JP2003047005A - Image encoding apparatus and method, program, and storage medium - Google Patents

Abstract

(57) [Problem] To efficiently perform image encoding when encoding image data including areas requiring different resolution levels. In addition, when decoding encoded image data, encoding is performed to generate a code string capable of specifying a target resolution area at an early stage. SOLUTION: An area determination unit 102 generates a flag based on high resolution area information input from a high resolution area information input unit 101, and a high resolution image input unit 103 and a low resolution image input unit 104 generate high, low based on the flag. The resolution image is read, and the discrete wavelet transform unit 105 performs a two-dimensional discrete wavelet transform on the high-resolution image. Coefficient synthesis unit 1
06 synthesizes the low-frequency subband of the high-resolution image with the low-resolution image. The discrete wavelet transform unit 107 performs a two-dimensional discrete wavelet transform on the synthesized subband, and the bit plane encoding unit 108 generates a code sequence.

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 image data, a method therefor, and a program,
It relates to a storage medium.

[0002]

2. Description of the Related Art In recent years, with the improvement of the technology of image input devices such as digital cameras and scanners, the resolution of image data captured by these input devices has been increasing. If the image has a low resolution, the amount of image data is small, and it does not hinder processing such as transmission and storage. However, as the resolution becomes higher, the amount of image data also becomes enormous, and it takes a lot of time for transmission and a large storage capacity is required for storage. For this reason, it is common to reduce the amount of data by using high-efficiency coding when transmitting and storing images to eliminate image redundancy or to process the image in a visually permissible range. . The coding method that can completely reproduce the original image by decoding is called lossless coding, and the coding method that obtains a visually close image but cannot completely reproduce the original image is called lossy coding. In the case of lossy encoding, it is important to reduce the code amount by changing the portion where deterioration is not noticeable visually, but this largely depends on the characteristics of the image. There are various types of image data, even if it's just a bit.Natural images, characters, and lines that are created by shooting people, landscapes, etc. with silver salt photos, reading them with a scanner, or shooting them directly with a digital camera. There are character / line images in which information is rasterized, computer-generated 2D image data and CG images in which 3D shapes are rendered, etc., and each has a different required resolution and required number of gradations to obtain good playback image quality. Is said.
Generally, character images and line images are required to have higher resolution than natural images.

Conventionally, a method utilizing wavelet transform has been used as one method of high efficiency coding. In the conventional method, first, an image to be coded is divided into a plurality of frequency bands (subbands) by using the discrete wavelet transform,
Next, the transform coefficients of each subband are quantized and entropy coded by various methods to generate a code string. As a method of wavelet transform of an image, as shown in FIG.
As shown in (b) and (c), a method of applying one-dimensional conversion processing in the horizontal and vertical directions to divide into four subbands is used. Furthermore, a method of repeatedly dividing only the low frequency subband (LL subband) is common. FIG. 5 shows an example in which the one-dimensional conversion is repeated twice.

One of the advantages of the image coding using the wavelet transform is that it is easy to realize the stepwise decoding of the spatial resolution. When the wavelet transform is performed as shown in FIG. 5 and the coefficients of the respective subbands are sequentially encoded and transmitted from the low frequency subband LL to the high frequency subband HH1, the decoding side receives the coefficients of the LL subband. To restore a 1/4 resolution image,
When the L, LH1, HL1, and HH1 are received, the restored image with a resolution of 1/2 is received, and when LH2, HL2, and HH2 are received, the restored image with the original resolution is gradually displayed. You can raise and decode the image.

As seen in a mixed image of characters and photographs,
When parts with different required resolutions are mixed in the image, the data required for decoding the high resolution only for the required part is encoded by using the spatial resolution of the wavelet transform. For, the method of discarding the data necessary for decoding the high resolution is used.

[0006]

However, in the conventional high-efficiency encoding method as described above, image data including areas having different required resolutions such as natural images and mixed images of character / line images are encoded. In that case, the image data had to be read once with a high resolution, which was not an efficient encoding method.

The present invention has been made in view of the above problems, and an object of the present invention is to perform efficient image encoding when encoding image data including areas requiring different resolution levels. To do.

It is another object of the present invention to perform coding for generating a code string that can specify a resolution area of interest at an early stage when decoding coded image data.

[0009]

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, wherein a first area of an original image is read at a first resolution and a first image including the read area is generated.
Reading means and a second area including a read area in the original image at a second resolution higher than the first resolution and different from the first area.
Second reading means for generating an image of the first image, a first frequency converting means for performing frequency conversion on the second image to obtain a coefficient for each sub-band, and a plurality of the first frequency converting means. A synthesizing unit for synthesizing the first image in a predetermined subband of the subbands to generate a synthetic subband, and second frequency conversion for the synthetic subband to obtain a coefficient for each subband. And a code string generating means for generating a code string from the coefficients of the sub-bands obtained by the first frequency converting means and the second frequency converting means.

