JP3048170B2 - Color image processing equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a color for converting coordinate data representing a color in a predetermined color space into one of a plurality of expression colors that can be represented by an output device. The present invention relates to an image processing device.

[Conventional technology]

2. Description of the Related Art Conventionally, in a color image processing apparatus such as a digital color copying machine, image data obtained by reading a document is subjected to pseudo halftone processing and recorded or displayed and output.

As such a pseudo halftone processing method, a dither method, an error diffusion method (ED method), and the like are known. In order to express a color image in this type of system, when a light-emitting display such as a CRT is used as an output device, input RGB multi-value data is subjected to pseudo halftone processing independently in each of R, G, and B color spaces. When a thermal recording device represented by ink jet, electrostatic recording, or thermal transfer is used as the output device, the input multi-value data is converted to R, G, B
Generally, after converting the color coordinates into C, M, Y (K) color coordinates, pseudo halftone processing is performed independently in each color space as described above.

[Problems to be solved by the invention]

However, for example, the density preservation type pseudo halftone processing (ED method) is performed independently as in the past because the basic idea is to preserve the density in a microscopic space in the color space. There is a drawback that the color tone is pseudo-expressed in the 3-4 color space and the synthesized color does not always match the color image of the document.

The present invention has been made in view of the above-described drawbacks of the related art, and calculates and inputs a plurality of pieces of predicted average value coordinate data from the expression color coordinate data after the conversion process and the predicted expression color coordinate data of the pixel of interest. The expression color represented by the predicted expression color coordinate data when the spatial distance between the coordinate data representing the color of the pixel of interest and the plurality of pieces of predicted average value coordinate data is the smallest is set as the expression color of the pixel of interest, and is generated at the time of conversion. A color image processing apparatus capable of faithfully reproducing the tint of an input color image by correcting the error data to be reproduced and limiting the amount of correction of the error data so that the edge portion can be clearly reproduced. Aim.

[Means for Solving the Problems] In order to achieve the above-described object, a color image processing apparatus according to the present invention uses coordinate data representing colors in a predetermined color space among a plurality of colors that can be expressed by an output device. A color image processing device for converting into one expression color, input means for inputting coordinate data representing a color in a predetermined color space, expression color coordinate data of a plurality of pixels having undergone conversion processing around a pixel of interest, A process of calculating predicted average value coordinate data by weighted averaging using a weighting coefficient with the predicted expression color coordinate data of a target pixel while changing the predicted expression color coordinate data to calculate a plurality of predicted average value coordinate data A space between the average value calculating means, the coordinate data of the pixel of interest input by the input means, and the plurality of respective predicted average value coordinate data calculated by the average value calculating means; Means for comparing the target distance, and the expression color represented by the predicted expression color coordinate data when the predicted average value coordinate data having the smallest spatial distance is calculated based on the comparison result of the comparison means as the expression color of the pixel of interest. Determining means for determining, and error correcting means for correcting by adding error data generated when the determining means determines the expression color of the pixel of interest to coordinate data of a newly input pixel; When the level of the error data is higher than a predetermined value, the correction means limits the level of the error data to the predetermined value.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, the description is made using L ^* a ^* b ^* color coordinate data as color coordinate data represented by lightness and chromaticity, but other color coordinate data, for example, L ^* u
^The present invention can also be applied to the color coordinate data of ^* v ^* , YIQ, and Yuv.

FIG. 1 is a block diagram showing an outline of an image processing apparatus according to one embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a color image reading unit constituted by a CCD, which converts a reflected light amount of an original into an electric amount and outputs the electric amount. The CCD 1 converts the R light, G light, and B light in the reflected light into electric signals for each color component by the filters of the three primary colors R, G, and B. These analog electric signals are quantized by the A / D converter 2 into digital signals. The digital image signals r, g, and b are corrected by a shading correction circuit 3 for variations between CCD elements and uneven light sources. The color system conversion circuit 4 converts the r, g, b signals into L ^* a ^* b ^* signals. L ^* a ^* b ^* represents the lightness ^a * ^{b *} chromaticity is ^{L *,} to form the color space to represent all the colors in the L ^* a * ^{b *.} The image correcting unit 5 may, for example, perform edge enhancement on the L ^* signal or a ^* b according to an external instruction.
^* Convert an input color image signal into a desired color image signal, such as color correction in space, color conversion and scaling.

Reference numeral 6 denotes a pseudo halftone processing unit for a color image for achieving the purpose of the present embodiment. The multivalued image signal represented by L ^* a ^* b ^* and one pixel can be represented by the recording unit 7. Pseudo halftone processing is performed for each pixel based on several types of expression color data.

