JPH04318762A - Dither matrix generating method and picture data processing unit - Google Patents

Abstract

PURPOSE:To realize an output picture with excellent gradation in which an intermediate tone is sufficiently reproduced by bringing a density characteristic of a picture outputted from a digital output device with respect to a gradation level to an ideal characteristic. CONSTITUTION:An analog picture inputted via a picture input section 1 of a CCD image sensor is converted into a digital multi-value picture signal at an A/D converter section 2. Then a dither processing circuit 3 uses a dither matrix in which a threshold level interval has an equal pitch to convert the signal into a binary picture signal. The binary picture signal is outputted to a digital output device 5 such as a laser printer from a binary picture output section 4. Thus, an output density characteristic with respect to a gradation level of an output picture is obtained and further, a characteristic curve in axial symmetry is obtained with respect to an output density ideal characteristic of a density characteristic curve. The threshold level of the dither matrix is decided by the characteristic curve. Thus, the input picture is corrected so that the output density characteristic of the digital output device 5 in use is close to the ideal characteristic and then binarized.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for creating a dither matrix used when converting a multivalued image signal into a binary image signal by a dither method, and an image using the dither matrix created by this method. The present invention relates to a data processing device.

0002

2. Description of the Related Art In the case of digital output devices such as digital copying machines and laser facsimiles, image data read by an image sensor, etc. is digitally converted into a multi-value image signal, and this multi-value image signal is further converted into a binary image signal. In this case, the systematic dither method is known as one of the methods for converting a multivalued image signal into a binary image signal. .

This systematic dithering method is a method of binarizing a multivalued image signal of an input image by comparing it pixel by pixel with a threshold value set in a dithering matrix. There were dot-dispersed types as typified by the proposal of , dot-concentrated types such as halftone dots and spirals.

However, in any dither matrix, the threshold values for each pixel are set at equal threshold intervals such as 0, 4, 8, 12, ..., 252 in the case of a dither matrix with a size of [8×8]. There are 64 levels of difference, and [
4×4] size is 0, 4,
8, 12, ..., 60, etc. 16 with equal threshold intervals
It was a stage value.

[0005]

[Problems to be Solved by the Invention] As described above, since the dither matrix used in the conventional systematic dither method has equal threshold intervals, it is easily influenced by the output density characteristics of the digital output device used, and for example, the output When reproducing a binarized input image using the organized teaser method using an output device whose density characteristics have shifted to the high density side, the result is an image with a dark halftone reproduction and an overall crushed impression. It became.

Accordingly, the present invention provides a method for creating a dither matrix capable of converting a multivalued image signal of an input image into a binary image signal so that the density characteristics with respect to the gradation level of an image output from a digital output device approach ideal characteristics. This is what we are trying to provide.

Another object of the present invention is to provide an image data processing device capable of obtaining an output image with excellent tonality and sufficient reproduction of halftones without being affected by the output density characteristics of digital output equipment. It is something to do.

[0008]

[Means for solving the problem] The invention corresponding to claim 1 is:
A first step of converting a multi-value image signal into a binary image signal using a dither matrix for converting a multi-value image signal into a binary image signal using a dithering method using a dither matrix with equal threshold intervals; A second step of determining the output density characteristic with respect to the gradation level when the binary image signal obtained in the first step is outputted by a digital output device, and outputting an output density characteristic curve with respect to the gradation level obtained in this second step. This dither matrix creation method includes a third step of determining a line-symmetric characteristic curve with respect to the ideal density characteristic, and a fourth step of determining a threshold value of the dither matrix based on the characteristic curve obtained in the third step. .

The invention corresponding to claim 2 includes a dither matrix storage unit that stores a dither matrix with asymmetric threshold intervals, and a dither matrix stored in this storage unit that converts a multivalued image signal into a binary image signal. This image data processing device includes a dithering circuit section that outputs a binary image signal from this circuit section to a digital output device.