Further, it is provided with a flag generating means for generating a flag designating an area to be read by the second reading means.

On the other hand, the first image further comprises a flag generating means for generating a flag indicating a predetermined area.

In this case, a mask indicating a coefficient associated with the second image among the coefficients forming the subband generated when the code string generating means frequency-converts the combined subband. It further comprises a mask generating means for generating.

[0014]

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. 1 shows the functional arrangement of an image coding apparatus according to this embodiment. 1 in the figure
Reference numeral 01 is a high resolution area information input unit, 102 is a high resolution scan area determination unit, 103 is a high resolution image input unit, and 104.
Is a low-resolution image input unit, 105 is a discrete wavelet transform unit, 106 is a coefficient synthesis unit, 107 is a discrete wavelet transform unit, 108 is a bit plane coding unit, 109 is a code string formation unit, and 109 is a code output unit.

Further, FIG. 10 shows the basic configuration of the image coding apparatus in this embodiment.

Reference numeral 1001 is a CPU, which is a RAM 1002 or R
The program and data stored in the OM 1003 are used to control the entire apparatus and also to perform encoding processing described later.

A RAM 1002 is an external storage device 100.
4 and a storage medium drive 1010, a program area, a data area, a processing target data area, and the like are temporarily stored, and a work area used by the CPU 1001 for processing is also provided.

A ROM 1003 stores programs and data for controlling the entire apparatus.

An external storage device 1004, such as a hard disk, stores programs and data read from the storage medium drive 1010 in a file format. Further, when the size of the work area used when the CPU 1001 executes the processing is no longer provided in the RAM 1002, it can be provided as a file.

Reference numerals 1005 and 1006 denote a keyboard and a mouse, respectively, which can input various instructions to the apparatus.

A display device 1007 is composed of a CRT, a liquid crystal screen, etc., and can display images and characters.

An image input device 1008 is composed of a digital camera, a scanner or the like, and can input an image as data. Further, a circuit for performing processing (γ correction, color correction, etc.) on an input image is also included in this.

A storage medium drive 1009 is a CD-R.
A drive that reads programs and data from a storage medium such as an OM and a DVD outputs the read programs and data to the external storage device 1004 and the RAM 1002.

A bus 1010 connects the above-mentioned units.

As described above, the image coding apparatus of this embodiment has the basic structure shown in FIG. 10, but the program having the functional structure shown in FIG.
9, or read from the external storage device 1004, C
By being executed by the PU 1001, the present image coding device may be a device having the configuration shown in FIG.

In the present embodiment, a black-and-white original will be described as being read and encoded as image data in which the brightness value of each pixel is represented by 8 bits. However, the present invention is not limited to this, and can also be applied to the case of reading and encoding as image data expressing a luminance value with a bit number other than 8 bits such as 4 bits, 10 bits, 12 bits. In addition, each pixel is R
It can also be applied to the case of reading as color image data represented by a plurality of color components such as GB and CMYK or luminance and chromaticity / color difference components such as YCrCb. In this case, each component in the color image data may be regarded as monochrome image data.

The function and operation of each unit in this embodiment will be described below with reference to FIGS. 1 and 10. In the present embodiment, a black-and-white original to be encoded is read at a predetermined resolution and encoded, but only a part thereof is read at double the resolution in both the horizontal and vertical directions. Hereinafter, the predetermined resolution will be referred to as low resolution, and the resolution obtained by doubling the predetermined resolution will be referred to as high resolution. It is assumed that the size of the black and white original is fixed, and the image size when the entire original is read at high resolution is represented as X and Y. Both X and Y are multiples of 4.

First, the high resolution area information input section 101 inputs high resolution area information for specifying which part of the document is to be read at high resolution. In this embodiment, the area to be read at high resolution is rectangular. In addition, the high resolution area information includes the pixel position at the upper left corner (Ulx, Uly) and the pixel position at the lower right corner (L) based on the image size when reading at high resolution.
Rx, LRy). FIG. 2 shows an example of a document and high resolution area information.

The high resolution scan area determination unit 102 specifies a pixel position to be actually read in high resolution from the upper left corner pixel position and the lower right corner pixel position of the high resolution area input from the high resolution area information input unit 101. F (x,
y) is generated. The generated resolution flag F (x, y) is temporarily stored in the RAM 1002. Regarding the pixel position that is actually read in high resolution, the resolution flag F (x,
y) is set to 1 and 0 otherwise. In particular, And FIG. 7 shows an example of the resolution flag F (x, y) when the high resolution area information is designated as shown in FIG.

