JPH09326931A - Image processing device - Google Patents

Abstract

(57) Abstract: When the number of gradation levels of an input image and the number of gradation levels of an output image are the same, the actual number of gradations of the output image decreases and the gradation property deteriorates. An image input unit 1 inputs 8-bit (256 gradation) image data. The gradation conversion unit 3 receives the input image data.
Data is converted into 29 gradation levels by the conversion characteristic curve stored in the conversion characteristic storage unit 2 and sent to the image output unit 3. The image output unit 3 outputs the image data whose gradation is converted into 29 gradation levels, and prevents the actual gradation number of the output image data from decreasing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus such as a digital copying machine for inputting a multivalued digital image, performing various kinds of processing, and outputting it.

[0002]

2. Description of the Related Art In an image processing apparatus such as a digital copying machine, a multi-valued digital image input may have different density distribution tendencies depending on imaging conditions and the like. In order to convert such an image into an image having a standard density distribution and output a high quality image, as shown in, for example, Japanese Patent Laid-Open No. 3-24674, the input image data is converted into a floor image. The key is converted and output. That is, as shown in FIG. 7, the input image data is converted by using a function or a look-up table showing the density conversion curve A that gives the correspondence between the input density value and the output density value, and the contrast is converted. Are being improved. When this gradation correction is performed, as shown in FIG. A, an 8-bit or 256-gradation input image is converted into an 8-bit (256-gradation) output image.

[0003]

When the number of gradations of the input image and the number of gradation levels of the output image are the same as described above, for example, the gradation levels are (0, 1, 2, 3, 4, 5, 6). , 7) and 8
As shown in FIG. 7, the gradation level of the output image when the density of the input image of the value is converted by the density conversion curve having eight output levels is (0, 3, 4, 5, 6, 6, 7, 7) and 6 values, one level of the output gradation is not used, the 6th level and the 7th level of the output gradation overlap, the actual number of gradations decreases, and the gradation property deteriorates. Further, in a device in which such gradation conversion is connected in several stages, the actual gradation number is further reduced to 256
The actual gradation of gradation cannot be obtained.

The present invention has been made in order to eliminate such disadvantages, and an object thereof is to obtain an image processing apparatus capable of forming a high quality image without substantially reducing the number of gradations. .

[0005]

The image processing apparatus according to the present invention is an image processing apparatus for inputting a multi-valued digital image, converting the gradation of the multi-valued digital image, and outputting the converted image. It is characterized in that gradation conversion is performed at a gradation level number larger than the number of levels and the gradation level is output.

A second image processing apparatus according to the present invention is characterized in that the density of an input image is converted into a density image which is greater than the density level of the input image and then output.

According to the third image processing apparatus of the present invention, when the input image is dithered and output, the density of the dither image is greater than the number of gradation levels of the input image. It is output by

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, an image processing apparatus has an image input section, a conversion characteristic storage section, a gradation conversion section, and an image output section. The image input unit inputs 8-bit (256 gradation) image data. The conversion characteristic storage unit stores in advance a conversion characteristic curve for converting an input value of an 8-value gradation level number into an output value of a 29-value gradation level number, for example. The gradation conversion unit performs gradation conversion of the image data input from the image input unit according to the conversion characteristic curve stored in the conversion characteristic storage unit and sends the image data to the image output unit. The image output section outputs the image data whose gradation is converted into 29 gradation levels, and outputs the output image data.
It is possible to prevent the actual gradation number of the data from decreasing.

Also, when converting the density of the input image data to increase or decrease the density of the input image data, 8-level image data is converted.
When the density of data is converted into 29-level image data, or when the input image data is subjected to dither processing and output, the density of the dither image to be output is converted into 29-value gradation level values, It is possible to prevent the actual gradation number of the output image data from decreasing.

[0010]

1 is a block diagram showing an embodiment of the present invention. As shown in the figure, the image processing apparatus includes an image input unit 1
It has a conversion characteristic storage unit 2, a gradation conversion unit 3, and an image output unit 4. The image input unit 1 inputs 8-bit (256 gradation) image data. For example, as shown in FIG. 2, the conversion characteristic storage unit 2 stores in advance a conversion characteristic curve A for converting an input value of 8-level gradation number into an output value of 29-level gradation number as a function or a lookup table. Has been done. The gradation conversion unit 3 converts the input image data by the conversion characteristic A stored in the conversion characteristic storage unit 2. Image output unit 4
Outputs 29 levels of image data.