A first embodiment of the pseudo halftone processing section 6 according to the present embodiment will be described in detail with reference to the block diagrams shown in FIGS.

<First Embodiment> The selection unit 62 shown in FIG.
Is selected for each pixel (hereinafter, referred to as requantization). In the embodiment, the recording device is a printer capable of binary recording of C, M, Y, and K4 colors, and therefore, the expression colors are C, M, and
Eight colors, Y, B (CM), G (CY), R (MY) and W, K (black).
The value (x) of the selected eight colors is represented by a value of 3 bits (0 to 7), and is converted into a C, M, Y, K2 value signal by the recording signal conversion unit 64. FIG. 4 shows the correspondence between the requantized signal x and the recording signal. This requantized signal x is input to the data storage unit 60. The data storage unit 60 two-dimensionally delay-holds the requantized data of the pixel in the vicinity of the pixel of interest that has just been requantized, and further calculates an average image color from the multiple pixel data in the average value calculation unit 61. Input to selection section 62. Note that the average value calculation unit 61 uses L ^* a ^* b
^* Because the operation is performed in space, the expression color based on the requantized data x stored in the data storage unit 60 based on the value is L ^* a ^* b ^*.
Convert to data. The error correction unit 63 receives the quantization error generated when requantization is performed by the selection unit 62, and corrects adjacent input pixel data. Therefore, the above-described requantization is performed on the input image signal that has been subjected to the period difference correction processing.

Hereinafter, the processing algorithm will be described in detail with reference to FIG.

In FIG. 3, the requantized signal x is input to the memory 600 in order to hold a delay of about one line, and this signal and the requantized signal one line before are stored in the RAMs 602, 601, 604, 603, 6
Input to the input address terminals of 06 and 605. This RAM stores the expression color data represented by the requantized signals x to x as L ^* a.
^* B ^* Convert to color coordinate data. In other words, the expression color x is RAM6
01, 602 to L ^* data, RAM 603, 604 to a ^* data,
RAMs 605 and 606 are converted to b ^* data. In this embodiment, this conversion table uses FIG.

Each L ^* space of the data converted into _{^{L * x a * x b *}} x respectively from the re-quantized signal x, a ^* space, b ^* data of the adjacent sly four pixels to the pixel of interest in space, respectively F / F610-1 610-2 610-3 and F / F611-1 611
-2 611-3, F / F612-1 612-2 Located at the output terminal of 612-3. That is, the target pixel position * shown in FIG.
, L ^* data of pixels A, B, C, and D adjacent thereto, that is, L ^* _A , L ^* _B , L ^* _C , and L ^* _D are respectively F / F610-3.
Output, F / F610-1 input, F / F610-1 output, F / F610-2 output. Similarly, a ^* data a ^* _A a of the above four pixels
^* _B a ^* _C a ^* _D is F / F611-3 output, F / F611-1 input, F /
Similarly obtained F611-1 output ^{B *} data ^{b *} _A ^{b * _B b *}
_C b ^* _D data F / F613-3 output, F / F612-1 input, F / F
It can be obtained with 612-2 input and F / F612-2 output. The averaging value calculation units 613, 614, and 615 respectively use the weighting coefficients shown in FIG. 7 independently at the positions of the four adjacent pixels and the target pixel (the pixel to be requantized) in the L ^* b ^* a ^* space. Average value
Obtain mL ^* ma * mb ^* .

mL ^* = 1 × L ^* _D + 4 × L ^* _C + 1 × L ^* _B + 4 × L
^* _A ma ^* = 1 × a ^* _D + 4 × a ^* _C + 1 × a ^* _B + 4 × a
_{^{* A mb * = 1 × b}} * D + 4 × b * C + 1 × b * B + 4 × b
^* _A The value of the average value mL ^* ma ^* mb ^* is input to the selection unit 622, and the weight coefficient 6 assigned to the target pixel position and requantization of the target pixel are predicted, and the average value calculation ends. That is, if the target pixel is predicted to be cyan now (x = 0), the average value mL ^* ₀ ma ^* ₀ mb ^* ₀ in the case where cyan is predicted is Becomes

Similarly, average value mL ^* xma ^* for which requantization is predicted as _x
x mb ^* _x is Then, eight sets of predicted average values for eight expressible colors are obtained. Selecting unit 622 the eight sets of predicted mean value and the error corrected input image data f '(i, j) the distance between (described later), a Delta] E _x ² obtained by the following equation, the value of x indicating the smallest value It is determined as requantized data.