When the high resolution scan area determining unit 102 determines the area to be read at high resolution, the high resolution image input unit 103 and the low resolution image input unit 104 determine the high resolution area based on the resolution flag F (x, y). , The low-resolution image is read. Actually, the image of the same document is input to the high resolution image input unit 103 and the low resolution image input unit 104, and the high resolution portion and the low resolution portion are read by the respective input units 103 and 104.

The high-resolution image input unit 103 reads pixel data from a black-and-white document only for the portion of the resolution flag F (x, y) = 1, and the high-resolution image data Ph (x,
y) is formed in the RAM 1002. Resolution flag F
For a position where (x, y) = 0, Ph (x, y) =
When 0 is set, the actual read operation is not performed.

The discrete wavelet transform unit 105 performs two-dimensional discrete wavelet transform while appropriately storing each pixel data Ph (x, y) of the high resolution image data input from the high resolution image input unit 103 in the RAM 1002.
It is decomposed into four subbands LL, LH, HL, and HH. Then, the coefficient of each subband is output again to the RAM 1002 in an area different from the area in which the image data Ph (x, y) is stored. Hereinafter, the coefficient of each subband is represented as C (S, x, y). S represents a subband and is any one of LL, LH, HL, and HH. Also,
x, y is the coefficient position of the upper left corner in each subband (0,
0) represents the coefficient positions in the horizontal direction and the vertical direction.

The two-dimensional discrete wavelet transform is realized by applying a one-dimensional transform (filter process) to each of the horizontal and vertical directions. FIG. 4 shows the two-dimensional discrete wavelet transform process. From the figure, in the two-dimensional discrete wavelet transform, first, the one-dimensional discrete wavelet transform in the vertical direction is applied to the image to be encoded (FIG. 4A), and the low frequency subband L and the high frequency subband H are applied. Disassemble (FIG. 4 (b)). Further, by applying a horizontal one-dimensional discrete wavelet transform to each of them, L
It is decomposed into four subbands of L, HL, LH, and HH (FIG. 4 (c)). In this image coding apparatus, the one-dimensional discrete wavelet transform for N one-dimensional signals x (n) (n is 0 to N-1) is performed by the following equation.

H (n) = x (2n) −x (2n + 1) (1) l (n) = floor {(x (2n) + x (2n + 1)) / 2} (2) where h (n) Is the coefficient of the high frequency subband, l (n)
Represents the coefficient of the low frequency subband. Also, floor
Let {R} be a function that obtains the maximum integer value that does not exceed the real number R.

On the other hand, the low resolution image input unit 104 refers to the resolution flag generated by the high resolution scan area determination unit 102, reads the image data at a low resolution for the pixel of F (x, y) = 0, and the low resolution Image Pl (lx, l
y) in the RAM 1002 (high resolution image data Ph
It is formed in an area different from the area in which (x, y) is stored. Here, lx is 0 to (X / 2-1) and ly is 0.
To (Y / 2-1). Four flags F (l corresponding to one pixel Pl (lx, ly) of the low resolution image
x × 2, ly × 2), F (lx × 2 + 1, ly × 2),
F (lx × 2, ly × 2 + 1), F (lx × 2 + 1, l
y × 2 + 1) is provided. Since the coordinate values of the four corners of the high-resolution area are even, these four flags all have the same value, and therefore F (lx × 2, ly × 2) is used as an index for determining the necessity of reading. Resolution flag F (lx
Pl for lx and ly with x2, lyx2) = 1
When (lx, ly) = 0, the actual read operation is not performed.

When the low-resolution image input in the low-resolution image input unit 104 and the sub-band decomposition of the high-resolution image in the discrete wavelet transform unit 105 are completed, the coefficient synthesizing unit 106 produces a resolution flag generated by the high-resolution scan area determining unit 102. The low-frequency subband C generated by the discrete wavelet transform unit 105 with reference to F (x, y)
(LL, x, y) and the low resolution image Pl (lx, ly) are combined. The low-frequency subband C (LL, x, y) has the same size (horizontal, horizontal) as the low-resolution image Pl (lx, ly).
Since the size in the vertical direction is X / 2, Y / 2), variables representing the coefficient position in the subband are changed from x, y to lx, ly.
Instead, the combining process in the coefficient combining unit 106 will be described.

The coefficient synthesizing unit 106 uses C (LL, lx, l
The resolution flag F (lx × 2, ly × 2) is checked for all the coefficients in y) in raster scan order, and F (lx ×
If 2, ly × 2) = 0, C (LL, lx, ly) is replaced with Pl (lx, ly). In this way C
When this replacement processing is performed for all the coefficients of (LL, lx, ly), the coefficient corresponding to the low-resolution image portion in C (LL, lx, ly) is converted into the low-resolution image Pl (x, ly).
y) can be replaced.

