JP2003101794A - Image encoding apparatus, image encoding method, program, and storage medium - Google Patents

Abstract

(57) Abstract: A character area and a non-character area in an image to be encoded are determined using an encoding process, and quantization is performed using a quantization step corresponding to each determined area. To provide an image encoding device and an image encoding method. A color conversion unit performs color conversion processing on input image data, and a discrete wavelet conversion unit performs discrete wavelet conversion on a luminance component and a color difference component of each tile. The area determination unit 104 performs area determination using the transform coefficient for each tile. In the case of a tile in a black character area, the quantization unit 105 uses a finer quantization step for the luminance component of this tile than in the initial setting, and in the case of a tile in a color character area, the luminance component and color difference component of this tile are initialized. In the case of a tile in a non-character area using a quantization step finer than that at the time of setting, quantization is performed on the luminance component and color difference component of this tile using the quantization step at the time of initial setting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image coding apparatus for coding an image, an image coding method, a program, and a storage medium.

[0002]

2. Description of the Related Art When a conventional image coding apparatus (first image coding apparatus) performs a coding process on image data,
For this image data, preprocessing (color conversion processing or thinning processing), frequency conversion processing (for example, discrete cosine conversion),
The respective processes are performed in the order of the quantization process, the encoding process (code string generation process), and the code string output process.

Further, an image coding apparatus in which the above-mentioned image coding apparatus is provided with a region judging section (a second image coding device is also conceivable. That is, each region (for example, a character region and a photograph region) in an image is judged. With reference to this determination result, quantization is performed by changing the quantization parameter, for example.

[0004]

However, the first image coding apparatus has the following problems. That is,
Since the same quantization parameter is used for one page of image data regardless of the character region or the photo region in the image to be encoded, the quantization parameter suitable for the photo region causes image deterioration of the character region. Inevitable,
When a suitable quantization parameter is used for the character area, there is a problem that the code amount of the entire image increases.

On the other hand, the second image coding apparatus has the following problems. In other words, since the region determination is performed by a process independent of the image coding process (the above-described preprocessing, frequency conversion process, quantization process, coding process (code string generation process), and code string output process), the entire device Increasing the circuit scale and complicating the processing system cannot be avoided.

The present invention has been made in view of the above problems, and determines a character region and a non-character region in an image to be encoded by using a process of encoding processing, and a quantum corresponding to each determined region. An object of the present invention is to provide an image coding apparatus and an image coding method that perform quantization by using a coding step.

[0007]

In order to achieve the object of the present invention, for example, an image coding apparatus of the present invention has the following configuration.

That is, an image coding apparatus for coding an image, in which a frequency conversion is performed on a luminance signal and a color difference signal in the image to generate a conversion coefficient of the luminance signal and a conversion coefficient of the color difference signal. A character area in the image using a conversion unit and a conversion coefficient of the luminance signal and a conversion coefficient of the color difference signal by the frequency conversion unit,
With respect to the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal by the frequency conversion means, the quantization step is changed according to the area determination means for determining the non-character area and the area determined by the area determination means. Quantizing means for performing quantization by means of, and entropy coding means for performing entropy coding on the quantization result by the quantizing means,
An image coding apparatus, which generates coded data based on a result of the entropy coding.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail according to preferred embodiments with reference to the accompanying drawings.

[First Embodiment] FIG. 5 is a block diagram showing the functional arrangement of a conventional image coding apparatus. A conventional image coding apparatus will be described with reference to FIG.

In the figure, 501 is an image input section, and C
It consists of a CD, contact sensor, etc., and reads the original image data. A color conversion / thinning processing unit 502 performs color conversion processing and thinning processing suitable for subsequent processing on the image data input by the image input unit 501. 503 is a discrete cosine transform unit (hereinafter referred to as a DCT unit)
Then, the color conversion / thinning processing unit 502 performs discrete cosine transform processing on the image data subjected to color conversion / thinning processing to generate conversion coefficients.

Reference numeral 504 is a quantizer for performing a quantization process on the transform coefficient generated by the DCT unit 503 to generate a quantized coefficient,
Reference numeral 505 is an encoding unit that performs encoding processing on the quantized coefficient by the quantization unit 504. The color space converted by the color conversion processing / thinning processing unit 502 is the La * b * color space, and the sampling ratio (the ratio of the luminance signal: the color difference signal 1: the color difference signal 2) by the thinning processing is thinned. Not 1: 1: 1 or as thinning rate
2: 1: 1 and 4: 1: 1 are often used. Also, the quantizer
The quantization parameter used in the quantization processing in 504 and the encoding parameter used in the encoding processing in the encoding unit 505 can be arbitrarily set.

