JPS5896459A - Quantizing method for picture luminance signal - Google Patents

Abstract

PURPOSE:To decrease the number of bits per one picture element without the deterioration in picture quality, by summing a residual luminance signal of pre- processed picture elements to a reading luminance signal of the picture element to be processed. CONSTITUTION:An m-bit quantizing device 21 receives an analog luminance signal through a signal line 20A and quantizes it to an m-bit digital luminance signal. An n-bit quantizing device 22 uses a residual luminance signal pre-processed before by one and a luminance signal of the device 21 is converted into an n-bit luminance signal. Taking a picture element processed by one as 33, a picture element under processing at present as 34, and a picture processed after by one as 35, the n-bit luminance signal of the picture element 34 is formed by summing a residual luminance signal read out from a memory device 23 before by one to the m-bit luminance signal of the device 21. The residual luminance signal is given to the memory device 23 for the processing of the next picture element 35.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for quantizing image luminance signals, and an object of the present invention is to reduce the number of bits per pixel with almost no deterioration in image quality.

According to the present invention, in media such as photographic film that can express halftones, when image information is quantized and exposed based on this, the number of bits carrying exposure amount information per pixel is It can be reduced with almost no deterioration. In halftone photographs, the number of dots within one halftone dot can be significantly reduced, and the resolution can be increased while maintaining tone characteristics.

2. Description of the Related Art Conventionally, when expressing a halftone image using a medium capable of expressing binary luminance, a method similar to a dither method and a halftone photographic method have been known. In the dither method, it is known that the brightness of a pixel is expressed by one bit per pixel, and the brightness error is allocated to two or more pixels in the vicinity of that pixel. However, in conventional dithering methods, the texture noise is not noticeable.

In order to obtain sufficient image quality when scanning and reading an image containing halftones, quantizing the obtained luminance signal, and storing and reproducing it with a storage device, or when transmitting image information digitally, , 8 bits per pixel (266 gradations)
~6 bits (64 gradations) are required. The number of bits per pixel required to obtain this sufficient image quality is assumed to be m bits. According to the present invention, the outline and main gradation are expressed by n pitches (n<m) in the relevant pixel, and the gradation of a small residual value is processed next to the surrounding pixels, especially the relevant pixel. Shared with pixels. As a result, visual image quality hardly deteriorates even if the number of bits allocated per pixel is reduced to n=2 to 4.

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram of a system using the present invention. Reference numeral 1 denotes a scanning reading device, and a quantizing device 2 quantizes an analog luminance signal of an image read by the scanning reading device.

Reference numeral 3 denotes a storage device such as a magnetic disk, in which the image signal quantized by the quantizer 2 is stored and read out as necessary. Reference numeral 4 denotes a digital-to-analog converter which converts the read quantized image signal into an analog signal, which is printed by a recording device 5 to restore the original image. Note that the storage device 3 can also be replaced with a facsimile transmission line. In this case, the scanning reading device 1. The quantization device 2 is a transmitting terminal, and the digital-to-analog conversion device 4. The recording device 5 becomes a receiving terminal. Furthermore, the digital-to-analog conversion device 4 may be considered as a device for generating halftone signals, rather than a conversion device for applying digital signals to photographic paper as luminance information.

FIG. 2 is a detailed block diagram of the quantization device 2 shown in FIG. 1.

The m-bit quantizer 21 receives and receives the analog luminance signal through the signal line 20A, and quantizes it into an m-bit digital luminance signal. If necessary, such as edge enhancement and 9-tone correction are performed in this block, and the m-bit digital luminance signal is sent to the n-bit quantizer through the line 21A to generate an n-bit luminance signal as follows. In FIG. 3, the squares indicate pixels, the arrow 31 indicates the main scanning direction of the reading device, and the arrow 3
2 indicates the sub-scanning direction. (2) The printing bed 34 has nine pixels currently being processed; pixel 33 is a pixel processed one before pixel 34; pixel 35 is a pixel processed one pixel after pixel 34; The residual luminance signal of pixel 33 is expressed as h33 (hereinafter referred to as b and t) by adding the suffix 33 to h.