In the discrete wavelet transform unit 107, the coefficient synthesizing unit 106 causes C (LL, lx, ly) and Pl (l
x, ly) to obtain a new C (LL, lx,
ly) is decomposed into four subbands by the same processing as that of the discrete wavelet transform unit 105. FIG. 5 shows a state in which the LL subband after synthesis is further decomposed into four subbands by the discrete wavelet transform unit 107. In the figure, the LH, HL, and HH subbands generated by the discrete wavelet transform unit 105 are respectively L
It is shown as H2, HL2, and HH2, and the LH, HL, and HH subbands generated by the discrete wavelet transform unit 107 are separately shown as LH1, HL1, and HH1. Note that the LL in FIG. 5 is obtained by combining the LL subband in FIG. 4C with the low-resolution image and then re-decomposing it.
Not the same.

The bit plane coding unit 108 has the LL and LH generated through the discrete wavelet transform unit 105, the coefficient synthesizing unit 106, and the discrete wavelet transform unit 107.
1, HL1, HH1, LH2, HL2, HH2 are coded for the coefficient values C (S, x, y) of the seven subbands to generate a code string. Divide the coefficient of each subband into blocks,
Although a method of facilitating random access by separately encoding is known, here, encoding is performed in subband units to simplify the description. Coding of the coefficient value C (S, x, y) of each subband expresses the absolute value of the coefficient value C (S, x, y) in the subband by a natural binary number, and the upper digit to the lower digit. It is performed by performing binary arithmetic coding on the digits in the bit plane direction. When the coefficient value C (S, x, y) of each subband is expressed in natural binary, the nth digit bit from the bottom is described as Cn (S, x, y). The variable n representing the binary digit is called a bit plane number, and the bit plane number n has the LSB as the 0th digit.

FIG. 6 is a flowchart showing the flow of processing for encoding the subband S in the bit plane encoding unit 108.

Step S601 is a step for obtaining the maximum value Mabs (S) of the absolute values of the coefficients in the subband S, and step S602 is the number of significant digits N _BP (S) required to represent the absolute value of the coefficients in the subband. ), Step S603 is a step of substituting the number of significant digits for the variable n,
Step S604 is a step of obtaining (n-1) and substituting it into n, step S605 is a step of encoding the bit plane of the n-th digit, and step S606 is a step of determining whether or not n is 0.

The bit plane coding unit 10 will be described with reference to FIG.
The flow of the subband S encoding process in No. 8 will be described.

First, in step S601, the absolute value of the coefficient in the subband S to be coded is checked, and its maximum value M
Calculate abs (S). Next, in step S602, Ma
Number of digits N required to represent bs (S) in binary number
_BP (S) is _calculated by the following formula.

N _BP (S) = ceil {log2 (Ma
bs (S))} where ceil {R} represents the smallest integer value equal to or greater than the real number R. In step S603, the number of significant digits N _BP (S) is substituted for the bit plane number n. In step S604, the bit plane number n is 1
pull. In step S605, the bit plane n is encoded using the binary arithmetic code. In this embodiment, QM-Coder is used as the arithmetic code. Regarding the procedure of coding a binary symbol generated in a certain state (context) using this QM-Coder, or the initialization procedure and termination procedure for arithmetic coding processing, the international standard ITU- T Recommendation T.
81 | ISO / IEC 10918-1 Recommendation etc. have been explained in detail, so the explanation is omitted here. Further, in order to simplify the explanation, in the present embodiment, each bit is arithmetically encoded with a single context. At the start of encoding each bit plane, the arithmetic encoder in the bit plane encoding unit 108 is initialized, and at the end, the termination processing of the arithmetic encoder is performed. Also, each coefficient is first coded '
Immediately after 1 ', the positive / negative sign of the coefficient is represented by 0, 1 and arithmetically coded. Here, 0 is set for positive and 1 for negative.

For example, when the coefficient is -5 and the number of significant digits N _BP (S) of the sub-band S to which this coefficient belongs is 6, the absolute value of the coefficient is represented by the binary number 00001, and the absolute value of each bit plane is Encoding is performed from the upper digit to the lower digit. At the time of encoding the second bit plane (in this case, the fourth digit from the top), the first "1" is encoded, and immediately thereafter, the positive / negative code "1" is arithmetically encoded.

In step S606, the bit plane number n is compared with 0, and n = 0, that is, in step S605 L
If the SB plane has been encoded, the subband encoding process is terminated, and otherwise, step S
The processing is moved to 604.