FIG. 6 shows a functional configuration of a conventional image coding apparatus different from the image coding apparatus shown in FIG. The functional configuration of the image encoding device shown in FIG. 6 is such that an area determination unit 507 is added to the image encoding device shown in FIG. The area determination unit 507 has a function of identifying the image attribute of the image data input by the image input unit 501 for each predetermined pixel unit or block unit,
It is determined whether the image in pixel unit or block unit is a character region or a photo region.

The method of area determination by the area determination unit 507 is not limited to one method. For example, a method of determining the character area by converting the input image data into a luminance component and a color difference component and detecting a sharp change (edge amount) of the luminance component on the image data, or a method of determining the luminance component amount and the color difference component amount. There is a method of comparing and discriminating by setting each threshold value, and a method of discriminating an area by taking a frequency distribution in a predetermined amount of input image data and comparing with a predetermined threshold value. The discrimination result by the discrimination method is input to the quantizer 504. The quantizer 504 uses fine quantization parameters for the area determined to be the character area. As a result, it is possible to finely perform the quantization processing up to the high frequency component peculiar to the character area. On the other hand, the high frequency component is roughly quantized in the area determined to be the photograph area. As a result, the code data amount can be suppressed,
Optimal encoding processing can be performed on the entire input image data.

However, the image coding apparatus shown in FIG. 5 has the following problems. That is, since the same quantization parameter is used for one page of image data regardless of the character area or the photo area in the image to be encoded, the image of the character area is used as the quantization parameter suitable for the photo area. There is a problem that the deterioration is unavoidable and that the code amount of the entire image increases when using a quantization parameter suitable for the character area.

On the other hand, the image coding apparatus shown in FIG. 6 has the following problems. That is, since the area determination process is performed by a process independent of the image encoding process (preprocessing, frequency conversion process, quantization process, encoding process (code string generation process), and code string output process described above), the entire device is processed. Inevitably an increase in the circuit scale and complication of the processing system.

FIG. 1 is a block diagram showing the functional arrangement of the image coding apparatus according to the first embodiment of the present invention for solving these problems.
Shown in. In the figure, 101 is an image input unit, 102 is a color conversion unit, 103 is a discrete wavelet conversion unit, 104 is a region determination unit, 105 is a quantization unit, 106 is an encoding unit, and 107 is a code output unit. The image input unit 101 is composed of a CCD or a contact sensor, and has a document table or AD (not shown).
The original image set to F is read, and the original image is input to the apparatus as electronic data (image data).

The color conversion unit 102 performs color conversion processing on the image data read by the image input unit 101. The color space after conversion is not particularly limited as long as it is a color space in which an input image signal is converted into a luminance signal and a color difference signal. That is, the YIQ color space in the NTSC system may be used, or the YCbCr color space that is often used in JPEG processing on a personal computer (hereinafter referred to as PC) may be used (in both cases, the Y signal corresponds to the luminance signal).

The discrete wavelet transform unit 103 reads the image data (luminance signal, color difference signal) read by the image input unit 101 and color-converted by the color transform unit 102 by 2
Dimensional discrete wavelet transform processing, luminance signal,
The conversion coefficient of each color difference signal is calculated and output. Figure 3
(A) shows the basic configuration of the discrete wavelet transform unit 103, and the input image signal is stored in the memory 301.
Stored in the memory 301, sequentially read by the processing unit 302, subjected to conversion processing, and written again in the memory 301. In the present embodiment, the processing configuration in the processing unit 302 is as shown in FIG. In the figure, the input image data is separated into even-numbered address and odd-numbered address signals by a combination of a delay element and a downsampler, and is filtered by two filters p and u. In the figures, s and d respectively represent the low-pass coefficient and the bypass coefficient when the one-level decomposition is performed on the one-dimensional image signal, and are calculated by the following equation.