0010

[Operation] In the invention corresponding to claim 1, a multi-valued image signal is converted into a binary image signal using a dither matrix with equal threshold intervals, and the digital output device that uses the binary image signal outputs the signal. The output density characteristic for the gradation level of the output image is determined using the method, and a characteristic curve that is axisymmetric with respect to the ideal output density characteristic of the output density characteristic curve is determined, and the threshold value of the teaser matrix is determined by that characteristic curve. . Therefore, the input image is corrected and binarized so that the output density characteristic of the digital output device used approaches the ideal characteristic.

[0011] In the invention corresponding to claim 2, a multi-valued image signal of an input image is converted into a binary image signal using a dither matrix with a non-uniform threshold interval. Therefore, it is less affected by the output density characteristics of the digital output device used.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an image data processing device, and as shown in the figure, a CCD (solid-state image sensor)
An analog image inputted through an image input unit 1 composed of an image sensor such as It is configured to convert into a binary image signal by a dither method and output the obtained binary image signal from a binary image output section 4 to a digital output device 5 such as a laser printer.

The dithering circuit 3 includes a dither matrix memory 31 comprising an EEPROM (electrically erasable read-only memory) or the like for storing a dither matrix in which a threshold value is set for each pixel corresponding area in a matrix.
a readout control unit 32 that controls readout of the dither matrix stored in the memory 31; and a multivalued image signal output from the A/D conversion unit 2 and readout from the memory 31 under the control of the control unit 32. The comparator 33 compares the dither matrix with the dither matrix for each pixel corresponding to the size of the dither matrix and binarizes the result.

The dither matrix memory 31
As shown in FIG. 2, a non-uniform halftone dot dither matrix with a size of [8×8] and threshold intervals of 5, 7, 13, 15, ..., 254 is stored. . Here, the halftone dither matrix is created according to the procedure shown in FIG. 4 and stored in the memory 31.

First, as a first step ST1, as shown in FIG. 3, the size is [8×8] and the threshold intervals are 0, 4,
Using an arithmetic halftone dither matrix of 8, 12, . . . , 252, a multivalued image signal of an input image represented by 256 gradations is converted into a binary image signal representing 64 gradations. In this case, since the threshold intervals of the dither matrix are equal, the gradation conversion characteristics when converting a multilevel image signal into a binary image signal are 256 gradation levels as shown by P0 in FIG. (multivalued image signal) has a characteristic corresponding to 64 gradation levels (binary image signal) with linearity.

Next, as a second step ST2, the first
The 64-gradation binary image signal obtained by the step is output using the digital output device 5 that is actually used,
The output density characteristics with respect to the gradation level of the binary image signal are determined.

Next, as a third step ST3, the second
A characteristic curve line-symmetrical to the ideal output density characteristic of the output density characteristic for the gradation level of the binary image signal obtained in the step is determined.

Finally, in the fourth step ST4, the gradation conversion characteristic when converting the multi-valued image signal of the input image represented by 256 gradations into a binary image signal representing 64 gradations is determined according to the third step ST4. A dither matrix is created by determining a threshold value for each pixel so as to match the characteristic curve obtained in the step.

[0020] The dither matrix thus created is not one in which the threshold intervals are fixed to an equal difference, but are mainly variable in a non-uniform difference depending on the output characteristics of the digital output device 5 actually used.

Now, it is assumed that the output density characteristic with respect to the gradation level of the binary image signal of the digital output device 5 obtained in the second step ST2 is shown by the solid line P1 in FIG. In this case, the ideal output density characteristic is a linear characteristic as shown by the broken line P2 in the figure. Therefore, when the multivalued image signal of the input image represented by 256 gradations is converted into a binary image signal representing 64 gradations using the dither matrix (see FIG. 2) whose threshold value is determined in the fourth step ST4, The gradation conversion characteristic has non-linearity as shown by the solid line P3 in FIG.

