JPS61288662A - Color picture signal processing method - Google Patents

Abstract

PURPOSE:To attain a color picture with high quality by compressing and mapping the saturation and lightness of an input color picture signal into the saturation and lightness of an output system in a prescribed function. CONSTITUTION:A compression ROM 10 applying compression and mapping consists of a RGB/Luv conversion section 11, a garadation (lightness) compression section 13, a saturation compression section 15, an Luv/R'G'B' conversion section 17. The RGB signal of each picture element is converted into the Luv by the conversion section 11 according to an equation I. Then the lightness is compressed. In taking the maximum and minimum values of the lightness of the input system of a hue as L4, L1 and those of the output system as L3, L2, then L, u, v are converted respectively into L'', u'' and v'' according to an equation II. Then the saturation is compressed. The lengths in the saturation of the region corresponding to the lightness LA are taken as l1, l2. In this case L'', u, V'' are converted into L', u', v' according to equation III. As a result, the input color picture signal having wide color reproducing range is subjected to compression and mapping in the natural form.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a color image signal processing method for processing color image signals, and is particularly applicable to color image signals of input systems such as television image signals having a wide reproduction range of brightness and saturation. The present invention relates to a color image signal processing method for converting into a color image signal for an output system with a narrow reproduction range, such as a color printer.

<Prior art> For example, as shown in Figure 2, the brightness L* is taken on the vertical axis,
Taking the saturation C line, Jd and J- on the horizontal axis, and comparing the color reproduction range RTv of the red hue (jan-1 = 70°) on a television and the reproduction range RIJ of an inkjet printer, the former is much larger. wide. Therefore, when reproducing a television image and an inkjet printer, the portion of the range RTY excluding the range RIJ is condensed on the boundary line, resulting in an extremely unnatural image.

This is an unavoidable problem especially in full-color printers because there is currently no ideal coloring material such as color ink or color toner.

Although this problem is a fundamental and serious problem in this type of device, it has never been seen before.

OBJECTIVES The present invention aims to provide a color image signal processing method that solves the above-mentioned problems. That is, the objective is to obtain a high-quality color image by compressing and mapping each point in the color-reproducible region of the output system to each point in the color-reproducible region.

Embodiment FIG. 1 shows a block diagram of signal processing when the present invention is applied to an inkjet video printer that prints color television signals using an inkjet printer.

In the figure, 5R, 5G, and 5B are R and G, respectively.

The color television signal input terminal B, lO is a compression ROM (Read Only Memory) that performs compression mapping.
In y), its functions are RGB+Luv)+C conversion section 11° gradation (lightness) compression section 13. Saturation compression section 15°Luv*'→
It is divided into an R'G'B' conversion section 17. 20 is additive color method 3
Subtractive color method for printing primary color signals R', G', and B', respectively, to three primary colors cyan C and magenta M.

Logarithmic conversion ROM for logarithmically converting each yellow Y signal, 30
is a masking ROM that performs asymmetric color removal, 40C940M
, 40Y is a digital/analog (D/A) that converts digital signals into analog signals for driving the inkjet head.
) Conversion unit, 50C, 50M, and 50Y indicate inkjet heads, respectively.

The operation will be explained below.

The RGB signal for each pixel of a television is calculated using the following formula (1)%) is converted to

These L*, u*, v* are processed by the gradation compression unit 13. according to predetermined constant mapping functions fL, fu, fγ. The saturation compression unit 15 converts it into L*'u*'y*', and compresses the brightness and saturation. fL, fu, and fγ will be described later.

Next, Luv*' + ty*' at the RGB converter t7
, u)k'.

By reversing and solving equation (1) from v*', R
', G', and B' are calculated, and R'G'B' is obtained.

Looking back on the above, we can see that the predetermined functions fL, fu
, fγ can be used to uniquely determine output R'G'B' from input RGB. Therefore, the above portion can be configured with a table conversion memory (ROMIO) into which the input RGB is input as an address.

This R'G'B' is a signal that has already been compressed into the inkjet reproduction range.
The M20 density signals are converted into C, M, and Y, and then the digital value V of the head drive voltage corresponding to this is converted in the masking section 30.
c, Vm, and Vy.

Note that logarithmic conversion and masking are performed by a table conversion ROM whose input is an address.

Furthermore, the D/A converters 40C, 40M, and 40Y convert the voltage into an analog voltage for driving the inkjet head 5.
Drive 0C, 50M, 50Y.

