JP2948147B2 - Image coding device - Google Patents

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 a binary image.

[0002]

2. Description of the Related Art A facsimile apparatus, which is a typical example of a conventional still image communication apparatus, employs a system in which images are sequentially scanned in a raster direction and encoded and transmitted. In this method, it is necessary to transmit the coded data of all the images in order to grasp the whole image of the image.
It has been difficult to adapt to video communication services such as video technology.

[0003] Hierarchical coding has been considered in order to quickly grasp the entire image of an image. FIG. 10 (a) shows a conventional hierarchical coding example. 101 to 104 are frame memories for storing 1, 1/2, 1/4, and 1/8 reduced images, respectively, and 105 to 107 generate 1/2, 1/4, and 1/8 reduced images, respectively. Reduction section, 108 to 111 are 1/8 each
, 1/4, 1/2, 1 are encoded.

A reducing section 105 reduces the image from the frame memory 101 by a method such as subsampling in both the main scanning direction and the sub-scanning direction by half, and generates a 1/2 size image.
It is stored in the frame memory 102. Further, the 1 / 2-size image is reduced by the reduction unit 106 to create a 1 / 4-size image and stored in the frame memory 103.
An 8-size low-resolution image is created and stored in the frame memory 104.

[0005] By transmitting codes sequentially from low-resolution ones, a rough overall image can be quickly grasped. In the example of FIG. 10 (a), the image is reduced to 1/2, 1/4, 1/8 in both the main scanning and sub-scanning directions, and the encoding is performed for 1/8, 1/4, 1/2, 1 This is an example in which the transmission is performed in the order of (actual size image) and the transmission is performed in this order. For encoding 1/8 image, 108 images stored in the frame memory 104 are sequentially scanned,
Entropy coding such as arithmetic coding is performed with reference to the target pixel to be coded by the encoder 108 and surrounding pixels. For the 1/4 image, the surrounding pixels of the pixel of interest from the frame memory 103 and the 1/8 image from the frame memory 104
The encoding is performed by the encoder 109 by referring to the surrounding pixels of the image to increase the encoding efficiency. Similarly, for a half image of the frame memory 102, the frame memory 103
The original image of the frame memory 101 is referred to as a 1/4 image of the
The encoding is performed in 1 respectively.

[0006] Further, other than the still image communication device, 2
Reduction of the value image has been performed. For example, this is a case where images are output from the same image database to printers having different output resolutions. When a binary image read at 400 dpi is output to a 300 dpi or 200 dpi printer, the image must be reduced to 3/4 and 1/2 in both the vertical and horizontal directions.

Conventionally, when such a reduction is performed, a sub-sampling method is used in which pixels are thinned out at regular intervals, or a low-pass filter is applied and then re-binarized and sub-sampled.

[0008]

In the hierarchical encoding method, as described above, the entire image can be transmitted at an early stage by sequentially transmitting the reduced images in the order of low resolution.
Therefore, it is necessary to leave information in the reduced low-resolution image so that the entire image can be easily grasped. If such reduction is performed by the conventional method, there is a disadvantage that information is lost. FIG. 10 (b) shows an example of sub-sampling the pixels marked with x in the original image (1) to obtain a reduced image (2) in the vertical and horizontal directions.

In the case of only sub-sampling, if one line L is in the middle of a sampling point (marked by x in the figure) as shown in the figure, this line disappears due to reduction. In order to solve such a disadvantage, a method of performing sub-sampling after performing filtering has been considered. An example is shown in FIG. 10 (c). In FIG. 10 (c), crosses indicate sampling points. In the example shown in FIG. 10 (c), a 3 × 3 low-pass filter having a coefficient as shown in (3) of FIG.
Perform value conversion. For example, binarization can be defined as 1 when the filter output is 8 or more, and 0 when it is less than 8. However, even in the method using filtering, the disadvantage that the line disappears when the single vertical line L2 in the original image is between the sub-samplings in the example of FIG. 10 (c) is not improved.

