JPS5839168A - Gamma compensation device - Google Patents

Abstract

PURPOSE:To perform always an optimum gamma compensation in accordance with the density level of an original image, by setting perliminarily plural density correction functions for every maximum density level of the original. CONSTITUTION:An image signal read by a CCD 1 is applied to an A/D converter 3 through an amplifier 2 and is converted to a digital signal and is applied to an ROM 6. Meanwhile, the output of the amplifier 2 has the peak value detected and held in a peak holding circuit 4, and the peak value is applied to a comparing circuit 5 and is compared with a reference value to detect a maximum density level of the output of the CCD. Gamma compensation values for input density levels are stored preliminarily in the ROM 6 in accordance with plural density compensation functions, and a density correction function is selected on a basis of the maximum density level detected by the comparing circuit 5, and thus, gamma compensation is performed in accordance with the output of the A/D converter 3.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gamma correction device that optimally adjusts the density level of image information of a document read by an integrated scanning element.

Generally, when a document image is read by a solid-state scanning device such as a CCD, the relationship between the output V of the solid-state scanning device and the document #1 is expressed by the following formula when the image state of Il, @ is photoelectrically converted by the solid-state scanning device. As shown in Figure 1, the output of the solid-state scanning element does not change linearly with respect to the KIN'114 concentration, and therefore the jIi image is reproduced based on the output of the solid-state scanning element. Photos etc.
In order to improve the reproducibility of intermediate tones, the solid-state scanning element output is subjected to gamma correction.

V W 10-D - (1) Conventionally, means for performing this type of gamma correction include:
For example, by using a ladder circuit as shown in FIG. 2(a) K to conduct the diodes DI and D2 in stages according to the magnitude of the input Vi (solid-state scanning element output),
Figure (b) Input (V4) and output (-vs
) K to perform gamma correction of broken line approximation based on % property.
However, it is better to perform gamma correction according to the fI & degree correction correction function set in this way, and the output at the solid-state scanning element also depends on the characteristics shown in (1) 2 above. There is a problem that gamma correction cannot be performed optimally. In other words, there is a wide range in the maximum at of the original image, for example, the image density is very low in newspapers and diazo copies, and high in the areas written with a nine-point pen, and the density varies depending on the manuscript, from level III in the calligraphy area to torsion in the background area. Because the difference between the adjustment level and the adjustment level is different, the density correction function during gamma correction is not constant, and the correction output may differ from the set value and overlap.For example, when the gamma When the correction is expressed in the form of MU lines (correction of gun f1), the density correction function set as shown in FIG. 2 cannot be applied.

Main 1j IF! The present invention has been developed in consideration of the above points, and provides a gamma correction device that can always perform optimal gamma correction of the output of a solid-state scanning element according to the density level state of an original image.

In the gamma correction device according to the present invention, a plurality of density correction functions are set in advance for each maximum density level of a document, and the density correction functions are applied to the detected maximum density level or image of the document from the output of the solid-state scanning element. The selected *@Il! The optimum gamma correction is performed according to the iA positive function.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The gamma correction device according to the present invention [K has a CCD that reads the image information of the original by photoelectric conversion, as shown in FIG.
1, an amplifier 2 that amplifies the output of the CCD I to a required level, a mu/D converter 3 that converts the amplified output into a digital signal, and a peak hold circuit that detects and holds the peak value of the output of the amplifier 2. 4, a ratio IIRFjA path 5 for detecting the highest concentration level of the COD output by comparing its peak hold output with a reference value, and a gamma correction value for the input #1 degree level stored in advance according to a plurality of density correction functions. The selection of the Ill correction function is indicated according to the highest density level detected by the comparator circuit 5, and the gamma correction value is determined according to the output of the A-D converter 3 according to the selected density correction function. ROM that outputs
It is made up of G and traps. Note that the comparator 5 is, for example, a comparator CMP1- which divides one reference voltage Vref into three stages and compares them with the inputs as shown in FIG.
A window comparator section consisting of CMP 3, the output of comparator CMPI and the output of comparator CMP 2 (Gl) and the output of comparator CMP 1 and CMP
The highest density level of the image information of the document read by the CCDIK according to the combination of the outputs of G2 and G2, and the gate section consisting of G2, Gl and G2. It has become possible to detect these in a step-by-step manner.

