JPH0135540B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a predictive conversion encoding device for encoding image information in a facsimile device for transmitting images with gradations such as photographs, an image storage device for storing images, etc. It is. Conventional configuration and its problems As a coding method for quantized multi-tone images,
Traditionally, DPCM (Differential Pulse Code)
Modulation) method is known. This method uses surrounding pixels to predict the value of the pixel to be encoded, and encodes the difference between the original pixel value and the predicted value. The number of codes for expressing the signal increases, making it difficult to obtain a high compression rate. In particular, one pixel is 8 bits (256 gradations), such as in commercial photo transmission.
It is difficult to obtain a high compression ratio when transmitting a multi-gradation image expressed in . On the other hand, in order to obtain a high compression rate, a method is used in which the difference is quantized with a small number of bits to reduce the number of codes, but this method has the problem that the quality of the reproduced image is greatly degraded. Purpose of the Invention In view of the drawbacks of the prior art, the present invention provides a prediction method that can obtain many gradations during commercial photographic transmission, minimize changes in image quality of reproduced images, and obtain a high compression rate. A transform encoding device is provided. Structure of the Invention The present invention provides a pixel density conversion unit that converts the pixel value of an original pixel of a multilevel quantized multi-gradation image and the pixel value of a previous pixel processed one step before the original pixel into density. , a density difference calculation unit that calculates a density difference between the density of the original pixel converted by the pixel density conversion unit and the density of the previous pixel, and a density range that each pixel of the multi-level quantized multi-tone image can take. is divided into a plurality of density regions, a density difference constant is determined in advance for each density region, and the density difference constant corresponding to the density region to which the density of the original pixel determined by the pixel density conversion section belongs is determined. a concentration difference constant output section to output, a concentration difference constant calculated by the concentration difference calculation section and a concentration difference constant outputted by the concentration difference constant output section;
an image conversion means comprising a determination selection unit that selects either the pixel value of the original pixel or the pixel value of the previous pixel according to the comparison result and outputs the selected pixel value as a converted pixel value; A prediction unit that predicts the converted pixel value output from the constituent judgment selection unit using a plurality of surrounding converted pixel values that have already been converted, and an encoding unit that encodes the error of the predicted value obtained by the prediction unit. The above object is achieved by providing a predictive encoding means having a section. DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a block configuration of an image system having a predictive transform coding device according to an embodiment of the present invention. In Figure 1, 1
is an image reading device that scans, samples, and quantizes multi-tone images such as photographic originals. 2
is an image conversion device that converts a quantized image signal; 3 is an encoding device that encodes the output image from the image conversion device 2; 4 is a transmission device that transmits a code;
5 is a receiving device that receives the transmitted code, 6 is a decoding device that restores an image from the code, and 7 is an image recording device that records the restored image. Although the system shown in FIG. 1 is an example of an image transmission system, the transmitting device 4 and the receiving device 5 may be used as a storage device and a reading device from the storage device. The configuration of the image conversion device 2, which constitutes the main part of the predictive transformation coding device, will be described in detail below with reference to FIG. First, in the image conversion device, an image signal 210 scanned and quantized by the image reading device 1 is stored in a one-pixel memory 21 . This image signal is a signal having a value (referred to as a brightness signal value) proportional to the reflectance of the image original when the reading device reads reflected light from the original. The output signal 211 from the one-pixel memory 21 is input to the density converter 23.
The concentration converter 23 converts the input value into concentration according to the following equation. d=-log ₁₀ (1-L/LMAX) (1) Here, LMAX is the maximum value of the read image signal. For example, when one pixel is quantized with 8 bits, the number is 255. L is the input value to the concentration converter.
d is the concentration. Equation (1) is applied to the case of a reading device where the relationship between the original and the read value is a large value in the dark areas of the original and a small value in the bright areas, and a small value in the dark areas and a large value in the bright areas. In the reading device, the following equation is obtained. d=-log ₁₀ (L/LMAX) ...(2) Also, in equation (1) or equation (2), the inside of the bracket is 0.
In this case, d is determined to be a sufficiently large finite value. The concentration converter 23 can be implemented using a logarithm operator, a subtracter, and a divider, but it can also be easily implemented using a ROM (read-only memory). In the case of an image in which one pixel is expressed by 8 bits, there are 256 types of luminance signal values, and therefore the density value d
There are also 256 types. By calculating the density value for 256 kinds of luminance signal values in advance and storing the value in ROM, by using the luminance signal value as the address input of the ROM, the density value d can be obtained as output data from this memory. It will be done. In this case, the required storage capacity of the ROM is 256 words. The word length is determined by the effective digits of the concentration d. For example, it is determined that the effective digits of the concentration d are up to the second decimal place, and an integer value obtained by multiplying the concentration d by 100 is stored, and in equation (1), If the value of d is set as 2.55 or 5.11 when the inside of the cutlet is 0,
The word length of the ROM is 8 or 9 bits, and a concentration converter can be realized extremely easily with a small capacity ROM. On the other hand, the luminance signal 212 of the pixel that was converted one pixel ago and stored in the one-pixel memory 22 is also converted into a density value by the same density converter 24 as described above. The output concentration signal 213 of the concentration converter 23 and the output concentration signal 214 of the concentration converter 24 are outputted by a subtracter 25.