The same applies to B), then the m-bit luminance signal b of the pixel 34
From the m-bit total brightness signal t34 obtained by adding the residual brightness signal h33 read out through the wire 23A of the memory device 23 to 34, the n-bit brightness signal B3.4 of the pixel 34 is obtained.
is generated, and its residual luminance signal h34 is sent to the memory device 23 through the line 22A. h34 is used when processing the next pixel 35. There are two ways to create an n-bit luminance signal from an m-bit total luminance signal t: one uses n pits from the MSB of the 0m-bit signal and uses the remaining <m--) bits as a residual luminance signal, and the other uses @(m-n ) bits, the LSB of the n-bit signal may or may not be incremented by +1. When +1 is added, the residual luminance signal becomes a negative value.

For the first pixel of one scanning line, a virtual pixel is provided in front of this pixel, and the value that becomes the residual luminance signal h8° from this pixel,
For example, there is a method of assigning 1 to all (m-n-1) bits from the LSB among m bits. For example, m=8. If n = 3 (oOoollll), then to illustrate the case of the above ■, m = 8 n = 3 h 33 = OO010101 b 34 = 10110011 t 34 = 11001000 B 34 = 110 h 34 = OOO01000, and the case of the above ■ To illustrate, m = B n = 3 h 33 = OO010101 b34 = 10111011 t 34 = 11010000 B34 = 111 h34 = -00010000 = (1111oooO...'t ')OOOO
Complement of OO) The luminance signal of n bits per pixel produced in the above manner is sent to the storage device 3 of FIG. 1 through the line 22B.

The image signal quantized to n bits per pixel by the method described above has the following characteristics.

(1) Although the image luminance signal is quantized with n bits per pixel, it has gradation characteristics close to m bits. contour,
The information for the main gray level is represented within the pixel, and only the residual luminance signal is added to the neighboring pixel. The residual luminance signal is information necessary for expressing tone in a large area, and even if it is shifted by one pixel, there is little visual deterioration in image quality.

(2) Since m bits of information per pixel has become n bits, the memory capacity for storage only needs to be n/m.

Since images have a large amount of information, this effect is great.

(3) Since the residual luminance signal may be added only to the next pixel to be processed, the device configuration is very simple.

(4) Even if the residual luminance signal is added only to the 'th order processing pixel, almost no texture noise as seen in the dither method or the one-bit-per-pixel representation method is generated.

This is because the amount of residual luminance signal is reduced to */2 n 1 compared to the case of 1 bit per pixel.

In the case of 1 bit per pixel, residual brightness is distributed to pixels after one main scan in order to reduce this texture noise.
The device configuration becomes significantly complicated.

(6) When the digital-to-analog converter 4 in FIG. 1 is used as a digital signal-dot signal converter and a dot signal is generated from n-pit signals, the shape of the dots forming the dot signal is constant. The halftone dot area is 2 (n-town. m, = 8
, when n = 3, the area of one halftone dot is 1/32. Therefore, in terms of one-dimensional resolution, the characteristics can be improved by a factor of 5.7. Note that it is also within the scope of the present invention to apply the residual luminance signal as an analog signal, add it to the read luminance signal, perform n-bit quantization, and create an analog residual luminance signal for the pixel.

As detailed above, the present invention adds the residual luminance signal (positive or negative) of the previously processed pixel to the read luminance signal of the pixel when quantizing the luminance signal of an image using two or more bits. This is a method of quantizing image luminance signals, in which the residual luminance signal generated at the pixel is added to the pixel to be processed later. It can be reduced with almost no effect.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a system implementing the method of the present invention, FIG. 2 is a block diagram of a quantization device, and FIG. 3 is a block diagram of a system implementing the method of the present invention.
It is a pixel arrangement explanatory diagram. 1...Reading device, 2...Quantization device,
3... Storage device, 4... D/A conversion device, 5... Recording device, 21... m-bit quantization device, 22... ...n-bit quantization device,
23...Residual luminance signal storage device. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Figure 1

Claims (1)

[Claims] Quantization of an image brightness signal, characterized by adding a residual brightness signal of a previously processed pixel to a read brightness signal of a pixel, and adding the residual and total brightness signals generated at the pixel to a subsequently processed pixel. Method.