JPS61214863A - Gradation compensating method - Google Patents

Abstract

PURPOSE:To reproduce a picture approximating the characteristic of an input picture signal by subjecting the input picture signal to a gradation compression processing if the input picture signal is not within a gradation reproducing range. CONSTITUTION:Histogram data and values for processing are stored in a RAM 15, and a histogram processing and a gradation compensation processing are performed in a CPU 12 by a processing program in a ROM 16. A histogram is obtained with respect to an input luminance signal Y, and a luminance of 1% of a minimum luminance out of all frequencies is defined as a dark point Y1% and a luminance of 99% is defined as a highlight point Y99% in accordance with the histogram. A reproducing range DOUT obtained by the difference between a maximum reproducing luminance Ymax and a minimum reproducing luminance Ymin of a printer is compared with a gradation range Din=Y99%-Y 1% of the input picture signal. If Din>=DOUT and Y1%<Ymin are true, the gradation is compensated while preserving the gradation range of the input picture signal in accordance with Y+(Ymin-Y1%) Y'.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a tone correction method for obtaining a recording signal for an output device whose tone reproduction range is limited from an input image signal.

<Prior art> Conventionally, for example, the gradation range of a television signal is extremely wide, but when making a hard copy of this television signal, the gradation reproduction range of the printer is narrower than the gradation range of the television signal, so the gradation range is extremely wide. For example, this gradation correction method takes a histogram for each gradation level of the input image signal, matches the maximum gradation level of the input image signal with the maximum gradation level of the printer, and then adjusts the gradation level of the input image signal. The minimum gradation level was made to match the minimum gradation level of the printer. Although it has always been possible to reproduce high-contrast images using this method, there have been drawbacks such as 1) contrast being added to a flat image that originally lacked contrast, and gradation image areas becoming dark and crushed.

(Objective) In view of the above-mentioned shortcomings of the prior art, it is an object of the present invention to provide a gradation correction method capable of reproducing an image having characteristics close to those of an input image signal.

<Embodiments> Hereinafter, five embodiments of the present invention will be described in which white and black television signals are recorded by a monochrome image printer.

FIG. 1 is a plot diagram of the black and white image printer of this embodiment. In FIG. 1, Y is the luminance signal of the input television signal;
11 is an analog/digital converter, and 12 is a CPU that executes histogram processing and gradation correction processing. 13 is a digital/analog converter, 14 is a recording head capable of recording images with gradation, and 15 is histogram data or CP.
RAM that stores values necessary for processing U12, L6 is CP
This is a ROM that stores the processing program of U12.

The operation will be explained below with reference to the flowchart shown in FIG.

First, in step S1, a histogram is taken for the input luminance signal Y. That is, the frequency distribution for each brightness level of each pixel on one screen is determined. In S2, the brightness of 1% from the minimum brightness of all frequencies from the histogram is taken as the dark point Y (1%), and 99
The process of steps S1 and S2 in which the brightness of % is set as the highlight point Y (99%) is detailed in Japanese Patent Application No. 58-193129 proposed by the present applicant.

In step S3, the printer's maximum reproduction brightness Ymax (
The reproduction range Dout obtained from the difference between the white background of the recording paper and the minimum reproduction brightness Ymi%, and the gradation range Di of the input image signal.
Compare the size with n = Y (99%) - Y (1%).

If Dout<Din in S3, that is, if the histogram is as shown in FIG. 3 (1), the tone reproduction range of the printer is narrower than the tone range of the input image signal, so tone correction is performed in step S4 according to the following formula. .

It becomes possible to perform gradation correction on an input image that originally has a high contrast while maintaining the high contrast.

Also, in S3, Dout≧Din, and in step S6, Y(1
%)≧Ymin, that is, if the histogram is as shown in FIG. 3 (2), the histogram of the input image signal is all within the gradation reproduction range of the printer, so gradation correction is performed as shown in step S7. The input luminance signal Y is outputted as the output signal Y without any processing.

In this way, since gradation correction is not performed on input image signals included in the gradation limit range of the printer, recording is performed while the characteristics of the input image signals are maintained as they are.

Also S3. In S6, Din≧Dout and Y(1%)<Ymi
In the case of n, that is, when the histogram is displayed as shown in FIG. 3(3), the tone is corrected in step S9 according to the following equation while preserving the tone range of the input image signal.

Y+ (Ymin-Y (1%)) -Y' With this configuration, it is possible to output a gradation input image with the same contrast and without changing the gradation.

In FIG. 3, in indicates the histogram of the input image signal, and out indicates the histogram of the output image signal.

<Other Embodiments> In this embodiment, a monochrome image has been described, but for a color image, similar processing may be performed for RGB signals. Furthermore, when using a luminance-one color difference signal, processing may be performed on the luminance signal. Further, although the explanation has been made using a printer as an example of an output device, the present invention can also be applied to a display device having a relatively narrow gradation reproduction range. Also, 5 Y (1%) and Y (99%) were used in the histogram, but Y (0%), y
Other percentage values may be used, including (ioo%).

<Effects> As explained in detail above, according to the present invention, it is possible to output an input image signal while making the most of its characteristics, and an extremely natural image can be reproduced.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the monochrome printer of this embodiment, FIG. 2 is a processing flowchart of the CPU 12 in FIG. 1, and FIG. 3 is a diagram showing an input/output histogram.・

Claims (1)

[Claims] In a gradation correction method for outputting a gradation image using an output device with a limited gradation reproduction range, it is determined whether an input image signal falls within the gradation reproduction range of the output device; A gradation correction method comprising performing gradation compression processing on the input image signal when the input image signal does not fall within a gradation reproduction range.