JPS60192460A - Picture signal processing unit - Google Patents

Abstract

PURPOSE:To set automatically the characteristic of gamma correction by obtaining a high light point and a dark point, and using the resulted values to select a suitable gamma correction curve. CONSTITUTION:A gamma correction circuit 24 consists of a ROM and in total 64 kinds of gamma correction reference tables are arranged in the ROM. That is, 8 dark processing points DP1-DP8 are provided, 8 processing high light points HP1-HP8 are provided and the 64 kinds of tables are supplied from the combinations. Then a reference table is decided by a control circuit 22 by a processing dark point nearest to the set dark point DP and a processing high light point nearest to the set high light point HP, and the result is outputted to each gamma correction circuit 24.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an image signal processing device that gamma-corrects an input image signal, and more particularly to an image signal processing device suitable for obtaining a high-gradation hard copy.

<Prior art> In a printer that obtains a hard copy from an input image signal with gradation such as a television signal, the gradation reproduction range is narrower than that of a television, so if the input image signal is printed out as is, the gradation will be An image with no tonality may be obtained, or an overall bright or dark image. Therefore, the input image signal is generally printed out after performing appropriate gamma correction. However, depending on the form of the input image signal, a desired image may not be obtained even after unique gamma correction. This problem can be solved by varying the characteristics of gamma correction, but the setting of the characteristics relies on the intuition of an expert or is achieved through trial and error.

<Object of the Invention> In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide an image signal processing device capable of automatically setting gamma correction characteristics.

<Description of Embodiment> FIG. 1 is a control block diagram of an image processing apparatus of this embodiment. The operation will be explained below.

The NTSCT V signal input to the NT8C input terminal 2 is demodulated into R, G, and B color signals by the N'r8C decoder 6 and input to the changeover switch 12. The NTSCTV signal is separated into vertical and horizontal synchronization signals by a synchronization separation circuit 8, and the synchronization signals are input to a changeover switch 14. Further, the LG and B color signals inputted to the RGB input terminal 4 are inputted directly to the changeover switch 12, and the synchronization signal 8Y is also inputted directly to the changeover switch 14. The input signal source specified by the manual input selection switch 10 is selected by the selector switch 12.
14 is selected. The selected nine R, G, and B color signals are processed by an analog-to-digital converter (hereinafter referred to as A/D converter) of 16 ft.
Input is 16G, 16B. On the other hand, the selected synchronization signal is input to the timing No. 14 generation circuit 18, and the A/
It forms a sampling signal for the D converter 16 and an input/output clock for line memories 2OR, 20G, and 20B, which will be described later. On the other hand, the analog color signal input to the A/D converter 16 is converted into a digital signal and input to each line memory 20. The output signals of each line memory 20 are then manually input to a gamma correction circuit 24 R, 24<1.24f3, where R-+C, G→M, B-+Y conversion, gamma correction, and normalization are performed simultaneously. . Here, the characteristics of the gamma correction IE are automatically controlled by the control circuit 22. This operation will be explained in detail later.

C and M whose characteristics have been converted by the gamma correction circuit 24.

The Y data is input to the masking circuit 26, where unnecessary color removal and undercolor removal processing is performed, and each color signal C, M',
Y', 13K are obtained, and each is connected to a digital-to-analog converter (hereinafter referred to as a D/A converter) 28C.

28M, 28Y, and 28BK are converted into analog signals. Each analog signal is applied to the inkjet head via the inkjet head driver circuit, for example, to obtain a desired image.

Next, the operation of the gamma correction circuit 24 will be explained.

The gamma correction circuit 24 operates as described above (from RGB to CMY).
Performs logarithmic transformation, gamma correction, and normalization.

This conversion is performed as follows prior to actual image formation.

FIG. 2 shows the television screen 30. 32 in the figure
is a horizontal scanning line, 34 is a sampling direction, 36 is a sampling point, X is a horizontal scanning direction, and Y is a vertical direction. Sampling by the A/D converter 16 is performed in a direction perpendicular to the horizontal scanning direction X, and by sequentially moving its position in the horizontal scanning direction, sampling of the entire screen is performed. 640 in the X direction during actual image formation
Point, in the Y direction: Sampling of 480 points is performed. However, all sampling information is not required for setting highlight points and dark points for gamma correction and normalization, so a total of 64 points are collected every 10 vertical lines in the X direction, and 480 points in the Y direction. Sampling is performed.