Therefore, in a system in which the reduction is repeated many times, a line having a width of one pixel must be preserved, and the line will eventually disappear in a low-resolution image. Therefore, it is necessary to save a thin line such as one pixel line regardless of the sampling point. Further, when such a reduction is performed in a binary pseudo halftone image such as a dither image, there is a disadvantage that density information is lost due to sampling points. In particular, in the case of low density or high density, when dots due to the pseudo halftone are discretely scattered, there is a problem that the halftone is suddenly lost or the density is reversed.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is intended to reduce the size of a binary image by preventing the disappearance of fine lines in a line drawing or the like and the loss of density or information in a dither image. It is an object of the present invention to propose an encoding device for encoding. For this purpose, the image encoding apparatus according to the present invention includes a first storage unit that stores original image data representing an original image, a first storage unit that reduces the original image data, and a first storage unit that represents the reduced image. First reducing means for outputting reduced image data, second storing means for storing the first reduced image data, and a second means for reducing the first reduced image data and representing the reduced image A second reduction unit that outputs the reduced image data of the original, the document image data output from the first storage unit, the first reduced image data output from the second storage unit, Encoding means for encoding the second reduced image data output from the second reducing means.

The first reduction means and the second reduction means comprise first reduced image data of arbitrary pixels forming a first reduced image and a second reduced image, and a second reduced image. To determine the data, (a-1) the image data of the pixel of interest to be reduced, (a-2) the image data of the pixels surrounding the pixel of interest, and (b-1) before the pixel of interest (B-2) reduced image data of an arbitrary second reduced pixel located above the pixel of interest, and (b-3) reduced image data of an arbitrary first reduced pixel located therein. Above the pixel and the second
, And reference is made to reduced image data of an arbitrary third reduced pixel located in front of the reduced pixel.

In order to achieve the above object, an image encoding apparatus according to the present invention comprises: frame means for storing original image data representing an original image; a reducing means for reducing the original image data; The image processing apparatus includes: a reducing unit that outputs image data; a holding unit that holds the reduced image data; and an encoding unit that encodes the reduced image data output from the reducing unit.

The reduction means reads out (a-1) the frame means in order to determine reduced image data of each pixel forming the reduced image.
Image data of the pixel of interest to be reduced, (a-2) read from the frame unit, image data of pixels around the pixel of interest, and (b-1) read from the holding unit, Reduced image data of any first reduced pixel located before the pixel of interest; and (b-2) any second reduced pixel located above the pixel of interest read from the holding unit. And (b-3) reduced image data of an arbitrary third reduced pixel that is read from the holding unit and located above the first reduced pixel and in front of the second reduced pixel. Are filtered.

[0015]

FIG. 1 shows the configuration of a binary image encoding apparatus to which the present invention is applied. In the present embodiment,
Here is an example of reducing both sides to half. Reference numeral 1 denotes a frame memory for storing a binary image to be reduced. The binary image is input to the frame memory 1 via an input I from an image reader or the like, for example. The image stored in the frame memory 1 is input to the filter operation unit 2 for each raster. The already reduced pixels are input to the filter operation unit 2 through the feedback line 71. The filter operation unit 2 performs an inter-pixel operation between the pixel of interest to be subjected to the reduction processing, the peripheral pixels, and the already reduced binary pixels.

FIG. 2 shows operation pixels used in the filter operation performed by the filter operation unit 2. FIG. 2A shows a target pixel Xij (i = 1 to M, j = 1, N; M) to be subjected to reduction processing.
N is an image size in the horizontal and vertical directions) and peripheral pixels, which are 3 × 3 pixels centered on the target pixel Xij. FIG. 2 (b)
Are the pixels A, B, and C that have already been reduced and the reduced pixels that are to be determined

[0017]

(Equation 1)

The following shows the positional relationship. In FIG. 2 (b), A, B and C are reduced pixels to be determined, respectively.