D (n) = x (2n + 1) -floor ((x (2n) + x (2n + 2)) / 2) (Equation 1) s (n) = x (2n) + floor (( d (n-1) + d (d)) / 4) (Formula 2) where x (n) is image data to be converted. Through the above processing, one-dimensional discrete wavelet transform processing is performed on the image signal. The two-dimensional discrete wavelet transform is a one-dimensional transform that is sequentially performed in the horizontal and vertical directions of an image, and the details thereof are publicly known, so description thereof will be omitted here. FIG. 3C is a configuration example of a two-level conversion coefficient group obtained by two-dimensional conversion processing, in which image data has coefficient sequences HH1, HL1, and LH of different frequency bands.
It is decomposed into 1, LL, LL. In the following description, these coefficient sequences will be referred to as subbands. The coefficient of each subband is output to the subsequent area discriminating unit 104 and quantizing unit 105.

Further, in the above-mentioned discrete wavelet transform processing, there is no special regulation on the size of image data to be processed. That is, the conversion process may be performed on the image data of one page, or the conversion process may be performed for each predetermined size (hereinafter, referred to as tile). However,
In the present embodiment, the discrete wavelet transform is performed for each tile composed of 128 pixels × 128 pixels for the purpose of discriminating the region in the input image data and for improving the coding efficiency using the discrete wavelet transform. I decided to. That is, the above-mentioned subbands for the luminance signal and the color difference signal exist for each processing tile composed of 128 pixels × 128 pixels.

At this time, when the tile of interest is image data such as a character in which the change in the luminance value of the pixel is steep, the transform coefficient after the discrete wavelet transform is a high-frequency subband (2 levels of the subband of the tile. Case HH1, HL1, LH
The value of the conversion coefficient of 1) becomes large. In addition, when the input image data is a natural image such as a photograph, the high frequency subband in the subband of the tile of interest does not take a particularly large value, and the low frequency subband (LL, HL2, LH in the case of 2 levels)
The value of 2) becomes large.

The area determination unit 104 performs area determination using the transform coefficient for each tile output from the discrete wavelet transform unit 103. Specifically, it is determined whether each tile is a character area or a non-character area (for example, the natural image area described above). As the area determination method, first, the average value of the conversion coefficients included in the high frequency subbands (of the luminance signal and the color difference signal) (HH1, HL1, LH1 in the case of 2 levels) is calculated for each tile. Then, the first threshold and the second threshold are set in advance, and the calculated average value is the first
The tiles that exceed the threshold of are estimated to be tiles in the character area.
In addition, the average value of the conversion coefficients included in the low-frequency subband (LL, HL2, LH2 in the case of 2 levels) is calculated for each tile, and the tile whose calculated average value is larger than the second threshold is non-character. Presumed to be the tile of the area. However, the method of determining whether each tile is a character area is not limited to this.

Further, the area determining unit 104 uses the conversion coefficient for the luminance signal and the conversion coefficient for the color difference component in the tile determined to be the character area, and this character area is an area including a black character (black character area). It is determined (area determination) whether or not there is an area (color character area) that does not include black characters. The area determination unit 104 executes the following processing when it is determined that the tile of interest is a character area in the above processing.

In the target tile of the character area, to determine whether this character area is a black character area or a color character area, the conversion coefficient of the color difference component (for example, Cb or Cb component) of the target tile is referred to and the conversion of this color difference component is performed. If the average value of the coefficients is equal to or less than the predetermined value, the character area has no color, that is, the black character area can be obtained. On the other hand, the conversion coefficient of the color difference component (for example, Cb or Cb component) of the tile of interest is referred to, and if the average value of the conversion coefficient of this color difference component is equal to or more than a predetermined value, it means that there is a colored character area, that is, the color character It can be a region. The determination result (determination result for each tile) in the area determination unit 104 is input to the quantization unit 105.

The quantizer 105 quantizes the transform coefficient for each tile output from the discrete wavelet transform unit 103 based on the determination result of the area determination unit 104. Specifically, when the area determination unit 104 determines that the tile output from the discrete wavelet transform unit 103 is a black character region, the quantization step of the transform coefficient of the luminance component of this tile is set to the default quantization. Quantization processing is performed by making it finer than steps. Further, as the quantization step of the conversion coefficient of the color difference component of the tile determined by the area determination unit 104 to be the black character area, the default quantization step is used.

On the other hand, when the determination result by the region determination unit 104 is a color character region determination, the quantization step of the conversion coefficient of not only the luminance component but also the color difference component is finer than the quantization step of the default setting. Give. Further, the non-character area is quantized by using the quantization step at the time of initial setting.