By the above-mentioned processing, all the coefficients of the subband s are coded, and the code string CS corresponding to each bit plane n is coded.
Generate (S, n). The generated code string is sent to the code string forming unit 109 and temporarily stored in the RAM 1002.

The bit plane coding unit 108 finishes coding the coefficients of all subbands, and all code strings are stored in the RAM1.
After being stored in 002, the code string forming unit 109 reads out the code string stored in the RAM 1002 in a predetermined order, inserts necessary additional information, and outputs a final code string to be the output of the present encoding apparatus. It is formed and output to the code output unit 110.

The final code string generated by the code string forming unit 109 is composed of a header and coded data hierarchically divided into three levels 0, 1, and 2. The coded data of level 0 is CS (LL, _NBP (LL) -1) obtained by coding the coefficients of the LL subband.
To CS (LL, 0). Level 1 is a code sequence CS (LH1, N _BP (LH) obtained by encoding the coefficients of each subband of LH1, HL1, and HH1.
1) -1) to CS (LH1,0), CS (HL1, N)
_BP (HL1) -1) to CS (HL1,0), and
CS (HH1, N _BP (HH1) -1) to CS (HH
1, 0). Level 2 is LH2, H
Code strings CS (LH2, N _BP (LH2) -1) to CS obtained by coding the coefficients of the subbands of L2 and HH2
_{(LH2,0), CS (HL2,} N BP (HL2) -
1) to CS (HL2,0) and CS (HH2, N
_{It is} composed of _BP (HH2) -1) to CS (HH2,0). FIG. 3 shows the structure of the code string generated by the code string forming unit 109.

The code output unit 110 outputs the code string generated by the code string forming unit 109 to the outside of the device. The code output unit 110 is, for example, an external storage device 1004 or a RAM.
When a storage device such as 1002 or an interface for a network line is newly provided in the image encoding device, this is the interface.

As described above, the image data is divided into the high resolution and the low resolution, and the low resolution sub-band obtained by sub-band decomposition of the high resolution image is combined with the low resolution image and encoded. By doing so, it becomes possible to read and encode only a necessary portion with high resolution.

[Second Embodiment] FIG. 8 shows the functional arrangement of an image coding apparatus according to this embodiment. It should be noted that parts common to those of FIG. 1 used in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. The basic configuration of the image coding apparatus according to this embodiment is the configuration shown in FIG.

In FIG. 8, reference numeral 801 is a low resolution image input section, 802 is an image area determination section, 803 is a high resolution image input section, and 804 is a coefficient synthesis section.

In this embodiment, as in the first embodiment,
The description will be made assuming that a black-and-white original is read and encoded as image data in which the brightness value of each pixel is represented by 8 bits.
However, not limited to this, 4 bits, 10 bits, 1
It can also be applied to the case of reading and encoding as image data expressing a luminance value with a bit number other than 8 bits such as 2 bits. It can also be applied to the case where each pixel is read as color image data represented by a plurality of color components such as RGB and CMYK or luminance and chromaticity / color difference components such as YCrCb. In this case, each component in the color image data may be regarded as monochrome image data. In the present embodiment, as in the first embodiment, a black-and-white original to be encoded is read and encoded at a predetermined resolution, but only a part of it is read at double the resolution in both the horizontal and vertical directions. Is. After that, the specified resolution is changed to low resolution, horizontal
The resolution obtained by doubling this in both the vertical direction is called high resolution. It is assumed that the size of the black and white original is fixed, and the image size when the entire original is read at high resolution is represented as X and Y. Both X and Y are multiples of 4.

The function and operation of each section of the image coding apparatus according to this embodiment will be described below with reference to FIG.

First, the low resolution image input unit 801
Read the entire black-and-white document to be encoded at half the resolution (for example, read every other pixel in both horizontal and vertical directions),
Pl (lx, ly) is generated. Where lx is 0- (X
/ 2-1) and ly are 0 to (Y / 2-1).

The image area determination unit 802 is a low resolution image input unit 8
The character / line drawing area is detected from the low resolution image Pl (lx, ly) input from 01, and the resolution flag Fl (lx, ly) is detected.
ly) is generated. The resolution flag Fl (lx, ly) indicates whether or not each pixel of the low resolution image Pl (lx, ly) is included in the portion determined to be the character / line drawing area, and Pl
1 if (lx, ly) is included in the character / line drawing area
Is set to 0 when Pl (lx, ly) is not included in the character / line drawing area. In the present embodiment, the image area determination unit 80
The specific method of character / line drawing area discrimination in 2 is not limited.