[0019]

(Equation 2)

The reduced pixel before (the binarized result of the operation Yij), the reduced pixel at the same position as the reduced pixel on the previous line, and the reduced pixel before it. The filter operation is performed as follows. If the filter output and _{Yij, Y ij = 4 × X} i, j +2 (X i, j-1 + X i-1, j + X i + 1, j + X i, j + 1) + (X i-1, _j-1 + Xi _{+ 1, j-1} + Xi _{-1, j + 1} + Xi _{+ 1, j + 1} ) -3 (A + B) -C (1) That is, as shown in FIG. Has a coefficient shown in FIG. 9A, and a recursive filter in which reduced pixels which have already been subjected to the filter processing are given the coefficient shown in FIG. The pixel of interest (Xi
Since the coefficient values are rotationally symmetric about j), the peripheral pixel values can be calculated uniformly. Also, as shown in FIG. 3 (b), after performing a filter operation and binarizing, among the subsampled pixels, the left (immediately before), the upper (one line before) and the upper left (immediately before the pixel one line before) ) Is calculated as the coefficient of the recursive section, it is possible to improve the preservability of thin lines in the vertical, horizontal and oblique directions. In addition, since the coefficient of the recursive unit is symmetrically fed back, it is possible to save and reduce the line of every other pixel, for example, the shape of a periodic pattern such as a checkered pattern.

FIG. 4 shows the relationship between non-reduced pixels centered on the sub-sampling point * and pixels A, B and C already reduced. As is clear from the filter coefficients in FIG.

[0022]

(Equation 3)

The non-reduced pixels having the strongest correlation are 3 × 3 pixels surrounded by a bold line, and all the reduced pixels have no deflection in the direction as shown in the figure. In the examples shown in FIGS. 2 and 3, a reduced pixel before a reduced pixel determined by the filter operation as a reduced image used for the filter operation, a reduced pixel located immediately above a target pixel on the previous line, and Is used, but the present invention is not limited to these three reduced pixels. For example, the true pixels of the pixel of interest on the line two lines before the reduced pixel to be further determined as these reduced pixels or the line before the reduced pixel are used. Four or more reduced pixels obtained by adding the reduced pixels and the like located above may be used for the filter operation. According to this, the characteristics of the original image can be stored more favorably in the reduced image, but the scale of the filter operation unit increases.

The value Yij (multi-value data) calculated by the filter operation unit 2 according to the equation (1) is input from the filter operation unit 2 to the comparator 4. The comparator 4 binarizes Yij as follows. That is,

[0025]

(Equation 4)

Let Yij> T be the value of Yij after binarization.
If

[0027]

(Equation 5)

If Yij <T,

[0029]

(Equation 6)

Binarization is performed as shown in FIG. The appearance probabilities of 1,0 with respect to input data by this binarization method are equal probabilities (0.
Assuming 5), the expected value of the filter operation output is 4.5. Therefore, when T = 5, the binarized output of the 1,0 symmetric pattern with respect to the input data pattern is also 1,0 symmetric. In this embodiment, T = 5. However, if the value of T is changed, the binary output, that is, the image quality changes. That is, as the value of T increases, the ratio of output 0 increases, and as the value of T decreases, the ratio of output 1 increases. Therefore, if 1 is black and 0 is white,
The former is particularly effective for negative images, and the latter is particularly effective for positive images.

The data binarized by the comparator 4 is selected by the selector unit 5 and input to the sub-sampling unit 6.
FIG. 5 shows the sampling positions of the sub-sampling unit 6. By sampling the data indicated by diagonal lines in the main scanning direction (horizontal direction) and every other timing in the sub-scanning direction (vertical direction), both the vertical and horizontal sizes are 1/2 (area ratio is 1/4). ) Can be formed. It is also possible to change the reduction ratio by changing the sub-sampling ratio.