_{^{ΔEx 2 = (L * f '}} -mL * x) 2 + (a * f' -ma *
^{_{x) 2 + (b * f}} '-mb * x) 2 ( where ^{_{L * f' a * f '}} b * f' is f '(i, j) of L
^{* A *} b ^* spatial data) X _ij = min (ΔE _x ² ) By the way, the expression color determined by the above processing (x = 0 to 7)
If, the input image f 'than that Delta] E _x ² is minimum, it defines the requantization error ΔL ^* Δa ^* Δb ^* generated at this time by the following equation.

_{^{ΔL * = L * f '-mL}} * x Δa * = a * f' -ma * x Δb * = b * f '-mb * x will be described in detail above requantization error correction.

The above error is basically determined by the value of the error that occurs when two adjacent pixels (that is, a pixel f (i, j + 1) to be re-quantized next) and a pixel f ′ (i + 1, j) adjacent to the next line are corrected. The value is adaptively limited as shown below.

| ΔL ^* |> α when _{^{L ΔL * = ± α L |}} Δa * |> α when Δa ^* = ± α _a of _a | _Δb ^* |> time of _{^{α b Δb * = ± α b}} (α L, α _a , α _b are constants) That is, by performing the above-described processing, blurring on the requantized image is prevented when the input image changes rapidly. This is a portion where the error between the average value and the input image is large is a portion where the input image changes abruptly. In this portion, it is determined that the image is an edge portion of the image, and the error is corrected to prevent blurring of the image.

By the way, the requantization error ΔL ^* Δa ^* Δb ^* is shown below assuming that the correction error to the next pixel position is e1 and the correction error to the next line pixel position is e2. FIG. 8 shows an error correction position with respect to the target pixel position.

Here, the correction error e2 to the next line pixel is input to the error memory 634 of FIG. 3, and at the same time, is input from the error memory 634 about one line before and held for one line, that is, e2
(I, j + 1) is read out, and e1 (i,
j + 1) is added. The output of this adder is the next pixel f (i, j +
This is a correction error for 1) and is added to the input image data by the adder 632. The equation is as follows.

f ′ (i, j + 1) = f (i, j + 1) + e1 (i, j + 1) + e2 (i, j + 1) The above error correction is performed by L ^{* in the} weighted average calculation units 613, 614, and 615 ^.
a ^* b ^* Execute independently in each space. The input image data f '(i, j + 1) for which the error correction has been completed is input from the F / F 631 to the selection unit 622 when the next pixel data clock is input, and the above-described series of requantization processing is performed. Be executed.
Note that the recording signal decode ROM 640 in FIG. 3 converts the recording binary signals C _X , M _X ,
It is a ROM for conversion to Y _X and K _X.

<Another Embodiment of Error Correction Unit> In this embodiment, the error is distributed to two adjacent pixel positions. However, for example, the error distribution is performed as shown in FIG. The occurrence of unusual texture at low frequencies in the image can be reduced.

Further, as another embodiment for achieving the same purpose, it is also effective to set the error distribution ratio to two pixels as shown in the figure below and to switch each pixel using a pseudo random number.

If a so-called M-sequence code is used as the 1-bit pseudo-random number generator, a pseudo-random number PN that does not generate periodicity even when processing the entire area of the 3A document can be constituted by approximately 25 stages of 1-bit shift registers.

<Embodiment of Error Correction Unit> Although the value of the generated error is limited by a predetermined value to prevent blur on the requantized image, edge information of the input image is more faithfully obtained on the requantized image. Has ΔL ^* Δa ^* Δ
It is effective to stop the error correction in the L ^* space indicating the brightness of b ^* .

In other words, | ΔL ^* |> β when the _{^{L ΔL * = 0 | Δa *}} |> α when Δa ^* = ± α _a of _a | _Δb ^* |> _α time of _{^{b Δb * = ± α b (}} α a, α _b , β _L are constants) In both embodiments, the constants α and β are set in accordance with the image, for example, when a silver halide photograph or the like is set in the original, α and β are set large, and conversely, when the halftone original is When set, α and β are set to be small, so that requantization more faithful to the original can be performed. Note that the values of α and β can be automatically changed based on the characteristics of the microscopic image (eg, edge information) while reading the document.

Although adaptive processing is performed based on the value of ΔL ^* Δa ^* Δb ^* , the error is defined as the difference between the input image signal after error correction and the determined predicted average value. Is more effective to define a new difference δL ^* from the input image signal f before the error correction and to perform the above-described adaptive processing according to this value.

 The formula is shown below.

_{^{δL * = L * f -mL *}} x | δL * |> β when the _{^{L ΔL * = 0 | Δa *}} |> α when Δa ^* = ± α _a of _a | Δb ^* |> _{α b} of time Δb ^* = ± α _b <Another Example of Prediction Average Value Calculating Unit> In the above-described embodiment, the average value was calculated from four re-quantized pixel data adjacent to each other in each L ^* a ^* b ^* space.
As shown in FIG. 10, the average value calculation area in L ^* space is a ^* b
^* By making it larger than that of the space, a requantized image faithful to the manuscript can be obtained.

