JP3932161B2 - Image processing apparatus and method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image processing apparatus and method for reproducing a color image using C, M, Y, and K recording color materials having a plurality of pseudo halftone processes including a dot concentration type and a dot dispersion type.
[0002]
[Prior art]
Conventionally, in a digital color image processing apparatus such as a digital color copying machine, an R (Red), G (Green), and B (Blue) signal of an original is read as reflectance data by a scanner, and the reflectance data is converted into a density value. Processing, color correction processing, black generation processing, under color removal processing (hereinafter referred to as UCR), pseudo halftone processing, etc., and four colors of C (Cyan), M (Magenta), Y (Yellow), K (Black) The image is reproduced and output from the printer with the recording color material.
[0003]
In the black generation process, a K signal is generated from the C, M, and Y signals, and in the under color removal process, an amount corresponding to the K signal is subtracted from the C, M, and Y signals. How much is set is one of the factors having a great influence on the image quality.
[0004]
The appropriate amount of black depends largely on the nature of the pseudo halftone processing used. For example, in error diffusion processing, “black” caused by the overlapping of CMY three-color toners and “black” due to K toner appear concentrated on a specific pixel due to the propagated quantization error. There is a case where no toner is attached to the pixel to which the toner is to be attached, and dark black appears sparsely in the highlight portion, which makes it extremely noticeable and rough. Black dots are particularly noticeable in color printers that have a large dot diameter and a small number of tones that can be expressed. Therefore, in the case of error diffusion processing, it is better not to add ink from highlights.
[0005]
On the other hand, such a phenomenon does not occur in multi-value dither processing using a dither matrix having a dot concentration effect such as screen processing. Therefore, it is possible to aim at improving the color reproducibility of gray highlights and low-contrast black characters by adding ink from highlights to some extent while taking into account the gradation and background stains of the photographic part. .
[0006]
[Problems to be solved by the invention]
By the way, in recent years, an apparatus having a plurality of halftone processes and selectively switching them has been proposed. In general, dot-concentrated processing such as a line screen has problems that (1) moire occurs in a halftone printing section and (2) edges are cut off by dither processing. On the other hand, dot dispersion processing represented by error diffusion is considerably more advantageous than the former in terms of resolution and moire, but has a problem that graininess noise is conspicuous.
[0007]
Therefore, there is a method that selects an optimal halftone process according to the nature of the input image. Further, there has been proposed one that switches other processing in accordance with switching of halftone processing. For example, when an image forming apparatus described in Japanese Patent Laid-Open No. 9-331450 selects and outputs halftone processing such as error diffusion processing and screen processing, the gamma table and filter processing are switched according to the halftone processing. Is.
[0008]
The present invention performs processing suitable for each halftone processing, similar to the above-mentioned publication.
SUMMARY OF THE INVENTION An object of the present invention is to provide an image processing apparatus and method in which the black generation start point is switched in accordance with halftone processing by paying attention to the difference in optimum black generation start point setting by halftone processing. .
[0009]
[Means for Solving the Problems]
In the present invention, the black generation start point is switched corresponding to each of the dot-concentrated halftone processing such as the line screen method and the dot dispersion type halftone processing such as the error diffusion method, so that it is optimum for each highlight portion. The ink generation process is performed.
[0010]
In the present invention, the highlight gray reproducibility is maintained in the dot-concentrated halftone processing such as the line screen method, and the K toner in the highlight is used in the dot dispersion halftone processing such as error diffusion. Roughness due to is suppressed.
[0011]
In the present invention, by selecting a dot-concentrated process with less graininess in the photographic paper section, an optimal halftone process is performed, and a black generation process suitable for the halftone process is performed. Yes.
[0012]
Further, according to the present invention, the halftone printing unit selects a dot dispersion type process that is less likely to cause moiré, thereby performing an optimum halftone process and performing a black generation process suitable for the halftone process. I have to.