The reduced images subsampled by the sampling section 6 are sequentially stored in the frame memory 7 for each raster. The pixel data already reduced and stored from the frame memory 7 is sent to the filter operation unit 2 through the line 71, and the filter operation unit 2 performs the filter operation as a recursive component. Next, the exception processing unit 3 will be described.
Even in the method of generating a reduced image by the method described above, there are cases where thin lines or the like are not stored. 6 and 7 show examples. Each figure corresponds to each pixel in FIG. 2, a small 3 × 3 pixel corresponds to a non-reduced pixel MI, and a large 3 pixel corresponds to a reduced pixel RI already reduced. ■ and □ are black pixel, white pixel,

[0033]

(Equation 7)

Is a Don't care pixel. FIG. 6 shows an example in which one pixel line disappears due to reduction when the sub-sampling point is at the center of the 3 × 3 non-reduced pixel MI. Further, similar disappearance occurs at the edge of the image and the like. In the example of FIG. 6, the filter operation value is 4 or less and the pixel is reduced to a white pixel regardless of the reduced pixel RI.

FIG. 7 illustrates a problem in reducing a pseudo halftone such as a dither image. Since the pseudo halftone image has periodicity in black pixels, there arises a problem that the halftone density significantly changes or the image disappears depending on the positional relationship with the sampling point. 7 (a), (b), (c), and (d), when all three of the reduced pixels RI are white pixels, the filter operation values are all 4 or less and are reduced to white pixels. However, in these cases, there is a high possibility that the image is an isolated point of the halftone image, and the above-described problem occurs unless the image is reduced to a black image.

Therefore, as shown in FIG. 1, an exception processing unit 3 is provided, and exception processing is performed separately from the filtering operation processing and the reduction processing by sub-sampling, whereby fine lines, edges,
Saves information such as isolated points. The image signal to be reduced is sequentially input to the exception processing unit 3 for each raster from the frame memory 1 as in the filter operation unit 2. Here, the same pixel is referred to at the same timing as the 3 × 3 pixel referred to by the filter operation unit 2. In addition, the pixels which have already been reduced and stored in the frame memory 7 (FIGS. 2A, B,
C) is also input to the exception processing unit 3 through the line 71.
That is, the filter operation unit 2 and the exception processing unit 3 perform completely parallel processing.

The non-reduced pixel 3 × input to the exception processing unit 3
If the three pixels and the three reduced pixels have a pattern as shown in FIG. 6 or FIG. 7, the result of binarization by the comparator 4 after the filter operation is not preferable as the reduced pixel. Therefore, in these cases, the reduction result (white pixel “0” or black pixel “1”) is output to the line 31 as an exception pattern, and at the same time, a signal “1” indicating the exception (“0” otherwise) is indicated. To line 32. The lines 31 and 32 are input to the selector unit 5. When the line 32 is "0", a signal from the comparator 4 is selected instead of an exception pattern. Conversely, if the line 32 is “1”, it is an exception pattern, and the result of the exception processing unit 3, that is, the line 31 is selected.

The pixel signal selected by the selector unit 6 is sub-sampled by the sub-sampling unit 6 as described above, and then stored in the frame memory 7 as a reduced image. As described above, since the apparatus includes the recursive filter operation unit 2 that uses already reduced pixels as a feedback coefficient and the comparator 4 that binarizes the operation result, a reduced image with good storability such as a fine line periodic pattern is generated. can do. Further, by providing the exception processing unit 3 for correcting the filter operation, it is possible to further improve the preservability of fine lines, edges, pseudo halftone images, and the like. [Second Embodiment] FIG. 8 is a block diagram when the reduction apparatus shown in FIG. 1 is applied to hierarchical coding.

Reference numerals 15, 17, and 19 denote frame memories, 1
Reference numerals 6 and 18 denote reduction circuits having the configuration shown in FIG.
4 is a reference pixel determination circuit, 21, 23 and 25 are encoders. First, the original image data I of the binary image signal is stored in the frame memory 15. Next, the original image data in the frame memory 15 is reduced by the reduction circuit 16 and stored in the frame memory 17. At this time, the reduction circuit 16 performs the reduction operation by using the original image data of the frame memory 15 and the reduced data already stored in the frame memory 17 to perform the filter operation of the above-described equation (1). The signal stored in the frame memory 17 is reduced to half of the original image. Similarly, the signal read from the frame memory 17 is reduced to a quarter of the original image by the reduction circuit 18 in consideration of the reduced data from the frame memory 19 and stored in the frame memory 19.