The high resolution image input section 803 has a resolution flag Fl (lx, l) generated by the image area determination section 802.
y), a black-and-white original is read at the resolution of the original image to form high-resolution image data Ph (x, y).
Resolution flag Fl (floor {x / 2}, floor
For the positions x and y where {y / 2}) = 0, the actual reading operation is not performed, and Ph (x, y) = 0 is set.
Here, x is 0 to (X-1) and y is 0 to (Y-1).

The discrete wavelet transform unit 105 subband decomposes each pixel data Ph (x, y) of the high resolution image data input by the high resolution image input unit 803 into subbands in the same manner as in the first embodiment. Subband coefficient C
(S, x, y) (S is any of LL, LH, HL, HH) is generated.

When the subband decomposition of the high resolution image in the discrete wavelet transform unit 105 is completed, the coefficient synthesizing unit 804 generates the resolution flag F generated by the image area determining unit 802.
By referring to l (lx, ly), the low-frequency subband C (LL, x, generated by the discrete wavelet transform unit 105.
y) and the low resolution image Pl (lx, ly) are combined.
The low-frequency subband C (LL, x, y) is the low-resolution image P
Since it has the same size as l (lx, ly), the variable representing the coefficient position in the subband is changed from x, y to lx, ly, and the synthesizing process of the coefficient synthesizing unit 804 will be described. The coefficient synthesizing unit 804 sets the resolution flags Fl (lx,
ly) is checked, and if F (lx, ly) = 0, then C (L
Replace L, lx, ly) with Pl (lx, ly).

After that, the process of encoding from the discrete wavelet transform unit 107 to the code output unit 110 is as described in the first embodiment.

As described above, image data is divided into a high resolution image and a low resolution image, and the low resolution image is combined with the low frequency image obtained by subband decomposition of the high resolution image and encoded. As a result, it is possible to read and encode only the necessary portion with high resolution. In the present embodiment, by once reading the original document at a low resolution, it is possible to select and read an area requiring a high resolution.

[Third Embodiment] FIG. 9 shows the functional arrangement of an image coding apparatus according to this embodiment. Parts common to FIG. 1 used in the first embodiment and FIG. 8 used in the second embodiment are denoted by the same reference numerals, and description thereof will be omitted. The basic configuration of the image coding apparatus according to this embodiment is the configuration shown in FIG.

In FIG. 9, reference numeral 901 denotes a mask generation unit, and 90
2 is a coefficient shift unit.

In the present embodiment, as in the first and second embodiments, a black-and-white document is read and encoded as image data in which the luminance value of each pixel is represented by 8 bits. However, the present invention is not limited to this, and can also be applied to the case of reading and encoding as image data expressing a luminance value with a bit number other than 8 bits such as 4 bits, 10 bits, and 12 bits. It can also be applied to the case where each pixel is read as color image data represented by a plurality of color components such as RGB and CMYK or luminance and chromaticity / color difference components such as YCrCb. In this case, each component in the color image data may be regarded as monochrome image data. In the present embodiment, as in the first embodiment, a black-and-white original to be encoded is read and encoded at a predetermined resolution, but only a part of it is read at double the resolution in both the horizontal and vertical directions. Is. Hereinafter, the predetermined resolution will be referred to as low resolution, and the resolution obtained by doubling the predetermined resolution will be referred to as high resolution. The size of the black and white manuscript shall be fixed,
The image size when the entire original is read in high resolution is represented as X and Y. Both X and Y are multiples of 4.

The function and operation of each section of the image coding apparatus according to this embodiment will be described below with reference to FIG.

The image coding apparatus of this embodiment is the image coding apparatus of the second embodiment to which a mask generation unit 901 and a coefficient shift unit 902 are added, and the functions and operations of other parts are the same as those of the second embodiment. Since it is the same as the embodiment described above, only the operations of the added mask generation unit 901 and coefficient shift unit 902 will be described.

The mask generation unit 901 uses the resolution flag Fl (lx, ly) generated by the image area determination unit 802 to calculate the C (LL, x, y) obtained by the coefficient synthesis unit 804 by the discrete wavelet transform unit 107. Each coefficient C (LL, x, y), C (LH) of each sub-band obtained by band decomposition
1, x, y), C (HL1, x, y), C (HH, x,
Mask information M (x, y) indicating whether or not y) is a portion related to the high resolution image Ph (x, y) is generated and output. The mask information is half the size of the information of the resolution flag, and is Fl (2 × x, 2 × y), Fl (2 × x +).
1, 2 × y), Fl (2 × x, 2 × y + 1), Fl (2
If either xx + 1, 2xy + 1) is 1, then M (x,
If y) = 1 and all 0, then M (x, y) = 0.