Therefore, in the gamma correction device according to the present invention configured as described above, the am image information multi-level quantized by the mu/D converter 3 and the highest density level signal detected by the comparator circuit 5 are stored in the ROM 6. 10M6 according to the highest concentration level signal]
One of the gamma correction functions is selected, and according to the selected density correction function, the output of the mu/D converter 3 is set as an address [by reading out the pre-stored gamma correction value]. , CCDIK will perform gamma correction on the image information of the read-pill document. At this time, the input of the comparator circuit 50 is set by the peak hold circuit 4 [This is the peak value of the detected image signal; 9 hours is set early, and the falling time t is set to be a sufficiently long time in accordance with the scanning speed WL.

The table below shows specific examples of gamma correction according to the density correction functions for the highest density levels of 1.5 and 1.0 when the image information of the original is 16-value quantized.

Table 5 also shows the characteristics of gamma correction in the above table. B3 shows the characteristics of the M/D conversion output, ROM output (gamma correction output), and density correction function in case I, and Ill and B2 in the figure
．． B3 is the A-D conversion output in case 2, RO
It shows each characteristic of M output and density correction function2)
71 For example, the original density of 0.6 is
If the D conversion is gamma-corrected to 6 using the coefficient correction function M3, it will not be possible.
It is then gamma corrected to 9 by the density correction function B3. In this way, in the present invention, C'
To select the density' correction function during gamma correction according to the highest density level of the image signal read by CDI.
It can be approximated as follows.

It should be noted that the gamma correction device according to the present invention is not limited to that of the above-mentioned embodiment; Instead of detecting etc-
s A switch means that can manually change the maximum density level step by step is provided so that the operator can change the original i! i
The highest density level may be determined from the density level 11 of the i image and the setting by the switch means may be made as appropriate. It goes without saying that the density correction function is not limited to one that makes the correction linear, and that various other density correction functions can be set depending on the desired gamma correction characteristics.

As described above, in the gamma correction device according to the present invention, a plurality of density correction functions for the multi-value quantization of the image signal of the original read by the solid-state scanning element and the highest density level of the original are stored in advance. and a means for selecting the density correction function according to the J[sI) highest density level to be read, and the image signal is quantized according to the selected fa degree correction function. By setting a predetermined gamma correction value 1 as an address, the solid-state scanning element performs gamma correction of the image signal of the read original, and the optimum table according to the density level of the original image is calculated. It has the excellent advantage that gamma correction can be carried out easily and quickly.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between general CCD output and original density, Figure 2 (a) is an electrical circuit diagram showing a conventional gamma correction device, and Figure 2 (b) shows its input/output characteristics. 3 is a block configuration diagram showing a gamma correction device according to an embodiment of the present invention, FIG. 4 is an electric circuit diagram showing a specific example of the configuration of a comparison circuit in the same embodiment, and FIG. FIG. 7 is a characteristic diagram showing a specific example of gamma correction in the example. 1...COD 2--Amplifier 3...A/D converter 4...Peak hold circuit 5...Comparison circuit 6
...ROM Applicant's agent Kiyoshi Torii

Claims (1)

[Claims] means for multi-level quantization of one signal of the original read by the solid-state scanning element; storage means for storing in advance a plurality of density correction functions at the highest density levels of Il and IIK; and means for selecting a density correction function according to the highest density level of the original, and reading out a predetermined gamma correction value using the quantized image signal as an address according to the selected density correction function. A gamma correction device characterized in that the gamma correction device performs gamma correction of an image signal of a document read by a solid-state scanning element.