The reference pixel determination circuits 20, 22, and 24 detect the sizes (number of pixels) of the image data stored in the frame memories 19, 17, and 15, respectively. , And a reference pixel position. The encoder 21 encodes a 1/4 image signal stored in the frame memory 19 using the reference pixel set by the reference pixel determination circuit 20 and outputs it as a first-stage signal 26. Similarly, the encoders 23 and 25 encode the half image and the original image signal stored in the frame memories 17 and 15 using the reference pixels set by the reference pixel determination circuits 22 and 24, respectively. The second stage signal 108 and the third stage signal 109 are output.

As described above, the image data from the first stage to the third stage is coded and transmitted in order from the image data having the lower resolution, so that the entire image of the image can be identified quickly, and if the data is unnecessary. , The subsequent transmission can be stopped. This enables an efficient image communication service. In addition, although only the third stage has been described here, it is needless to say that the present embodiment can be easily extended to an arbitrary stage.

Encoders 21, 23 and 25 shown in FIG. 8 perform entropy coding such as arithmetic coding for predictively coding the value of a target pixel from the reference pixels obtained by the reference pixel determination circuits 20, 22, and 24, respectively. Can be configured. Arithmetic codes are generally known and will not be described. As described above, by applying the reduction device shown in FIG. 1 to stepwise encoding, progressive image encoding with little image quality degradation can be performed. [Third Embodiment] FIG. 9 is a block diagram when the reduction device shown in FIG. 1 is used as an image input / output device. That is, it is adapted to resolution conversion when the resolution of the output device is lower than the resolution of the image input device.

Reference numeral 91 denotes an image input device, which is an image reading device or an image receiving terminal; 92, a reduction circuit shown in FIG. 1; and 93, an image output printer or an image transmitting terminal. For example, when the image input by the image input device 91 is 400 dpi and the image output by the image output device 93 is 200 dpi,
By performing 1/2 reduction in the configuration shown in FIG. 1, a reduced image with less image quality degradation can be generated. It can be adapted by repeating the reduction operation at another resolution or another reduction ratio, or by changing the interval between subsamples. Further, the present invention can be applied to image reduction between disks or the like storing an image database or the like instead of the image input / output device.

[0044]

As described above, according to the encoding apparatus of the present invention, since the reduction processing is performed in consideration of the state of the image after the reduction, the conventional binary image coding performs The information of the pseudo halftone image such as the edge, the thin line, or the dither image of the lost image can be stored even after the encoding. Furthermore, by applying the present invention to hierarchical coding, a low-resolution image with good information preservation can be obtained, and early transmission of the entire image can be performed more effectively. Further, when the resolution of the output device is lower than that of the input device, by performing the encoding of the present invention, a reduced image with little deterioration can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a binary image reducing device to which the present invention is applied.

FIG. 2A is a diagram showing a filter operation pixel.

FIG. 2B is a diagram showing a filter operation pixel.

FIG. 3a is a diagram showing filter operation coefficients.

FIG. 3B is a diagram showing filter operation coefficients.

FIG. 4 is a diagram illustrating a positional relationship between a sub-sampling position and a reduced pixel.

FIG. 5 is a diagram showing positions of sub-sampling.

FIG. 6A is a diagram illustrating an example in which a thin line disappears when only a filter operation is reduced.

FIG. 6B is a diagram showing an example in which a thin line disappears when only the filter operation is reduced.

FIG. 7A is a diagram illustrating an example in which an isolated point such as a dither image disappears when only a filter operation is reduced.

FIG. 7B is a diagram illustrating an example in which an isolated point such as a dither image disappears when only the filter operation is reduced.

FIG. 7C is a diagram showing an example in which an isolated point such as a dither image disappears when only the filter operation is reduced.