Next, how to determine the mappings fL, fu, and fγ that achieve compression will be explained.

First, the brightness is compressed. As shown in Figure 2, the maximum brightness value of the input system for a certain hue.

Set the minimum value to L4. Ll, and those of the output system are L3゜L2
Then, inputs l*, +1*, and y* are converted into L*, 11*, and v*, respectively, according to the following formulas.

Convert. In other words, only the brightness is compressed according to the ratio of the reproduced brightness of the input/output system.

If this is done for all hues (θ = tan snow field), the reproduction range RTV of the input system will be compressed while preserving the saturation C* as shown by the arrow in the range R', as shown in Figure 3. .

Next, compress in the direction of saturation.

First, consider a certain hue θ. (θ= j a n −I
N) At this time, let the length of the area corresponding to a certain brightness LA in the saturation direction be Qx, 12. At this time, Ll'.

u*, ■*” are l*’, u*’, y* as follows.
'Converted.

In this way, the saturation is compressed according to the ratio of the reproduced saturation of the input/output system.

According to the above method, conversion from R' as shown in FIG. 4 to the reproduction range RIJ of the inkjet printer is performed while preserving the brightness after and before the saturation conversion.

After all, when looking at a certain hue, the color range RTV of the color television signal is compressed and mapped to the reproduction range RIJ of the inkjet printer, as shown in FIG.

Note that the compression is performed in the order of brightness and saturation, but this means that if the saturation is compressed first, the brightness will be compressed from L1 to L2 and L3 to L.
In the area No. 4, there is no RIJ reproduction saturation, so the saturation is compressed to zero, and even if this is further compressed in terms of brightness, the zero saturation does not change. That is, if compression is performed on saturation first, a distorted compression mapping will be performed and the image quality will deteriorate.

On the other hand, if the brightness is first compressed as in this embodiment, uniform compression can be performed at each point on the chromaticity diagram, and there is little deterioration in image quality.

The above is one method of compression.
, v*1) (i=1.2...-N), determine the colors (L*i', u)ki', v*f') to be converted, and apply them using the least squares method to obtain fL, It is also possible to determine fu and fγ.

In addition, as a color system, not only Luv* but also other luminance -
The same applies to the use of a color system based on color difference.

Furthermore, in the above-mentioned embodiment, the compression ROM 1O, the logarithmic conversion ROM 20, and the masking ROM 30 were separated and each consisted of a ROM, but since the output is uniquely determined by the input RGB, they can be combined into one ROM. You can also configure it, or you can reverse the order and after the logarithmic transformation,
Compression processing may also be performed.

In this way, since the input television signal is compressed once within the reproduction range of the inkjet printer before being recorded, the color range that cannot be reproduced by the inkjet printer is naturally compressed into the reproduction range of the printer. The resulting color image will also be extremely natural and of high quality.

Note that the color image signal of the input system is not limited to a television signal, but may be an output signal from a solid-state image pickup device, etc., as long as the color reproduction range is wider than that of the output system.

Furthermore, the output system is not limited to inkjet printers, but can also be applied to various printers such as electrophotographic printers and thermal printers, or display devices with a relatively narrow color reproduction range.

Effects> As described above, according to the present invention, it is possible to compress and map input color image signals with a wide color reproduction range to the output system in a natural manner, which is optimal for obtaining high-quality color images. It has become possible to provide a processing method.

[Brief explanation of drawings]

FIG. 1 is a block diagram of color image signal processing of this embodiment, FIG. 2 is a diagram showing color television signals and the color reproduction range of an ink color jet printer, FIGS. 3 and 4,
FIG. 5 is an explanatory diagram for explaining compression mapping. In the figure, 10 is a compression ROM, 11 is RGB→Luv*
Conversion unit, 13 is gradation compression unit, 15 is saturation compression unit, 17 is Luv*'+R'G'B' conversion unit, 20 is logarithmic conversion RO
M and 30 indicate masking ROMs, respectively.

Claims (2)

[Claims] (1) If the range of saturation and brightness of the input color image signal is larger than the reproduction range of saturation and brightness of the output system,
A color image signal processing method characterized in that an output color image signal is obtained by compression mapping the saturation and brightness of an input color image signal to the saturation and brightness of an output system using a predetermined function. (2) The color image signal processing method according to claim 1, wherein when performing the compression mapping, brightness is first compressed and then saturation is compressed.