Furthermore, since the clean (G) component accounts for approximately 60% of the luminance signal, only the sampling data of the green signal G is used for the setting.

Assuming that the number of output bits of the A/D converter 16 is 8 bits,
Green digital data takes values from 0 to 255. The frequency 1W distribution is shown, for example, as shown in FIG. From this frequency distribution, calculate the integral value, and the integral value is, for example, 1% of the total.
The point at 99X is the highlight point (the highest value of the density that can be reproduced).
The minimum value of reproduced density (ODP, ) (HP) and π (π, which sets P) can be the same as the image quality control shown in FIG.

It is desirable to perform gamma correction and normalization according to the setting points (DP and HP set here), but a high-speed computer is required to perform the calculations during actual image formation. In the embodiment, the gamma correction circuit 24 is constituted by a ROM, and a total of 64 types of reference tables for gamma correction are arranged in RO+v[. That is, it has 8 points from DPz to DP8 as processing dark points, 8 points from HPI to HP8 as processing highlight points, and 64 types of tables as combinations thereof. Then, a reference daple is determined by the control circuit 22 based on the processing dark point closest to the set dark point L-DP, and the processing highlight point closest to the set highlight point (IP), and the reference daple is determined by the control circuit 22. Output. This determined dark point for processing is D
When the highlight point is HPB, the gamma correction circuit 24 receives input data JG expressed by the following formula.

B and output data C, M, Y are stored.

(Gamma correction circuit 24R) However, in the case of R≦(DPB), C=255(HPB)
≦R, #c=.

(24G) C, G≦(DPB) M=255 (HPB) ≦G IV=0 (24B) However, B≦('[)PI3) Y=255(HP, I3
)≦B In this way, input color signals R, G, B, YMC conversion, gamma correction, and 1F normalization processing are performed simultaneously.

FIG. 4 shows a correction curve expressed by the above equation.

In the figure, the horizontal axis is the data of the input color signal JG, 13, and the vertical axis is the data of the output color signal C1+S,Y. In the figure, when t-i P 5 is selected as the HPB, correction curves for each DPB are shown. In this embodiment, as described above, 64 types of correction curves are provided.

Next, the control program stored in the control circuit 22 of FIG. 1 will be explained with reference to FIGS. 5 to 7.

First, with reference to FIG. 5, a method of determining the luminance distribution of the luminance signal for the entire one frame will be explained. In Fig. 5, the brightness distribution (0 to 255) is A/D converted (
0 to 255), and this data is stored in the RAM in the control circuit 22.

First, in step 101, the data area in which data indicating this generated variable is stored is cleared. Next step 1
In step 02, initial settings are made for the X direction and the loop count in the X direction for the entire screen, and then in step 104, 1 is added to the frequency of occurrence of the above luminance level. Step 105
Perform loop counting in the X direction at step 107, and if y>4so (step 106), then perform loop counting in the X direction at step 107, and if x) reaches 640, The process to add the data is completed. Step 109 is the initial setting of the loop count in the X direction.

FIG. 6 is a flowchart for determining the set dark point (DP) and set highlight point (HP) for the input signal from the luminance distribution obtained in the above-mentioned process, and here step 201
~206 is the cumulative frequency of the brightness distribution, that is, the cumulative total (θ~2
55), and the input signal levels DP and HP are determined in steps 207 to 215. The cumulative total (255) here indicates the total cumulative frequency, and the number of pixels is 64 x 480.
I'm smiling. Furthermore, in this embodiment, the points i) p and ilp are each 1% of the total cumulative frequency of the luminance distribution.
The point is smaller than 99X, and the point is also larger than 99X.

In detail, DP searches the cumulative total (loop) from the bottom and finds the brightness level just before the total cumulative frequency exceeds 1.
HP searches the cumulative total (loop) from the top and finds the brightness level immediately before the total cumulative frequency becomes 99γ or less. In this embodiment, the DI) and HP determined in the above processing are used as the common DP and HP for the L, G, and B signals, respectively.