FIG. 7D is a diagram showing an example in which an isolated point such as a dither image disappears when only the filter operation is reduced.

FIG. 7E is a diagram showing an example in which an isolated point such as a dither image disappears when only the filter operation is reduced.

FIG. 8 is a block diagram when the present invention is applied to hierarchical coding.

FIG. 9 is a block diagram when the present invention is applied to an input / output device.

FIG. 10a is a block diagram showing conventional hierarchical coding.

FIG. 10b is a diagram showing an example of reduction by conventional sub-sampling.

FIG. 10c is a diagram showing a conventional low-pass filter and an example of reduction by sub-sampling.

[Explanation of symbols]

 1, 7: frame memory, 2: filter operation unit, 3: exception processing unit, 4: comparator, 5: selector unit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naoto Kawamura 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hisaharu Kato 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo Country (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 1/393 G06T 3/40 H04N 1/41 H04N 1/411

Claims (13)

(57) [Claims] A first storage unit that stores original image data representing an original image; a first reduction unit that reduces the original image data and outputs first reduced image data representing the reduced image Means, second storage means for storing the first reduced image data, and second storage for reducing the first reduced image data and outputting second reduced image data representing the reduced image. Reduction means, the original image data output from the first storage means, the first reduced image data output from the second storage means, and the output from the second reduction means. Encoding means for encoding the second reduced image data, wherein the first reducing means and the second reducing means include an arbitrary one for forming a first reduced image and a second reduced image. The first reduced image data and the second reduced image data of the pixel (A-1) image data of a target pixel to be reduced, (a-2) image data of pixels around the target pixel, and (b-1) before the target pixel. (B-2) reduced image data of an arbitrary second reduced pixel located above the target pixel; and (b-3) reduced image data of an arbitrary first reduced pixel located therein. A reduced image data of an arbitrary third reduced pixel located above the pixel and in front of the second reduced pixel. 2. The image encoding apparatus according to claim 1, wherein reduced image data is determined by filtering image data of the plurality of pixels referred to. 3. Frame means for storing document image data representing a document image; reducing means for reducing the document image data and outputting reduced image data representing the reduced image; and holding the reduced image data And a coding unit that codes the reduced image data output from the reduction unit, wherein the reduction unit determines reduced image data of each pixel forming the reduced image. (A-1) image data of the pixel of interest to be reduced, read from the frame means, and (a-2) image data of pixels around the pixel of interest, read from the frame means. Image data; (b-1) reduced image data of an arbitrary first reduced pixel located before the pixel of interest, read from the holding unit; and (b-2) read from the holding unit. On the pixel of interest And (b-3) any reduced image data read from the holding unit and located on the first reduced pixel and in front of the second reduced pixel. An image encoding apparatus that refers to reduced image data of a third reduced pixel. 4. The image encoding apparatus according to claim 3, wherein reduced image data is determined by filtering image data of the plurality of pixels referred to. 5. The image encoding apparatus according to claim 1, wherein said first storage means and said second storage means are frame memories. 6. The image encoding apparatus according to claim 1, wherein the document image is a binary image. 7. The image encoding apparatus according to claim 1, wherein the first reduction unit and the second reduction unit reduce an input image by half. 8. The image encoding apparatus according to claim 3, wherein said reduction unit reduces an input image by half. 9. The image encoding apparatus according to claim 1, wherein said encoding means performs entropy encoding. 10. The image encoding apparatus according to claim 9, wherein an arithmetic code is used in the entropy encoding. 11. The image encoding apparatus according to claim 1, wherein said encoding means has a reference pixel determination circuit for setting the number and position of pixels to be referenced. apparatus. 12. The image encoding apparatus according to claim 11, wherein the reference pixel determination circuit determines the reference pixel according to a size of the document image. 13. The image processing apparatus according to claim 3, further comprising a correction unit configured to correct the reduced image data output from the reduction unit when the referenced image data has a predetermined pattern. The image encoding device according to claim 1.