[0013]
Further, according to the present invention, by selecting dot dispersion type processing that is advantageous for resolution in the character portion, optimal halftone processing is performed, and black generation processing suitable for halftone processing is performed. .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
[0015]
FIG. 1 is a block diagram of an embodiment of the present invention. First, the LOG conversion unit 2 performs LOG conversion on the RGB reflectance signal (each color is, for example, an 8-bit digital signal) of an image read by the color scanner 1 to convert it into a density signal. Next, the color correction unit 3 performs color correction processing on the R′C′B ′ density signal to convert it into a CMY signal. The conversion formula for the color correction processing is expressed as in Formula (1) using color correction parameter groups a0 to a3, b0 to b3, and c0 to c3.
[0016]
C = a0 + a1 × R ′ + a2 × G ′ + a3 × B ′
M = b0 + b1 × R ′ + b2 × G ′ + b3 × B ′
Y = c0 + c1 × R ′ + c2 × G ′ + c3 × B ′
Formula (1)
The black generating unit 4 generates a K signal according to the CMY data. Here, the K signal is generated from the value of Min (C, M, Y) by table conversion, but may be calculated directly by an equation. A plurality of conversion tables from Min (C, M, Y) to K are held, and one of the plurality of conversion tables is selected in response to the selection signal 8. The UCR unit 5 performs the under color removal process using the CMY and K signals according to Equation (2).
[0017]
C ′ = C−K
M ′ = M−K
Y '= YK
Formula (2)
The halftone processing unit 6 performs halftone processing on the C′M′Y ′ signal and the K signal thus derived, and the signal C ″ M ″ Y ″ K ″ after the halftone processing is obtained. And output from the color printer 7. The halftone processing unit 6 is provided with a plurality of halftone processes including both a dot concentration type and a dot dispersion type, and receives one of the plurality of halftone processes in response to the selection signal 8. Select according to the method. Similarly, a plurality of conversion tables in the black generation unit 4 described above are held and selected in response to the selection signal 8. Since the conversion table is set corresponding to the halftone process, it is determined uniquely when the halftone process is determined.
[0018]
FIG. 2 shows a configuration example of switching of halftone processing. On the operation panel 11, the photographic paper portion 15 and the character portion 17 are designated as areas, and the other portions not designated are designated as the halftone dot printing portion 16. Then, AM screen processing is performed in the area designated as the photographic paper portion 15 in order to prioritize smoothness. For example, a 2 × 1 dither process is used. If it is a 600 dpi printer, it will be a 300-line multi-line screen type.
[0019]
On the other hand, in the area of the character 17 and the halftone print 16, the error diffusion processes 13 and 14 are performed in consideration of resolution priority and moire prevention. FIG. 3 shows a block diagram of error diffusion processing. As shown in FIG. 3, the error diffusion process is a process for generating an output image by distributing an error between input data and output data to pixels other than the target pixel. As the weighting matrix 24 for determining the distribution weight, for example, the matrix shown in FIG. 4 may be used.
[0020]
The same error diffusion process 14 for characters and halftone dot error diffusion process 13 in FIG. 2 may be used, but the number of quantizations 21 in FIG. 3 may be different, or in FIG. If the matrix size is varied, a more suitable process can be performed. FIG. 3 shows an example in which binarization is performed with one threshold value. However, it is also possible to provide a plurality of threshold values and perform multi-leveling. The error diffusion process 13 for halftone dot printing may increase the number of quantizations with more emphasis on gradation than the error diffusion process 14 for characters. In the error diffusion process 14 for characters, the resolution can be further improved by using a matrix having a size smaller than the matrix size shown in FIG.
[0021]
In the above example, the user designates an area using the operation panel 11, but a method of automatically detecting the type of image may be used. For example, according to the method described in Japanese Patent Application No. 11-182554 filed earlier, the photographic paper and the halftone dot region can be automatically identified. In addition to switching for each area, switching may be performed according to the mode, such as a photographic paper mode, a character mode, and a character / print photo mode.
[0022]
FIG. 5 is a diagram for explaining an example of conversion table switching in the black generating unit 4. As shown in FIG. 5, when the black generation start point of the line screen processing table 31 is a and the black generation start point of the error diffusion table 32 is b, a <b is set.
