present invention is to obtain an optimum quantization step width by responding to a target compression rate, and to improve compression efficiency while maintaining image quality.
[0083]
  According to human visual characteristics, the lower the frequency, the smaller the quantization step width. Therefore, in order to quantize the frequency component characteristics with the optimum quantization step width, a different quantization step width is used for each frequency component.
[0084]
  Therefore, in the present invention, the lower limit value is set different for each frequency conversion coefficient that is a coefficient of the frequency-converted image data. Further, by using the lower limit value according to the frequency characteristics, it is possible to increase the compression efficiency while maintaining the image quality.
[0085]
  When setting a limit value for the wave number component, it is easier to control and to simplify the configuration by grouping together similar frequency components and setting a limit value that matches the characteristics of the frequency components of the group.
[0086]
  Therefore, the present invention divides the frequency conversion coefficient into a plurality of groups. By dividing into a plurality of groups, it is easy to control and the configuration can be simplified.
[0087]
  When an image is composed of a plurality of components (three components of R, G, and B, three components of luminance Y, color difference Cb, and color difference Cr), the visual characteristics are different for each component. It is known. For example, the color difference Cb and Cr components are less likely to be perceived by the human eye than the luminance Y component. For this reason, the color difference Cb and Cr components generally make the quantization step width coarser than the luminance Y component.
[0088]
  For example, when encoding, if the target compression rate is low, such as the high image quality mode, the quantization step width of the color difference component is made finer than when the target compression rate is low, such as in the low image quality mode. . However, since the color difference component is difficult to perceive, even if the amount of codes that can be encoded increases, the compression efficiency is deteriorated even though it does not contribute much to the image quality.
[0089]
  Here, the luminance Y component, the color difference Cb component, and the color difference Cr component are components of luminance and color difference obtained by converting the three components of RGB by the following formula, and are widely used in JPEG. Color conversion.
Luminance Y component = 0.29R + 0.587G + 0.114B
Color difference Cb component = 0.5R-0.4187G-0.08113B
Color difference Cr component = -0.1687R-0.3313G + 0.5B
[0090]
  Therefore, the present invention sets a limit value for each component. By considering the component characteristics, the compression efficiency can be increased while maintaining the image quality.
[0091]
  There are cases where images having different properties or images having different needs are mixed in one screen. When a mixed image is targeted, it is necessary to perform quantization using a quantization step width that matches the properties or needs of each image. For this reason, an image in a predetermined area is identified, and the quantization step width is reduced to achieve high image quality or the quantization step width is increased depending on the nature and necessity of the image.
[0092]
  Therefore, the present invention includes determination means for determining the nature of the image in the predetermined area, and sets a lower limit value according to the result determined by the determination means. Further, the image quality can be further maintained by using a quantization step width that takes into account the characteristics of the region.
[0093]
  For example, conventionally, there is a case where a region from a high importance to a low region is mixed in one screen. When such an image is targeted, there is a demand for higher image quality in a region of higher importance. Decrease the quantization step width in the important area and increase the quantization step width in the unimportant area. That is, the quantization step width is reduced in the region of higher importance.
[0094]
  However, when the quantization step width is made too small, there is a problem that the information in the low frequency region becomes too much and the compression efficiency is deteriorated. Therefore, it is necessary to improve a region where the quantization step width is reduced, such as a region having high importance.
[0095]
  Therefore, according to the present invention, the determination unit determines the importance of the image. In addition, the image quality can be improved in a region having a high degree of importance.
[0096]
  In addition, for example, there may be a case where a character or line drawing area, a photo area, a natural image area, or a computer graphics area is mixed in one screen. When such an image is a target, there is a demand for improving the image quality in a region where image quality degradation is conspicuous.
[0097]
  Therefore, it is necessary to vary the quantization step width depending on the type of input image. For example, the quantization step width of a photographic area where image quality deterioration is relatively unnoticeable is increased, and the quantization step width of a character or line drawing area where image quality deterioration is relatively conspicuous is reduced. Similarly, it is known to increase the quantization step width of a natural image region where image quality degradation is relatively inconspicuous and to reduce the quantization of a computer graphics region where image quality degradation is relatively conspicuous.
[0098]
  However, when the quantization step width is made too small, there is a problem that the information in the low frequency region becomes too much and the compression efficiency is deteriorated.
[0099]
  Therefore, in the present invention, this determination means determines the type of the input image. Further, the image quality can be maintained more by using the quantization step width corresponding to the type of the input image.
