JP2575362B2 - Image processing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image processing method for converting image data into luminance data, hue data, and saturation data and performing color correction.

(Prior art)

FIG. 2 is a block diagram of a main part of a conventional ink jet printer for forming a color image from a video signal.
Input R (Red), G (Green) and B (Blue) signals are A / D converted 1
After being converted into a digital signal in step 1, they are input to gamma converters 12R, 12G, and 12B that perform gradation conversion and complementary color conversion. Here, the digital R, G, and B signals are converted into complementary color signals C, M, and Y whose gradation has been converted.

The complementary color signals C, M, and Y are subjected to masking processing by a masking ROM 14 to remove asymmetric color components of the ink, and become signals C ', M', and Y '. Signals C ', M', Y 'are signals indicating the optical color density (OD value) of the ink, and are OD value-voltage converters.
At 16 the OD value is converted to an analog drive voltage for the heads 18C, 18M and 18Y necessary to obtain the OD value. This driving voltage is applied to each head, and droplets 20C, 20M, and 20Y are ejected from the head, and a desired color image is formed on a recording medium (not shown).

[Problems to be solved by the invention]

However, reducing the capacity of the table relating to color correction affects the color reproduction accuracy, so that the capacity could not be reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems and to enable a table relating to color correction to be configured with a small capacity without reducing the color reproduction accuracy as much as possible. The purpose is to be able to obtain.

[Means and Actions for Solving the Problems] To achieve the above object, the present invention converts the data format of given image data representing an input image into luminance data, hue data, and saturation data, The quantization is performed so that the number of bits of each of the hue data and the saturation data is smaller than the number of bits of the luminance data, and the luminance data is subjected to gradation conversion according to the luminance distribution of the input image. And performs color correction according to the output characteristics of the output means and conversion to a data format suitable for the output means by converting the luminance data subjected to the gradation conversion and the quantized hue data and saturation data. It is characterized by using a table as input.

According to the present invention, a table relating to color correction is configured with a small capacity without reducing the color reproduction accuracy as much as possible, and this table is converted into a color correction corresponding to the output characteristics of the output unit and a data format suitable for the output unit. , A high-quality output image can be obtained.

〔Example〕

FIG. 1 shows an ink jet printer for obtaining a color image from a color video signal to which the present invention is applied.

In FIG. 1, components having the same functions as those in FIG. 2 are denoted by the same reference numerals. In FIG. 1, 19 is an input analog R, G, B
A converter 21 converts the signal into an (analog) luminance signal Y, a hue signal H, and a saturation signal C ^* .
A / D converter that inputs H and C ^* and converts it to a digital signal, 20
Is a gradation conversion characteristic setting unit for setting gradation conversion characteristics, and 22 is A /
This is a gradation converter that performs gradation conversion of the luminance signal Y ^* from the D converter 21 based on the conversion characteristics set by the setting device. Reference numeral 24 denotes a hue signal H and a saturation signal C ^* from the A / D converter 21 and a luminance signal subjected to gradation conversion by the gradation converter 22.
^This is a masking ROM for masking by inputting ^** .

Hereinafter, description will be given along the flow of signals. In this embodiment,
The converter 19 converts the R, G, B (analog video) signals into a luminance signal Y ^* , a hue signal H, and a saturation signal C ^* .

From the analog video R, G, B signals, Y ^* = 0.30R + 0.59G + 0.11B, and the R, G, B signals are converted into tristimulus values X, Y, Z by the following matrix operation.

The tristimulus values are converted into X and y as signals representing chromaticity by the following equation.

Then, assuming that the chromaticity of the white point is (xo, yo), it is converted into a hue signal H by the following equation.

The proviso when _{x ≠ x o x = x o} , into a chroma signal C by the y-y _o> H = 0 when _{H = π y-y o <} 0 when 0 further following equation.

The luminance signal converted into a digital signal by the A / D converter 21 is input to a gradation conversion characteristic setting unit 20 and a gradation setting unit 22.
In the gradation converter 22, for example, a plurality of conversion tables having input / output characteristics as shown in FIG. 3 are prepared.
One of the conversion tables is selected based on a command from the setting device 20. The setting unit 20 selects the conversion characteristic by detecting, for example, the frequency distribution or the maximum value and the minimum value of the luminance signal Y. Luminance signal Y gradation-converted by gradation converter 22
^** is input to one input terminal of the masking ROM 24 as an address signal. On the other hand, the hue signal H and the saturation signal C ^*
Is converted into a digital signal by the A / D converter 21 and is directly input to the input terminal of the masking ROM 24 as an address signal.

The masking ROM 24 performs three processes: inverse conversion from the signals Y ^** , H, C ^* to R, G, B signals, complementary color conversion from the R, G, B signals to complementary color signals C, M, Y, and masking processing. Do. These three processes can be performed at once by table conversion. The contents of this processing will be described below.

First, the conversion of the signals Y ^** , H, C ^* into R, G, B signals is performed as follows.

Complementary color conversion is performed according to the following equation.

_{C = -log (R / R o} ) M = -log (G / G o) Y = -log (B / B o) However R _o, G _o, B _o are constants.

Further, the masking process from the signals C, M, and Y is performed as follows.

In this way, values C ', M', Y 'indicating the OD value can be uniquely obtained from the signals Y ^** , H, C ^* .

In the masking ROM 24, the digital values of the signals Y ^** , H, and C ^* are respectively set as upper, middle, and lower addresses, and the values of the signals C ', M', and Y 'corresponding to the addresses are stored. I have. Masking matrix parameters a ₁₁ ~a ₃₃ performs color printing in advance implementation example, it can be determined using the least squares method by measuring the result. Since the masking ROM 24 uses the signals Y ^** , H, C ^* for the input, it is possible to give a large number of bits to the luminance in consideration of, for example, the visual characteristics, and to give a small number of bits to the hue and the saturation. And thus the capacity of the table can be reduced. In this example, 7 bits are given to the signal Y ^** and 5 bits are given to the signals H and C ^* .

In the above embodiment, the signals x and y representing the chromaticity are converted into the hue signal and the saturation signal, but the (RY, BY) signals, U *, V *, a *, b *, etc. May be used to perform the same conversion.

〔effect〕

As described above, according to the present invention, a table relating to color correction can be configured with a small capacity without reducing the color reproduction accuracy as much as possible, and a high quality output image can be obtained using this table.

[Brief description of the drawings]

 FIG. 1 is a signal processing block diagram of an embodiment of the present invention, FIG. 2 is a block diagram of a conventional color image signal processing device, and FIG. 3 is a diagram showing input / output characteristics of a gradation converter. 19 …… Converter, 24 …… Masking ROM.

Claims (1)

(57) [Claims] 1. A data format of given image data representing an input image is converted into luminance data, hue data, and saturation data, and the hue data and the saturation data are converted with respect to the number of bits of the luminance data. The quantization is performed so that the number of bits of each is reduced, and gradation conversion is performed on the luminance data according to the luminance distribution of the input image. An image processing method, wherein conversion to a suitable data format is performed using a table with the luminance data subjected to the gradation conversion and the quantized hue data and saturation data as inputs.