JPH0137064B2 - - Google Patents

Description

[Detailed description of the invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a predictive coding device for image information in a facsimile device for transmitting images with gradations such as photographs, an image storage device for storing images, and the like. 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 above-mentioned drawbacks, the present invention provides a multi-gradation system that is capable of achieving high compression rates while minimizing changes in the image quality of reproduced images when many gradations are required, such as in commercial photographic transmission. The present invention provides a predictive encoding device having an image conversion function for performing an image conversion process with little change in the image quality of a toned image, and a predictive encoding function for predictively encoding the converted image. Structure of the Invention The present invention provides pixel density conversion for converting 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 difference calculation unit that compares the density difference calculated by the density difference calculation unit with a predetermined threshold value. and a determination selection unit that selects one of 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 determination selection unit constituting the conversion means based on a plurality of converted pixel values that have already been converted around the converted pixel value;
The above object is achieved by providing a predictive encoding means including an encoding section that encodes the error of the predicted value obtained by the prediction section. DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a block diagram of an image transmission system having a predictive coding device according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an image reading device that scans, samples, and quantizes a multi-tone image such as a photographic original. 2 is an image conversion device that converts the quantized image signal; 3 is an encoding device that encodes the output image from the image conversion device 2; 4 is a transmitter that transmits the code; and 5 is the receiver that receives the transmitted code. 6 is a decoding device for restoring an image from the code; 7 is a decoding device for restoring an image from the code;
is an image recording device that records restored images. Although the system shown in FIG. 1 is an example of an image transmission system, it may also be used as an image storage system by using the receiving device 4 and the decoding device 5 as a storage device and a reading device from the storage device. Next, the image conversion device 2 will be explained in detail with reference to FIG. FIG. 2 shows the configuration of the image conversion device. In FIG. 2, an image signal 210 scanned and quantized by the image reading device 1 is stored in a 1-pixel memory 21. In FIG. When the image reading device 1 reads the reflected light from the original, this image signal has a value proportional to the reflectance of the image original (referred to as a brightness signal value).
This is a signal with . 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. In equation (1), the relationship between the original and the read value is applied to the case of the image reading device 1, where the value is large in the dark areas of the original and small in the bright areas, and the value is small in the dark areas and large in the bright areas. In the image 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.
This concentration converter 23 can be realized using a logarithm operator, a subtracter, and a divider, but it can also be easily realized 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 there are also 256 types of density values d. 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 when the number in the box is 0 is set to 2.55 or 5.11, the word length of the ROM becomes 8 bits or 9 bits, and a density 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 density signal 213 of the density converter 23 and the output density signal 214 of the density converter 24 are input to a subtracter 25 where subtraction is performed, and the absolute value of the result is output as a density difference signal 215. The concentration difference signal 215 is compared with the judgment value K determined by the judger 26, and if it is less than or equal to K, the judgment result O is given, and if it is greater than K, the judgment result 1 is given as the judgement output signal 216.
is output as This determination value K is a parameter that affects the image quality and compression rate of the reproduced image. If the value of K is increased, the compression rate will improve, but the image quality deterioration will be greater; if the value of K is decreased, the image quality deterioration will be less, but the compression rate will not be improved much. As a result of evaluating the reproduced image using this example, we found that the value of K should be 0.01 to 0.03 (1 to 3 if the density converter outputs a value obtained by multiplying the density d by 100) when used for commercial photographic transmission. Sufficient image quality can be obtained by setting the range. 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 judgment value K, and when the judgment result is 0, that is, the density difference is larger than the judgment value K. If it is less than the judgment value K, 1 pixel memory 2
Outputs the value of the pixel stored in 2 that was output one pixel ago. The output image signal 217 is simultaneously input to the 1-pixel memory 22, and the contents of the 1-pixel memory 22 are updated every time one pixel is converted. Image signal 217 output from image conversion device 2