The coefficient shift unit 902 obtains the maximum value Nmax of the number of effective bits of each subband, and outputs LL, LH1, HL1, H output from the discrete wavelet transform unit 107.
The mask information M (x, y) generated by the mask generation unit 901 is referred to for the coefficient value C (S, x, y) of the H1 subband, and if M (x, y) is 1, the coefficient C of the subband is calculated.
(S, x, y) is shifted up by Nmax bits. Therefore, when the coefficients of all subbands are referred to, the coefficient whose effective bit number is Nmax or more is M (x, y) = 1.
Can be regarded as a coefficient corresponding to, and a high-resolution area can be specified at an early stage.

After that, the processing up to the code output section 110 is as described in the first and second embodiments. However, in the present embodiment, when the code string forming unit 109 inserts the additional information into the encoded data, the number of bits Nmax shifted up by the coefficient shifting unit 902 is inserted into the header.

In the case of this embodiment, in addition to the effect described in the second embodiment, it becomes possible to early decode the character / line drawing information on the receiving side of the code string.

<Modification> The present invention is not limited to the above embodiment. For example, in the above-described first to third embodiments, an example of encoding using the discrete wavelet transform according to Expressions (1) and (2) has been shown, but the discrete wavelet transform is not limited to that used in this embodiment. There is no limitation, and the type of filter and the adaptation method may be changed. For example, the filter may be changed to a filter having a larger number of taps than the 9/7 filter, or the two-dimensional discrete wavelet transform may be repeatedly applied in addition to the low frequency subband. However, in this case, it is necessary to consider the influence range of the filter in the process of coefficient synthesis.

QM-Co is used as a coefficient coding method.
Although the bit plane coding method using der is shown,
The present invention is not limited to the above-mentioned embodiment, and for example, M
An arithmetic coding method other than QM-Coder such as Q-Coder may be applied, or another binary coding method such as MELCODE may be applied. In addition, the bit plane may be categorized into a plurality of sub-bit planes according to the state of the coefficient near the coefficient of interest, and may be encoded by a plurality of passes. Furthermore, the Golomb code or the like may be applied to entropy-encode the multi-value as it is without dividing the coefficient into two values.

Further, in order to simplify the explanation, in each of the above-mentioned embodiments, bit-plane coding in sub-band units has been explained, but in order to improve random accessibility, each sub-band is further divided into small blocks. Bit plane coding may be applied in units of this small block.

Further, in forming the code string, the images are arranged so that the image can be restored by gradually increasing the resolution on the receiving side. However, the present invention is not limited to this, and the coefficients are arranged in order from the largest value so that the image quality is gradually improved. A code string may be formed.

Even when the present invention is applied as a part of a system composed of a plurality of devices (for example, host computer, interface device, reader, printer, etc.), a single device (for example, copying machine, facsimile machine, It may be applied to a part of a device including a digital camera).

The present invention is not limited to the apparatus and method for realizing the above-mentioned embodiment,
Computer (CPU or
MPU), a software program for realizing the above-described embodiment is supplied, and the computer of the system or the apparatus operates the above-mentioned various devices according to the program to realize the above-described embodiment. Included in the category.

Further, in this case, the software program itself realizes the functions of the above embodiments, and the program itself and a means for supplying the program to the computer, specifically, the program is stored. Such storage media are included in the scope of the present invention.

As a storage medium for storing such a program, for example, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a magnetic tape, a non-volatile memory card, a ROM or the like can be used.

Further, not only when the computer controls the various devices only according to the supplied program to realize the functions of the above-described embodiments, but also when the program runs on the computer. Such a program is also included in the scope of the present invention when the above embodiment is implemented in cooperation with (operating system) or other application software.

Further, after the supplied program is stored in the memory provided in the function expansion board of the computer or the function expansion unit connected to the computer, the function expansion board or function expansion unit is instructed based on the instruction of the program. It is also within the scope of the present invention that the CPU and the like included in the above perform a part or all of the actual processing and the above embodiment is realized by the processing.

[0083]

As described above, according to the present invention, efficient image encoding can be performed when encoding image data including areas requiring different resolution levels. Further, when decoding the coded image data, it is possible to perform coding for generating a code string that can identify the resolution region of interest at an early stage.

[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 an example of a document and high resolution area information.

FIG. 3 shows a structure of a code string generated by a code string forming unit 109.

FIG. 4 is a diagram showing a two-dimensional discrete wavelet transform process.

FIG. 5 is a diagram showing how the LL subband after synthesis is further decomposed into four subbands by the discrete wavelet transform unit 107.

FIG. 6 shows a subband S in the bit plane encoding unit 108.
3 is a flowchart showing a flow of a process of encoding the.

FIG. 7 shows an example of a resolution flag F (x, y) when high resolution area information is designated as shown in FIG.