[0023]
As described above, according to the present embodiment, when the dot concentration type line screen processing is selected, the dot generation type error diffusion processing is selected using the black generation table 31 whose black generation start point is a. Uses a black generation table 32 whose black generation start point is b (> a), and thus suppresses highlight roughness in the error diffusion processing unit while maintaining the gray reproducibility of highlights in the line screen processing unit. can do. Even if the halftone processing is selectively switched in the photographic paper portion, character portion, and halftone printing portion, the black start point is also switched in accordance with the halftone processing, so that the highlight portion is not hindered.
[0024]
In this embodiment, a line screen is taken as an example of the dot concentration type and error diffusion is taken as an example of the dot dispersion type, but the present invention is not limited to this. For example, as the dot dispersion type, dither processing using a Bayer type matrix having a dot dispersion effect or FM screen processing may be used. In this embodiment, a scanner and a printer are assumed as input / output devices, but the present invention is not limited to this. Furthermore, the present invention can also be applied when image data acquired via a network is used as an input.
[0025]
【The invention's effect】
As described above, according to the present invention , in an apparatus or method that has a plurality of halftone processes including a dot concentration type and a dot dispersion type and selectively switches between them, the dot concentration type and the dot dispersion type can be used. Since the black generation start point is switched, optimum black generation processing can be performed for each of the highlight portions.
[0026]
According to the present invention , since the dot dispersion type black generation start point b is delayed from the dot concentration type black generation start point a (a <b), the gray reproducibility of the highlight of the dot concentration type processing unit is maintained. In addition, it is possible to suppress the roughness due to the K toner at the highlight of the dot dispersion processing unit.
[0027]
According to the present invention , since the dot concentration type is selected in the photographic paper processing unit, it is possible to obtain an image with less graininess and good gray reproducibility in the highlight portion.
[0028]
According to the present invention , since the dot dispersion type processing is selected in the halftone print processing unit, it is possible to obtain an image in which moire does not occur and the roughness of the highlight portion due to K toner is small.
[0029]
According to the present invention , since the dot dispersion type processing is selected in the character processing unit, an image with high resolution and less roughness due to K toner in the highlight portion can be obtained.
[Brief description of the drawings]
FIG. 1 shows a configuration of an embodiment of the present invention.
FIG. 2 shows a configuration example for switching halftone processing.
FIG. 3 shows a configuration example of error diffusion processing.
FIG. 4 shows an example of a weight matrix for performing error diffusion.
FIG. 5 shows characteristics of a black generation table selected according to halftone processing.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Color scanner 2 LOG conversion part 3 Color correction part 4 Black generation part 5 UCR part 6 Halftone processing part 7 Color printer 8 Selection signal

Claims (4)

Image processing that outputs to the image data a pseudo halftone process selected from a plurality of pseudo halftone processes including at least a dot-concentrated pseudo halftone process and a dot dispersion type pseudo halftone process When the dot dispersion type pseudo halftone process is selected when performing black generation from the image data, the apparatus may delay the start point of black generation compared to the dot concentration type pseudo halftone process. A featured image processing apparatus. Select dot-concentrated pseudo halftone processing for photographic paper portion of image data, select dot dispersion type pseudo halftone processing for halftone dot printing portion and character portion of image data, and An image processing apparatus that performs the selected pseudo-halftone process on the image data and outputs the image data, and when dot generation type pseudo-halftone process is selected when generating black from the image data, An image processing apparatus characterized by delaying the starting point of black ink generation compared to dot-concentrated pseudo halftone processing. Image processing that outputs to the image data a pseudo halftone process selected from a plurality of pseudo halftone processes including at least a dot-concentrated pseudo halftone process and a dot dispersion type pseudo halftone process In the method of generating black ink from the image data, when dot dispersion type pseudo halftone processing is selected, the start point of black generation is delayed from dot concentration type pseudo halftone processing. A featured image processing method. Select dot-concentrated pseudo halftone processing for photographic paper portion of image data, select dot dispersion type pseudo halftone processing for halftone dot printing portion and character portion of image data, and An image processing method for performing the selected pseudo halftone process on the image data and outputting the image data, and when performing black generation from the image data, when a dot dispersion type pseudo halftone process is selected An image processing method characterized in that the starting point of black generation is delayed as compared with the dot-intensive pseudo halftone processing.

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