[0100]
  The present invention also proposes an image processing method for executing the above-described contents. By setting the limit value, both image quality and compression can be achieved.
[0101]
  The image processing method can be executed by a program.
[0102]
  Furthermore, the program can be recorded on an information recording medium that can be read by a computer.
[0103]
  Although the embodiment of the present invention has been described in detail as described above, the above-described embodiment is a preferred embodiment of the present invention, and is not limited thereto, and does not depart from the gist of the present invention. It is possible to implement with various modifications.
[0104]
【The invention's effect】
  As described above, according to the embodiment of the image processing apparatus of the present invention, it is possible to achieve both image quality and compression by setting a limit value.
[0105]
  Further, according to the embodiment of the image processing apparatus of the present invention, by using the lower limit value, the quantization step width can be controlled, and the compression efficiency at an image quality level that cannot be perceived can be increased.
[0106]
  In addition, according to the embodiment of the image processing apparatus of the present invention, an optimal quantization step width can be obtained according to the performance of the output device, and the compression efficiency can be increased while maintaining the image quality.
[0107]
  Further, according to the embodiment of the image processing apparatus of the present invention, an optimal quantization step width can be obtained by responding to a target compression rate, and compression efficiency can be improved while maintaining image quality. .
[0108]
  According to the embodiment of the image processing apparatus of the present invention, the compression efficiency can be improved while maintaining the image quality more by using the lower limit value according to the frequency characteristic.
[0109]
  Further, according to the embodiment of the image processing apparatus of the present invention, by dividing into a plurality of groups, it is easy to control and the configuration can be simplified.
[0110]
  Further, according to the embodiment of the image processing apparatus of the present invention, by setting only a part of a plurality of groups, it is easy to control and the configuration can be simplified.
[0111]
  Further, according to the embodiment of the image processing apparatus of the present invention, the compression efficiency can be improved while maintaining the image quality more by considering the component characteristics.
[0112]
  Further, according to the embodiment of the image processing apparatus of the present invention, the compression efficiency can be increased while maintaining the image quality by using only the color difference component.
[0113]
  Further, according to the embodiment of the image processing apparatus of the present invention, the image quality can be further maintained by using the quantization step width that takes into consideration the nature of the region.
[0114]
  Further, according to the embodiment of the image processing apparatus of the present invention, it is possible to improve the image quality in a region having a high degree of importance.
[0115]
  Further, according to the embodiment of the image processing apparatus of the present invention, the image quality can be further maintained by using the quantization step width corresponding to the type of the input image.
[0116]
  Further, according to the embodiment of the image processing method of the present invention, it is possible to achieve both image quality and compression by setting a limit value.
[0117]
  Further, according to the embodiment of the present invention, in a computer-readable information recording medium on which an image processing method is recorded, it is possible to increase compression efficiency while maintaining image quality by each process of the image processing apparatus. System can be provided.
[0118]
  Further, according to the embodiment of the present invention, in the computer-readable information recording medium program recording the image processing method, the compression efficiency is improved while maintaining the image quality by each process of the image processing apparatus. A system can be provided that enables it.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an embodiment of an image processing apparatus of the present invention.
FIG. 2 is a block diagram showing the configuration of the first embodiment of the image processing apparatus of the present invention.
FIG. 3 is a block diagram showing a configuration of a second embodiment of the image processing apparatus of the present invention.
FIG. 4 is a block diagram showing a configuration of a third embodiment of the image processing apparatus of the present invention.
FIG. 5 is a quantization table of luminance Y components used in the image processing apparatus of the present invention.
FIG. 6 is a quantization table of color difference Cb and Cr component components used in the image processing apparatus of the present invention.
FIG. 7 is a quantization table of luminance Y components in a low compression rate region used in the image processing apparatus of the present invention.
FIG. 8 is a quantization table of color difference Cb and Cr component components in a low compression rate region used in the image processing apparatus of the present invention.
FIG. 9 is a quantization table of luminance Y components in a high compression ratio region used in the image processing apparatus of the present invention.
FIG. 10 is a quantization table of color difference Cb and Cr component components in a high compression ratio region used in the image processing apparatus of the present invention.
FIG. 11 is a frequency component division diagram in which frequency components are divided into low frequency components and low frequency components in the image processing apparatus of the present invention.
FIG. 12 is a frequency component division diagram in which frequency components are divided into higher frequency components than higher frequency components in the image processing apparatus of the present invention.