As an example of a coded sequence in JPEG2000 currently being recommended, the quantization step determined for each tile is added to the coded data of each tile in the coded sequence shown in FIG. Included in tile header. FIG. 4 is a diagram showing the structure of a code string in JPEG2000. Specifically, Main Header or Tile part in Figure 4
It is indicated by the QCD or QCC marker in the header.

The coefficient quantized by the quantizer 105 is entropy coded by the encoder 106,
A predetermined header is added to form encoded data (code string) (for example, a code string having the configuration shown in FIG. 4 is formed). In the code string, the determination result for each tile determined by the area determination unit 104 is included in the header (tile header) of each tile. Therefore, by referring to the tile header, it can be confirmed whether or not the tile corresponding to this tile header is a character area.
Further, it is possible to confirm whether the tile of the character area is a black character area or a color character area.

The generated coded sequence is output by the code output unit 107. The code output unit 107 is configured by an external storage device such as a hard disk, an interface for connecting to a network such as a LAN or the Internet, and stores the generated code sequence described above and stores it in the code output unit 107. This code string is output to the connected device, or the code string generated for the above network is output.

FIG. 7 shows a flowchart of the above-mentioned image coding method, and the flow of processing will be described. Since the detailed description of each step is as described above, the detailed description of each step is omitted.

First, image data is input to the image input unit 101 (step S701), and color conversion processing is performed on the input image data by the color conversion unit 102 (step S702). Next, in the color-converted image data, the discrete wavelet transform unit 103 performs discrete wavelet transform on the luminance component and the color difference component for each tile (step S703). Next, the area determination unit 104
Performs the above-described area determination for each tile (step S70
4) If the area is a character area, the process proceeds to step S706.
If the area is a non-character area, the process proceeds to step S709.
Move to.

In step S706, step S704
It is determined whether the tile determined to be the character area by the area determination in 1 is a black character area or a color character area. If it is a black character area, the process proceeds to step S707,
The quantization unit 105 quantizes the luminance component of this tile using a quantization step finer than the quantization step at the time of initialization (step S707). On the other hand, when it is determined in step S706 that the area is the color character area, the process proceeds to step S708, and the quantization unit 105 sets both the luminance component and the color difference component of this tile at the time of initial setting. Quantization is performed using a quantization step finer than the quantization step (step S708).

On the other hand, if it is determined in step S704 that the tile is a non-character area, the process proceeds to step S709, and both the luminance component and the color difference component of the tile determined to be the non-character area are processed. , Quantizer 105
Performs quantization on this tile using the quantization step at the time of initialization (step S709).

The coding unit 106 performs entropy coding on the tiles quantized by the quantizing unit 105 (step S710). As described above, the processing from step S704 to step S710 is performed on all tiles. When all tiles are entropy coded, coded data is generated using the result of this entropy coding (step S712), and the generated coded data is output to the outside via the code output unit 107. (Step S713).

As described above, the image coding apparatus (image coding method) according to the present embodiment, when coding image data, creates a character area (color character, black character) area and a photograph area. Since the quantization is performed using the corresponding quantization step, the deterioration of the image quality of the encoded image is better than that of the image encoding device (image encoding method) that quantizes the entire image using the same quantization step. In addition to being able to reduce the number of bits, the code amount of the entire image can be reduced.

Further, the image coding apparatus (image coding method) in this embodiment performs the above-mentioned area determination using the transform coefficient generated in the process of coding the image data. A device for performing area determination by independent processing (for example, a device having the functional configuration shown in FIG. 6)
The circuit scale of the device can be reduced, and the complexity of the area determination and encoding processing can be reduced.

[Second Embodiment] In the present embodiment, an image encoding apparatus for changing the tile size according to the original to be processed will be described. In the first embodiment, the description has been given on the premise that the tile has 128 pixels × 128 pixels, but it is desirable that the tile size to be processed has a variable setting function depending on the original. An example of changing the tile size in this embodiment will be described with reference to FIG. FIG. 2 is a diagram illustrating a configuration example of an operation unit that changes the tile size.

In FIG. 2, reference numeral 201 denotes an operation section for setting an operation mode. The basic configuration of the image coding apparatus according to the present embodiment is the basic configuration shown in FIG. Prepare The following operation modes set by this operation unit are the discrete wavelet transform unit 103.
Entered in.