Returning to FIG. 6, first, in step 201, the cumulative total (θ~2
55) Clear the data area in the RAM where it is stored. Next, in step 202, the cumulative frequency distribution is initialized, that is, the frequency at which the brightness is O is set, and step 2
Step 03 initializes the loop count. Step 20
In step 4, the luminance distribution in the loop (the frequency of occurrence of the luminance level) is added to the cumulative frequency for the immediately preceding loop, that is, the cumulative total (loop). In step 205, 1 is added to the loop count, and these processes are repeated until loop>255 is reached (step 206). ' Next, to calculate 1) P and 1-I P, step 207
Initialize DP and HP in step 208, 1)1 in step 208
) to initialize the loop count. In steps 209 and 210, the cumulative total (loop) is calculated as cumulative total (255), and while this value is smaller than 0.01, it is 1;
Next, 1 is added to the loop count and the above calculation is repeated, and if the value becomes 0.01 or more, the immediately previous loop count value is set as DP (step 211). Next, in step 212, a loop count is initialized to increase the HP, and in steps 213 and 214, the same calculation as above is repeated as long as the value is greater than 0997, and when the value becomes 0.99 or less, , the previous loop count value is set as I-TP (step 215).

Next, from the above DI”, HP to each processing point DPB, H
The procedure for setting the PB will be explained with reference to FIG.

First, in step 301, the magnitude relationship between DP and DP8 is checked, and 1) when P is greater than DP8, step 30
In 2, I) PB and D-P are set to 8. Next I) P and D
The size relationship of PI is checked in step 303, and DP is I)
When P is smaller than 1, DPB is changed to DPI in step 304.
shall be. Next, the calculation in step 505 is performed. Here 1
0 is the difference between each value of DPB. That is, in this embodiment, the fifth
As shown in the figure, DP8-DP7=DP7-DP6-...
... is set as ρP2-DPI=10.

I) P B is determined by rounding off the DPA determined by this calculation. The dark point for processing closest to the dark point DP set by the processing described above])
PB is recognized.

In steps 307 to 312, the processing highlight point HPB closest to the set HP is similarly determined.

Then, in step 313, these DPB and HPB are output to the gamma correction circuits 24J, 24G, and 24B. After this processing, actual image formation is performed.

In the above example, the cumulative distribution is based on the green signal, but the actual luminance signal Y is Y=0.59G+0.3 OR+
0. Although it is more desirable to use Y, there is a problem in that it takes a long time to process because it requires multiplication and addition.

Green alone contains approximately 60% of the luminance signal component, and an experiment in which images of multiple shades were printed showed that there was almost no significant difference between Y and G.

Furthermore, if a capacity of 2 bits is required for one type of reference table, the gamma correction circuit 24 having 64 types of reference tables can be constructed from a 128 bit ROM. Also, if a reference table is created by ignoring the least significant bit of the 8 bits of the input color signal, the capacity of IK bits is sufficient for one type of reference table, for example, if the processing dark point Dl) B is 16 point, highlight point) l
-f PB are provided at 8 points, and a total of 128 kinds of tables can be provided. Of course, increasing the ROM size allows for more detailed changes.

Although the setting points D iP and HP have been explained as the 1% and 99% points of the total, various points can be selected using the manual image quality adjustment input switch 29.

Alternatively, one of the HPB and IPB may be fixed and the other may be made variable.

<Description of Effects> As described above in detail, according to the present invention, a gamma correction curve can be determined uniquely from the determined highlight points and dark points, so there is no need to rely on human intuition or trial and error. It is possible to obtain high-quality, sieve-graded images without the need for gradation. Furthermore, since one of the plurality of correction means is automatically selected for the gamma correction curve, the processing time for gamma correction can be shortened.

[Brief explanation of the drawing]

FIG. 1 is a control block diagram of the image processing circuit of this embodiment, FIG. 2 is an explanatory diagram of sampling, FIG. 3 is a diagram showing a cumulative frequency distribution curve, FIG. 4 is a diagram showing a gamma correction curve, and FIG.
7 to 7 are diagrams showing program flowcharts stored in the control circuit 22 of the stripe 1 diagram. In the figure, 16 is an A/D converter, 22 control circuits, 2
4 is the gamma correction circuit, DP is the setting dark point, DP
B indicates a dark point for processing, HP indicates a set highlight point), and HPB indicates a highlight point for processing.

Claims (1)

[Claims] A plurality of correction means each having different correction characteristics for gamma-correcting an input image signal, a setting means for setting a highlight point and a dark point in one screen from the input image signal, and a setting means for setting a highlight point and a dark point in one screen from the input image signal, and a setting means for setting a highlight point and a dark point in one screen from the input image signal, An image signal processing apparatus 0 characterized in that it has a selection means for selecting one of the correction means from both.