subtraction is performed, and the absolute value of the result is output as a density difference signal 215. On the other hand, the density signal 213 of the original pixel is also input to the density difference constant output device 28. The density difference constant output device 28 has the divided density regions and density difference constants determined in advance for each density region, and calculates the density difference constant corresponding to the density region to which the density signal 213 belongs and outputs the density difference constant signal 218. Output. An example of concentration range division is shown in Table 1. Note that Table 1 is an example of the case of four divisions.

[Table] For example, when the value of the density signal 213 of the original pixel is 1.00, a value of 0.10 is output as the density difference constant signal 218. Although the concentration difference constant output device 28 can be configured using a comparator, it can be easily realized using a ROM (read-only memory).
For example, if the effective digit of the concentration d is set to the second decimal place, the concentration d
By inputting the value obtained by multiplying 100 by 100 as the ROM address input and storing the value obtained by multiplying the concentration difference constant by 100 at each address (that is, concentration), the concentration difference constant can be immediately obtained. If the original image is 8 bits/pixel, 256 words is sufficient for the ROM capacity.
For example, in Table 1, addresses 0 to 29 are 2,
5 is stored for addresses 30 to 94, 10 is stored for addresses 95 to 129, and 20 is stored for addresses 130 to 255. By using the ROM in this way, even if the number of divisions of the concentration region is increased, the concentration difference constant output unit 28
is not complicated. The density difference signal 215 is compared with the density difference constant signal 218 in the judger 26, and if it is less than or equal to the density difference constant, a judgment result of 0 is output, and if it is greater than the density difference constant, a judgment result of 1 is output as the judger output signal 216. be done. The image quality of the image after conversion is affected by the number of divisions of the density range, the boundary values of the divisions, and the density difference constant. If the density difference constant of each density range is made smaller, image quality deterioration will be reduced, but the compression ratio will not be improved much. As a result of actually evaluating images, sufficient image quality can be obtained with each constant shown in Table 1. Determiner output signal 216 from determiner 26 is input to selector 27 . The selector 27 selects the original pixel value as the output image signal 217 of the image conversion device 2 when the judgment result is 1, that is, when the density difference is larger than the density difference constant, and when the judgment result is 0, that is, the density difference. When is less than or equal to the density difference constant, the value of the pixel stored in the one-pixel memory 22 and output one pixel before is output. The output image signal 217 is simultaneously input to the 1-pixel memory 22,
The contents of 2 are updated every time one pixel is converted. The image signal output from the image conversion device 2 is encoded by the encoding device 3. The configuration of the encoding device 3 is shown in FIG. The output image signal 217 from the image conversion device 2 is stored in the image memory 31. The image memory 31 also stores scanning lines necessary for prediction when a reference pixel is included in a scanning line scanned in the past when predictive encoding is performed. From the image memory 31, reference pixel signals 310 and 31 are used to predict the value of the pixel to be encoded.
1,312 is output to the predictor 32. Predictor 3
An example of the second prediction method will be explained below using FIG. FIG. 4 is a diagram showing the relationship between reference pixels for prediction and pixels to be predictively encoded. 41 is the current scanning line, and 42 is a pixel X that was scanned one scan earlier than the current scanning line and has x=p and r=q in the scanning method.
The appearance probability is higher than that of the original image, and therefore the appearance probability that the difference value Δx is 0 is higher. The difference signal 315 is encoded by the encoder 34 and a code 316 is output. The difference signal output from the subtracter 33 has 511 types of values, and is encoded by assigning, for example, a Huffman code to each value. At this time, as mentioned above, the difference value Δx
Since the probability of occurrence of =0 is high, by shortening the code assigned to the difference value 0, the total number of codes of the encoded image can be reduced, that is, the compression rate can be increased. The output code 316 passes through the code transmitting device 4 and the code receiving device 5 to the decoding device 6.
is transmitted to. The decoding device 6 converts the code into a difference value, performs prediction using the same prediction method as the predictive coding device, restores the image by adding the difference value to the predicted value, and records it using the image recording device. recorded and played back. The table below shows the image conversion according to this example,
This is the scan line of the code length per pixel when predictive encoding is performed. Pixel X is a pixel to be predictively encoded, and P, Q, and R are pixels scanned in the past and are called reference pixels. The predictor 32 uses the reference pixels P, Q, and R to output a value x' expressed by the following equation as a prediction signal 313 for the pixel X. x' =P =p+(r-q) =p (When p+(rq)<0) (When 0p+(rq)255) (When p+(rq)>255) ......(3) However, p , q, r are reference pixels P, Q, respectively.