Also, the weight coefficient at each pixel position is not limited to the present embodiment, and a good requantized image can be obtained if there is a tendency that the weight coefficient increases as the pixel position approaches the target pixel position and decreases as the pixel position increases.

<Another embodiment of the selection unit> The above embodiment is based on the premise that C, M, Y, and K4 color binary recording is performed.
This is an example of requantization into eight representative colors of G, B and W. To express more faithful black, in addition to the above eight colors, CK = C + K, MK = M + K, YK = Y + K C, M , Y, and black, CK, MK, and YK. At this time, the conversion to the recording signal is C = 1, K = 1 for CK, M = 1, K for MK.
= 1, YK, Y = 1, K = 1. This processing reduces the recording of the recording color dots in a single color in an area close to black, so that it can be requantized as dark gray with a more faithful color.

In order to requantize light gray more faithfully, if the recording apparatus is an ink jet recording using dark and light inks or an electrophotographic method using pulse width energy modulation, recording at an intermediate level is assumed, and In the middle, WK, WK = 0.5K, that is, black recording is ternary recording, the former is the L ^* a ^* b ^* value when light black ink is used, the WK is the value, and the latter is black dot recording at half dot size In this case, the L ^* a ^* b ^* value may be used as the WK value and requantized to 9 colors in addition to the 8 colors.

Similarly, if it is extended to other C, M, and Y, a higher quality re-quantized image can be obtained because the recording expression color in the intermediate color is further increased.

As described above, according to this embodiment, the quantized data that has already been requantized and the requantization of the pixel of interest are predicted and L
The average value in the ^* a ^* b ^* space is obtained, the difference between this value and the input image signal is obtained by ΔE, the closest recording color is selected, the recording signal is obtained for each pixel, and the generated requantization error is calculated. By adaptively controlling the error value generated when correcting on adjacent pixels, any input image signal of a halftone image or a character image can be faithfully reproduced.

〔The invention's effect〕

As described above, according to the present invention, a plurality of pieces of predicted average value coordinate data are calculated from the expression color coordinate data for which conversion processing has been completed and the predicted expression color coordinate data of the pixel of interest, and coordinates representing the color of the input pixel of interest. The expression color represented by the predicted expression color coordinate data when the spatial distance between the data and the plurality of predicted average value coordinate data is the smallest is used as the expression color of the pixel of interest, and further, error data generated during the conversion is corrected. Accordingly, the color of the input color image can be faithfully reproduced, and the edge portion can be reproduced clearly by limiting the correction amount of the error data.

[Brief description of the drawings]

FIG. 1 is a block diagram of a color image processing apparatus according to an embodiment of the present invention, FIGS. 2 and 3 are block diagrams showing details of a halftone processing section, and FIG. 4 is determined by halftone processing. FIG. 5 shows the relationship between the expression color and L ^* a ^* b ^* , and FIG. 6 shows the reference pixel when performing halftone processing. 7 and 10 are diagrams showing weighting factors for obtaining an average value, and FIGS. 8 and 9 are diagrams showing distribution positions of errors. In the figure, 1 is a sensor, 2 is an A / D converter, 3 is a shading correction circuit, 4 is a color system conversion circuit, 5 is an image correction unit, 6 is a pseudo halftone processing unit, and 7 is a recording and display unit.

Claims (1)

(57) [Claims] 1. A color image processing apparatus for converting coordinate data representing a color in a predetermined color space into one of a plurality of expression colors that can be expressed by an output device, comprising: An input means for inputting coordinate data representing a color; a predicted average value coordinate obtained by a weighted average of weighted coefficients of expression color coordinate data of a plurality of pixels having undergone conversion processing around the target pixel and predicted expression color coordinate data of the target pixel. A process of calculating data, changing the prediction expression color coordinate data, executing the prediction average value calculation means for calculating a plurality of prediction average value coordinate data, and the coordinate data of the pixel of interest input by the input means;
Comparing means for comparing a spatial distance with a plurality of respective predicted average value coordinate data calculated by the average value calculating means; and a predicted average value coordinate data having a smallest spatial distance based on a comparison result of the comparing means. Determining the expression color represented by the predicted expression color coordinate data as the expression color of the pixel of interest; and newly inputting error data generated when the determination unit determines the expression color of the pixel of interest. Error correction means for correcting by adding to the pixel coordinate data, wherein the error correction means limits the error data level to a predetermined value when the error data level is larger than a predetermined value. Color image processing apparatus.