FIG. 13 is a quantization table of color difference Cb and Cr component components in which lower limit values used in the image processing apparatus of the present invention are set.
FIG. 14 is a quantization table of color difference Cb and Cr component components in which a lower limit value is set in a character area used in the image processing apparatus of the present invention.
FIG. 15 is a quantization table of color difference Cb and Cr component components in which a lower limit value is set in a photographic region used in the image processing apparatus of the present invention.
FIG. 16 is a quantization table of color difference Cb and Cr component components in which lower limit values are set in areas of high importance used in the image processing apparatus of the present invention.
FIG. 17 is a quantization table of color difference Cb and Cr component components in which a lower limit value is set in a low importance area used in the image processing apparatus of the present invention.
FIG. 18 is a flowchart showing an encoding operation in the first embodiment of the image processing apparatus of the present invention;
FIG. 19 is a flowchart showing an operation of setting and encoding a lower limit value for an area according to different types of image areas such as a character area and a photographic area in the second embodiment of the image processing apparatus of the present invention.
FIG. 20 is a flowchart showing an operation of performing region designation and encoding in the third embodiment of the image processing apparatus of the present invention.
[Explanation of symbols]
10 HDD
20 RAM (inside PC)
30 CPU (in PC)
40 Printer
50 CPU (inside printer)
60 RAM (in the printer)

Claims (12)

A quantization table storing pre Me set quantization step width,
Setting means for setting the image quality mode;
Orthogonal transformation means for performing orthogonal transformation on image data composed of a luminance component and a color difference component ;
An image area separating means for image area separation processing orthogonally transformed data by the orthogonal transform means into a character area and a photographic area,
Based on the quantization table and the scaling factor value set according to the image quality mode, a quantization step using a limit value set only in the frequency component of the color difference component in the quantization table according to the image area separation result Limit value setting means for setting the width;
Quantizing means for quantizing the based-out the set quantization step width limit value setting means, orthogonally transformed data by the orthogonal transformation means,
An image processing apparatus having encoding means for encoding quantized image data ,
An image processing apparatus, wherein a lower limit value used for a color difference component of the character area is larger than a lower limit value used for a color difference component of the photograph area . The limit value setting means includes:
The image processing apparatus according to claim 1, wherein setting the orthogonally transformed different limit values for each transform coefficient is a coefficient of the image data in the orthogonal transform means. The limit value setting means includes:
Grouping the coefficients of the orthogonal transformed image data in the orthogonal transform unit for each same frequency component, the image processing apparatus according to claim 1, wherein setting the limit value corresponding to a characteristic of the frequency components in the group . The image data comprises a plurality of components, the image processing apparatus according to claim 1, characterized in that by setting the limit value for each said component. The image processing apparatus according to claim 4 , wherein the plurality of components include luminance and color difference components, and the limit value is set only for the color difference components. A storage step of storing a quantization step width preset for each region in the quantization table;
A setting process for setting the image quality mode;
An orthogonal transform process for performing orthogonal transform on image data composed of a luminance component and a color difference component ;
And image separation step of image area separation processing orthogonally transformed data into a character region and a photograph region in said orthogonal transformation step,
Based on the quantization table and the scaling factor value set according to the image quality mode, a quantization step using a limit value set only in the frequency component of the color difference component in the quantization table according to the image area separation result Limit value setting process to set the width;
A quantization step of quantizing the based-out the set quantization step width in the limit value setting step, orthogonally transformed data by the orthogonal transformation step,
A images processing method that have a an encoding step of encoding the image data quantized,
An image processing method, wherein a lower limit value used for a color difference component of the character area is larger than a lower limit value used for a color difference component of the photograph area . In the limit value setting step,
The image processing method according to claim 6 , wherein a different limit value is set for each transform coefficient that is a coefficient of image data orthogonally transformed in the orthogonal transform step. In the limit value setting step,
7. The image processing method according to claim 6 , wherein the coefficients of the image data orthogonally transformed in the orthogonal transformation step are grouped for each same frequency component, and a limit value is set according to the characteristic of the frequency component in the group. . The image processing method according to claim 6, wherein the image data includes a plurality of components, and the limit value is set for each component. The image processing method according to claim 9, wherein the plurality of components include luminance and color difference components, and the limit value is set only for the color difference components. The program which performs the image processing method of any one of Claim 6 to 10 . A recording medium that records the program according to claim 11 and is readable by a computer.

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