For example, when a multivalued image signal located at a level of 48 out of 256 gradations is binarized using a dither matrix with equal threshold intervals shown in FIG. 3, it is located at a level of 12 out of 64 gradations. This 64-bit image signal is converted into a binary image signal.
The output density characteristic of the binary image signal located at the level of the 12th gradation in the digital output device 5 is shifted to the high density side of 0.40 from the ideal density of 0.34. In this case, the intermediate tones are reproduced darkly, resulting in an image with an overall crushed impression. The gradations of the binary image signal corresponding to the density of 0.34 are 8 out of 64 gradations.

On the other hand, when a multivalued image signal located at a level of 48 out of 256 gradations is binarized using a dither matrix with an arithmetic threshold interval shown in FIG. It is converted into a binary image signal located at . That is, the output density characteristics of the digital output device 5 are as follows:
This corresponds to 0.34, which is the ideal characteristic of a multivalued image signal located at a level of 48 out of 256 gradations.

As described above, according to this embodiment, the output density characteristic P4 of the digital output device 5 matches the ideal characteristic P2 having linearity with respect to the gradation level of the multivalued image signal, as shown in FIG. It becomes a characteristic. Therefore, without being affected by the output density characteristics of the digital output device 5,
An output image with excellent gradation and sufficiently reproduced halftones can be obtained.

[0025] In the above embodiment, the present invention was applied to a halftone dither matrix, but it goes without saying that the present invention can also be applied to other dither matrices. In particular, the threshold value of the dither matrix can be freely set variably even for dot configurations with high spatial frequencies that significantly impair gradation adjustment, such as in laser printers, so it is possible to obtain finely defined images, which is the advantage of the dot dispersion type. You can expect excellent results.

[0026]

As described in detail above, according to the invention corresponding to claim 1, the density characteristics with respect to the gradation level of the image output from the digital output device approach ideal characteristics.
It is possible to provide a method for creating a dither matrix that can convert a multivalued image signal of an input image into a binary image signal.

Further, according to the invention corresponding to claim 2, image data is provided that allows an output image with excellent tonality and sufficient reproduction of halftones to be obtained without being affected by the output density characteristics of a digital output device. Processing equipment can be provided.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the device of the present invention.

FIG. 2 is an example diagram of a dither matrix created by the method of the present invention.

FIG. 3 is an example diagram of a conventional dither matrix.

FIG. 4 is a flowchart showing an embodiment of the method of the present invention.

FIG. 5 is a gradation conversion characteristic diagram in a conventional dither matrix.

FIG. 6 is an output characteristic diagram of a conventional digital output device.

FIG. 7 is a gradation conversion characteristic diagram in the dither matrix of this embodiment.

FIG. 8 is an output characteristic diagram of the digital output device according to this embodiment.

[Explanation of symbols]

1... Image input section, 2... A/D conversion section, 3... Dithering circuit section, 4... Binary image output section, 5... Digital output device, 3
1... Dither matrix memory, 32... Readout control unit, 3
3... Comparison section.

Claims (2)

[Claims] Claim 1: A dithering matrix for converting a multivalued image signal into a binary image signal using a dithering method is a dithering matrix for converting a multivalued image signal into a binary image signal using a dithering matrix with equal threshold intervals.
A first step of converting into a value image signal, a second step of determining output density characteristics with respect to gradation level when the binary image signal obtained in this first step is outputted by a digital output device, and this second step. a third step of determining a line-symmetrical characteristic curve with respect to the ideal output density characteristic of the output density characteristic curve for the gradation level obtained by;
A method for creating a dither matrix, comprising: a fourth step of determining a threshold value of the dither matrix based on the characteristic curve obtained in the third step. 2. A dithering matrix storage section that stores a dithering matrix with non-uniform threshold intervals, and a dithering circuit section that converts a multi-valued image signal into a binary image signal using the dithering matrix stored in this storage section. , and a binary image output section that outputs the binary image signal from the circuit section to a digital output device.