is encoded by the encoding device 3. Next, the encoding device 3 will be explained in detail with reference to the block diagram of the encoding device 3 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. An example of the prediction method of the predictor 32 will be described below using FIG. 4. FIG. 4 is a diagram showing the relationship between reference pixels for prediction and pixels to be predictively encoded. Reference numeral 41 indicates a current scanning line, and 42 indicates a scanning line scanned one scan earlier than the current scanning line. 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' shown in the following equation as a prediction signal 313 for the pixel X. x'= p p+(r-q) p (When p+(r-q)<0) (When 0p+(r-q)255) (When p+(r-q)>255) ......(3 ) (However, p, q, r are the reference pixels P,
These are the values of Q and 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 the predictor 32, the input image signal is the converted image signal from the image conversion device 2. Therefore, in equations (3) and (4), the probability of appearance of pixel . 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, since the probability of appearance of the difference value Δx=0 is high, by shortening the code assigned to the difference value 0, it is possible to reduce the total number of codes of the encoded image, that is, increase the compression rate. can. The output code 316 is transmitted to the encoding device 6 via the transmitting device 4 and the receiving device 5. The decoding device 6 converts the code into a difference value, performs prediction using the same prediction method as the predictive encoding device 3, and restores the image, which can be recorded and reproduced by the image recording device 7. According to the predictive coding described above in this embodiment,
There is no image quality deterioration in the encoding device and decoding device,
Image quality can be controlled by the determination value K of the image conversion device. Now, the table shows the code length per pixel when image conversion according to the above embodiment is performed and predictive encoding is performed, and the code length per pixel when the same predictive encoding is performed without image conversion. It shows the improvement rate with respect to the length and the S/N ratio of the image after image conversion with respect to the original image. The images in the table below are 256-gradation images 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 this embodiment, a multi-tone image is converted,
In order to encode the image after conversion, the original pixel to be converted and the pixel after conversion one pixel before each are converted to density, and if the difference in density is within a certain range, the converted value of the pixel to be converted is calculated. is the same value as the converted value of the previous pixel, and if the density difference is outside a certain range, the converted value is made the same as the original pixel value, and the converted image is predictively encoded. It is configured as follows. In other words, when converting by the difference value of the read value between pixels, the value of the read pixel is a value proportional to the brightness of the document, so the read value (brightness signal value) between pixels in the dark part of the image ) and the difference value of the read value (luminance signal value) between pixels in the bright area are the same value, the density difference between pixels in the dark area is larger than the density difference between pixels in the bright area. The reading value (luminance signal value) is different and visually not the same.
Image conversion based on this has the disadvantage that image quality deterioration in dark areas is greater than image quality deterioration in bright areas, and image quality deterioration is visually greater.However, according to the image conversion according to this embodiment, it is possible to Since the density difference after converting the brightness value (luminance signal value) into density is used, there is less deterioration in image quality in both dark and bright areas, and compared to the case where the original image is predictively encoded, the converted image can greatly improve the compression rate due to the improved prediction matching rate. 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 required for transmission or storage is smaller than in a method that does not perform image conversion. Therefore, it is possible to shorten the transmission time and reduce the storage capacity.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an image transmission system having a predictive coding device according to an embodiment of the present invention, Fig. 2 is a block diagram of an image conversion device in the predictive coding device, and Fig. 3 is a block diagram of the predictive coding device. FIG. 4 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, 31... Image memory,
32...Predictor, 33...Subtractor, 34...Encoder.

Claims (1)

[Scope of Claims] 1. A pixel density 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 conversion unit; 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 difference calculation unit that calculates the density difference calculated by the density difference conversion unit and a predetermined threshold value; an image conversion means comprising: a determination selection unit that compares and selects one of the pixel value of the original pixel or the pixel value of the previous pixel according to the comparison result and outputs it as a converted pixel value; a prediction unit that predicts the converted pixel value output from the determination selection unit constituting the image conversion means, based on a plurality of converted pixel values that have already been converted around the converted pixel value;
1. A predictive transformation encoding device comprising: a predictive encoding means having an encoding section that encodes an error in a predicted value obtained by the prediction section.