In FIG. 3, reference numeral 202 denotes a display unit composed of an LCD or the like, 203 to 209 are setting keys for setting the following operation modes, 203 is a key to be pressed when the processing original is a character original, and 204 Is a key to be pressed when the document to be processed is a photo document, 205 is a key to be pressed when the document to be processed is a document in which character and photo are mixed, 206 is a menu key for displaying an operation menu, and 207 is for changing the menu A setting key, 208 is a start key that activates the operation of the image coding apparatus, and 209 is a stop key that is pressed to stop the operation of the image coding apparatus.

As a first setting example for changing the tile size, the key to be pressed depending on the document to be processed is made to correspond to the tile size. Specifically, when the character manuscript key 203 is pressed, the tile configuration of 128 pixels × 128 pixels is associated, and when the photo manuscript key 204 is pressed, the tile configuration of 512 pixels × 512 pixels is associated, When the photograph key 205 is pressed, the tile configuration of 256 pixels × 256 pixels is made to correspond.

In the second setting example, the menu key 206 is pressed to display the menu on the display unit 202, and the setting key 20
7, the number of tile constituent pixels is set for each document to be processed.

By varying the number of tile constituent pixels according to the original to be processed as described above, it is possible to perform processing with further improved coding efficiency.

In the above description, the case where the image coding apparatus is configured as a single unit has been described, but the present invention is not limited to this. That is, in the configuration having the connection function with the PC, the PC sets the above-described first threshold value, the second threshold value, or the pixel configuration of the tile corresponding to each image attribute to the image encoding device. May be.

[Other Embodiments] Furthermore, the present invention is not limited only to the apparatus and method for realizing the above-described embodiment, and the above-described embodiment can be applied to a computer (CPU or MPU) in the above-mentioned system or apparatus. A program code (software program code according to the flowchart shown in FIG. 7) for realizing the form is supplied, and the computer of the system or the apparatus operates the various devices according to the program code to realize the above-described embodiment. Realization is also included in the scope of the present invention.

Further, in this case, the program code itself of the software realizes the function of the above embodiment, and the program code itself and means for supplying the program code to the computer, specifically, the above-mentioned. A storage medium storing the program code is included in the scope of the present invention.

As a storage medium for storing such a program code, for example, a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, a CD
-ROM, magnetic tape, non-volatile memory card, ROM, etc. can be used.

In addition to the case where the functions of the above-described embodiments are realized by the computer controlling various devices only in accordance with the supplied program code, the OS on which the program code runs on the computer. The program code is also included in the scope of the present invention when the above embodiment is realized in cooperation with (operating system) or other application software.

Further, the supplied program code is stored in the memory provided in the function expansion board of the computer or the function expansion unit connected to the computer, and then the function expansion board or function is instructed based on the instruction of the program code. The case where the CPU or the like included in the storage unit performs some or all of the actual processing and the above-described embodiment is realized by the processing is also included in the scope of the present invention.

[0050]

As described above, according to the present invention, the character region and the non-character region in the image to be encoded are determined by using the process of the encoding process. Therefore, the region determination is performed by the process independent of the encoding process. It is possible to reduce the circuit scale of the entire apparatus and the complexity of the processing system, as compared with the above.

[Brief description of drawings]

FIG. 1 is a diagram showing a functional configuration of an image encoding device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration example of an operation unit for changing a tile size according to the second embodiment of the present invention.

3A is a diagram showing a basic configuration of a discrete wavelet transformation unit 103, FIG. 3B is a diagram showing a basic configuration of a processing unit 302, and FIG. 3C is a diagram obtained by two-dimensional discrete wavelet transformation. It is a figure which shows the structural example of the conversion coefficient group of a level.

FIG. 4 is a diagram showing a configuration of a code string in JPEG2000.

FIG. 5 is a block diagram showing a functional configuration of a conventional image encoding device.

6 is a diagram showing a functional configuration of a conventional image encoding device different from the image encoding device shown in FIG.