FIG. 8 is a diagram showing a functional configuration of an image encoding device according to a second embodiment of the present invention.

FIG. 9 is a diagram showing a functional configuration of an image encoding device according to a third embodiment of the present invention.

FIG. 10 is a diagram showing a basic configuration of an image encoding device according to the first to third embodiments of the present invention.

Claims (24)

[Claims] 1. An image coding apparatus for coding an image, comprising: first reading means for reading a first area of an original image at a first resolution and generating a first image including the read area. And a second higher than the first resolution in the original image.
Second reading means for reading a second area different from the first area and generating a second image including the read area at a resolution of, and performing frequency conversion on the second image. , A first frequency conversion unit for obtaining a coefficient for each subband, and combining the first image with a predetermined subband of the plurality of subbands by the first frequency conversion unit to generate a combined subband. A combination means, a second frequency conversion means for further performing frequency conversion on the combined subband to obtain a coefficient for each subband, the first frequency conversion means, and the second frequency conversion means. An image coding apparatus, comprising: a code string generation unit that generates a code string from the coefficients of the subband. 2. The image coding apparatus according to claim 1, further comprising flag generation means for generating a flag designating an area to be read by the second reading means. 3. The first reading means refers to the flag, reads an area other than the area read by the second reading means at a first resolution, and generates the first image. The image coding apparatus according to claim 2, wherein the image coding apparatus is a video coding apparatus. 4. The image coding apparatus according to claim 1, wherein the size of the first image is equal to the size of the predetermined subband. 5. The image coding apparatus according to claim 1, wherein the first resolution is half of the second resolution. 6. The synthesizing means, using the flag,
The image coding according to claim 2, wherein a coefficient included in a region corresponding to the first region in the predetermined subband is specified, and the specified coefficient is replaced with the first image. apparatus. 7. The image coding apparatus according to claim 1, further comprising flag generation means for generating a flag indicating a predetermined area in the first image. 8. The image coding apparatus according to claim 7, wherein the predetermined area is a character or line drawing area. 9. The second reading unit refers to the flag to set a second region of the predetermined area in the original image.
9. The image coding apparatus according to claim 7, wherein the second image is generated by reading the image with the resolution of. 10. The size of the first image is equal to the size of the predetermined subband.
The image coding device according to any one of claims 1 to 9. 11. The image coding apparatus according to claim 7, wherein the first resolution is half of the second resolution. 12. The synthesizing unit uses the flag to identify a coefficient included in a region corresponding to the first region in the predetermined subband, and the identified coefficient is used as the first image. 12. The image coding apparatus according to claim 7, wherein the image coding apparatus is replaced with. 13. A mask showing a coefficient relating to the second image among coefficients constituting a subband generated when the code string generation means frequency-converts the combined subband. The image coding apparatus according to claim 7, further comprising a mask generation unit that generates the mask. 14. The image coding apparatus according to claim 13, wherein the mask generation means specifies a coefficient associated with the second image using the flag. 15. The code string generation means obtains a maximum value of the number of effective bits included in each subband generated by the code string generation means, and a coefficient related to the second image is divided by the maximum value. The image coding apparatus according to claim 1, wherein the bit shift is performed only by the bit shift. 16. The image coding apparatus according to claim 15, wherein the code string includes the maximum value. 17. The image coding apparatus according to claim 1, wherein the first frequency transforming means and the second frequency transforming means use discrete wavelet transforms. 18. The image coding apparatus according to claim 1, wherein the predetermined subband is an LL subband. 19. An image encoding method for encoding an image, comprising a first reading step of reading a first region of an original image at a first resolution and generating a first image including the read region. And a second higher than the first resolution in the original image.
A second reading step of reading a second area different from the first area and generating a second image including the read area at a resolution of, and performing frequency conversion on the second image. , A first frequency conversion step of obtaining a coefficient for each subband, and combining the first image with a predetermined subband of the plurality of subbands by the first frequency conversion step to generate a combined subband And a second frequency conversion step of further performing frequency conversion on the composite subband to obtain a coefficient for each subband, and a first frequency conversion step and a second frequency conversion step. And a code string generating step of generating a code string from the coefficients of the obtained subbands. 20. The image coding method according to claim 19, further comprising a flag generating step of generating a flag designating an area to be read in the second reading step. 21. The image encoding method according to claim 19, further comprising a flag generating step of generating a flag indicating a predetermined area in the first image. 22. A mask showing a coefficient relating to the second image among coefficients forming a subband generated when frequency conversion is performed on the combined subband in the code string generating step. The image coding method according to claim 21, further comprising a mask generation step of generating. 23. A program for executing the image coding method according to any one of claims 19 to 22 on a computer. 24. A computer-readable storage medium that stores the program according to claim 23.