This is the value of R. The prediction signal 313 is the predicted pixel signal 314
is subtracted by the subtracter 33, and the result is the difference signal 3
It is output as 15. That is, a value Δx expressed by the following equation is output as the difference signal 315. Δx=x−x′ (4) where x is the predicted value of pixel X. In this predictor, since the input image signal is the converted image signal from the image conversion device 2,
It shows the improvement rate for the code length per pixel and the S/N ratio of the image after image conversion with respect to the original image when the same predictive encoding is performed without image conversion. The image in the data in the table below is a 256-gradation image in which one pixel is expressed with 8 bits. Further, the S/N ratio is a value calculated from the mean square error between the original image and the converted image.

[Table] According to the predictive coding method explained in this example,
Image quality deterioration does not occur in the encoding device 3 and decoding device 6, and the image quality is controlled by the number of density region divisions of the image conversion device 2, the division boundary value, and the density difference constant determined for each density region. be able to. That is, in this embodiment, in order to convert a multi-tone image and predictively encode the converted image, the original image is divided into a plurality of density regions, a density difference constant is assigned to each density region, and the transformed image is Convert the value of the original pixel and the value of the converted pixel one pixel before each to density, and if the difference in density is within the range of the density difference constant assigned to the density region to which the density value of the original pixel to be converted belongs. For example, an image converter that uses the converted value of a pixel to be converted as the value of the pixel that was converted one pixel before, and if it is outside the range of the density difference constant, the converted value is the same as the original pixel value. and an encoder that predicts the converted image output from this image converter and encodes the prediction error value, and a prediction method that is the same as that of the encoder, and a A decoder is provided for reproducing an image from the error signal obtained by the decoder. Therefore, when converting a multi-tone image using the difference value of the read value between pixels, the value of the read pixel is a value proportional to the brightness of the original, and the value of the read pixel between pixels in the dark part of the image is a value proportional to the brightness of the document. (luminance signal value) and
Even if the difference value of the read value (luminance signal value) between pixels in the bright area is the same value, the density difference between pixels in the dark area is significantly different from the density difference between pixels in the bright area, and it is visually are not based on the same density difference, and image conversion based on such read values (luminance signal values) results in image quality deterioration in dark areas being greater than that in bright areas, resulting in images with visually large image quality deterioration. becomes. The image conversion according to this embodiment uses the density difference after converting the value (luminance signal value) read from the image into density, and also takes advantage of the fact that the density difference in dark areas and the density difference in bright areas are perceived differently. Because the original image is divided into multiple density regions and a density difference constant is assigned to each density region, there is less deterioration in image quality in both dark and bright areas, and the probability that the prediction error is 0 during predictive encoding is reduced. Since this is a conversion that increases the conversion rate, the compression rate can be greatly improved because the prediction matching rate is improved compared to when the original image is predictively encoded. Effects of the Invention As described above, according to the present invention, when a multi-gradation image such as a photograph is encoded and transmitted or stored, there is less deterioration in image quality, and the number of codes can be transmitted or stored with fewer codes compared to a method that does not perform image conversion. It is possible to shorten the time and reduce the storage capacity, which has great effects.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an image transmission system having a predictive transformation coding device according to an embodiment of the present invention, FIG. 2 is a block diagram of an image conversion device that is a main part of the device, and FIG. FIG. 4, a block diagram of the main part of the encoding device, is a diagram showing the relationship between reference pixels and pixels to be predictively encoded. 1... Image reading device, 2... Image converting device, 3
...Encoding device, 6...Decoding device, 7...Image recording device, 21, 22...1 pixel memory, 23,
24...Concentration converter, 25...Subtractor, 26...
Determiner, 27... Selector, 28... Concentration difference constant output device, 31... Image memory, 32... Predictor, 3
3...subtractor, 34...encoder.

Claims (1)

[Scope of Claims] 1. Pixel density conversion that converts the pixel value of an original pixel of a multilevel quantized multi-gradation image and the pixel value of a previous pixel processed one step before the original pixel into density. a density difference calculation unit that calculates a density difference between the density of the original pixel converted by the pixel density conversion unit and the density of the previous pixel; a density that each pixel of the multi-level quantized multi-tone image can take; The range is divided into a plurality of density regions, and a density difference constant is predetermined for each density region, and the density difference constant corresponding to the density region to which the density of the original pixel determined by the pixel density converter belongs is determined in advance. a concentration difference constant output section that outputs a concentration difference constant, which compares the concentration difference calculated by the concentration difference calculation section with a concentration difference constant output from the concentration difference constant output section;
an image conversion means comprising a determination selection unit that selects either the pixel value of the original pixel or the pixel value of the previous pixel according to the comparison result and outputs the selected pixel value as a converted pixel value; A prediction unit that predicts the converted pixel value output from the constituent judgment selection unit using a plurality of surrounding converted pixel values that have already been converted, and an encoding unit that encodes the error of the predicted value obtained by the prediction unit. What is claimed is: 1. A predictive transform encoding device comprising: a predictive encoding unit;