FIG. 7 is a flowchart of an image coding method according to the first embodiment of the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5C059 KK01 LB05 MA23 MA24 MC11                       PP01 PP16 PP20 PP24 SS20                       UA02 UA15 UA39                 5C077 LL17 MP06 MP08 PP27 PP28                       PP32 PP34 PP46 PP49 PP68                       PQ08 RR11 RR18 RR21 TT06                 5C078 AA09 BA53 BA57 CA25 DA00                       DA01 DA02 DB07 EA00                 5C079 HB04 HB11 LA03 LA06 LA27                       LA31 NA09 PA02

Claims (19)

[Claims] 1. An image coding apparatus for coding an image, wherein a frequency is applied to a luminance signal and a color difference signal in the image to generate a conversion coefficient of the luminance signal and a conversion coefficient of the color difference signal. A conversion unit, a region determination unit that determines a character region and a non-character region in the image by using the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal by the frequency conversion unit, and determination by the area determination unit Quantizing means for varying the quantizing step in accordance with the generated region and quantizing the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal by the frequency converting means, and the quantizing by the quantizing means. And entropy encoding means for performing entropy encoding on the result, wherein encoded data is generated based on the result of the entropy encoding. The image coding apparatus. 2. The color conversion means for performing color conversion on the image, the color conversion means converting the image into a luminance signal and a color difference signal. Image encoding device. 3. The frequency conversion means performs frequency conversion on the image for each area of a predetermined size, and generates a conversion coefficient of a luminance signal and a conversion coefficient of a color difference component for each area. The image coding device according to Item 1 or 2. 4. The image coding apparatus according to claim 3, wherein the frequency transforming unit performs a discrete wavelet transform on the image for each region of a predetermined size. 5. The area determination means further uses the conversion coefficient of the luminance signal and the conversion coefficient of the color difference component by the frequency conversion means to determine whether or not a character area in the image
The image coding apparatus according to claim 1, wherein a black character area and a color character area are determined. 6. The area determining means further determines whether or not the area of interest is a character area by using a conversion coefficient included in a high frequency area of a luminance signal and a color difference signal of the area of interest in the image. The image coding apparatus according to claim 5, wherein when the rectangle of interest is a character area, it is determined whether the character area is a black character area or a color character area. 7. The image coding apparatus according to claim 5, wherein the area determination unit makes an area determination for each area of a predetermined size in the image. 8. For the area determined to be a black character area by the area determination means, the quantization means uses different quantization steps for the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal in the area. The image coding apparatus according to claim 1, wherein the image area is quantized. 9. A quantization step that is finer than a preset quantization step for the conversion coefficient of the luminance signal of the area determined to be a black character area by the area determination means,
9. The image coding apparatus according to claim 8, wherein the quantization unit performs quantization on a conversion coefficient of the color difference signal of the region using a preset quantization step. 10. The quantization unit is finer than a preset quantization step with respect to a conversion coefficient of a luminance signal and a conversion coefficient of a color difference signal in an area determined to be a color character area by the area determination unit. The image coding apparatus according to claim 1, wherein quantization is performed using a quantization step. 11. For a region determined by the region determination unit to be a non-character region, the quantization unit sets a predetermined quantization for a conversion coefficient of a luminance signal and a conversion coefficient of a color difference signal of the area. The image coding apparatus according to claim 1, wherein quantization is performed using steps. 12. The image coding apparatus according to claim 11, wherein the area includes a photograph area. 13. The image coding apparatus according to claim 1, wherein the coded data includes information regarding the area determined by the determination unit. 14. The image coding apparatus according to claim 1, further comprising size changing means capable of changing the predetermined size. 15. An image encoding method for encoding an image, wherein a frequency is applied to a luminance signal and a color difference signal in the image to generate a conversion coefficient of the luminance signal and a conversion coefficient of the color difference signal. A conversion step, an area determination step of determining a character area and a non-character area in the image using the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal in the frequency conversion step, and the determination in the area determination step A quantization step in which the quantization step is changed in accordance with the generated region, the quantization coefficient is applied to the conversion coefficient of the luminance signal and the conversion coefficient of the color difference signal by the frequency conversion step, and the quantization by the quantization step. And an entropy encoding step of performing entropy encoding on the result, wherein encoded data is generated based on the result of the entropy encoding. Image encoding method. 16. The method according to claim 15, further comprising a color conversion step of performing color conversion on the image, wherein the color conversion step converts the image into a luminance signal and a color difference signal. Image coding method. 17. The image coding method according to claim 15, further comprising a size changing step capable of changing the predetermined size. 18. A program for executing the image coding method according to claim 15. Description: 19. A computer-readable storage medium which stores the program according to claim 18.