In the image input unit 1 of the image forming apparatus configured as described above, for example, as shown in FIG. 2, the gradation levels are (0, 1, 2, 3, 4, 5, 6, 7). 8-value image data
When the image data is input, the gradation conversion unit 3 stores the conversion characteristic A in which each pixel of the input image data is stored in the conversion characteristic storage unit 2.
Tone conversion based on (0, 10, 16, 20, 23,
25, 27, 28) and the image data is converted to image data of 29-value gradation levels and sent to the image output unit 4. The image output unit 4 outputs the sent 29-value gradation level image data. In this way, since the 8-value input image data is converted into the 29-value image data and output, the actual gradation number of the output image can be set to 8 levels, and the gradation of the gradation-converted image can be changed. It can be output without reducing the number of keys.

In the above embodiment, the case where the inputted image data is subjected to gradation conversion to improve the contrast has been described. However, as shown in the block diagram of FIG. The same can be applied to the density conversion in which the density of the image data is made darker, or conversely, the density of the inputted image data is made lighter and the image data is outputted.

In this case, the conversion characteristic storage unit 2a stores in FIG.
As shown in the density conversion characteristic diagram of, the density conversion curve for converting the input density of the 8-value density level into the output density of the 29-value density level is stored in advance as one or more kinds as a function or a lookup table. I'll do it. In FIG. 4, a density conversion curve B shows an example of a conversion curve for increasing the density of the input image data, and a density conversion curve C shows an example of a conversion curve for decreasing the density of the input image data. When the image input unit 1 inputs 8-level image data, the density conversion unit 5 selects a density conversion curve corresponding to the density selected by the density selection unit 6 and converts the density. For example, when converting the input image data into a lighter density,
The density conversion curve B shown in is selected and converted into a gradation level number higher than the gradation level number of the input image data. Further, when converting the input image data to a darker density, the density conversion curve C shown in FIG. 4 is selected and converted to a gradation level number lower than the gradation level number of the input image data. To do. When this density conversion is performed, since the 8-level input image data is converted into the 29-level output image data, for example, the gradation levels are (0, 1, 2, 3, 4, 5, 6). ，
7) and octal input image data are converted by the density conversion curve B to output a bright image, the output image data is (0, 10, 16, 20, 23, 25, 27, 28). ) Image data of 29 levels, the actual gradation number of the output image can be set to 8 levels, and the output image data when the output gradation number shown in parentheses in FIG. (0,
3, 4, 5, 6, 6, 7, 7), it is possible to prevent the actual gradation number from decreasing to 6 levels. In addition, the gradation level is (0, 1, 2, 3, 4, 5, 6, 7)
Also when the 8-value input image data is converted by the density conversion curve C into a dark image and is output, the output image data is (0, 1, 3, 5, 8, 12, 17, 17). 28), the actual gradation number of the output image can be set to 8 levels, and the output image data (0, 0, 1, 1,) when the output gradation number is 8 levels. 2, 3, 4, 7), it is possible to prevent the number of real gradations from decreasing to 6 levels.

In the above-described embodiment, the case where the input image data is subjected to the gradation conversion or the density conversion has been described. However, as shown in FIG. 5, the input image data is dithered by the dither processing section 7. Similarly, in the case of outputting the same, it is possible to output without substantially reducing the gradation.

In this case, as shown in FIG. 6, the conversion characteristic storage unit 2b corresponds to 29-level dither image densities as the density characteristic curve D with respect to input image data having 8 gradation levels. I will let you. When 8-level image data is input to the image input unit 1, the dither processing unit 7 dither-processes the input image data, and the dither image density of 29 levels is obtained according to the density characteristic curve D shown in FIG. To the image output unit 4. The image output unit 4 outputs image data having a dither image density of 29 levels. For example, the dither image data which is obtained by converting the input image data having the gradation level of (0, 1, 2, 3, 4, 5, 6, 7) and eight values by the density conversion curve D and outputting it is (0 , 1, 3, 8, 16, 25, 27, 28)
Are converted to 29 levels, and the actual gradation number of the output image can be set to 8 levels, and the output image data (0, 0, 1, 2,
4, 6, 7, 7), it is possible to prevent the actual number of gradations from being reduced to 6 levels.

In each of the above embodiments, the case where the input image having the gradation level of 8 levels is converted into the output image of the 29 levels and outputted is described. The number of gradation levels to be outputted depends on the density conversion characteristic and the like. It can be selected arbitrarily.

[0017]

As described above, according to the present invention, the number of gradation levels of an output image is made larger than the number of gradation levels of an input image when the gradation conversion of an input image is performed and the converted image is output. Therefore, gradation conversion can be performed without reducing the number of gradations, and an image with gradation can be obtained.

Also, when converting the density of the input image data to increase or decrease the density, the number of density levels of the output image is set to be larger than the number of density levels of the input image. The density can be converted without reducing the number of tones, and a high-quality image can be output.

Furthermore, when the input image data is subjected to dither processing and output, the density of the dither image to be output is set to a number of gradation levels higher than the number of gradation levels of the input image. Dither processing can be performed without reducing the number of gradations, and an image with gradation can be obtained.

[Brief description of drawings]

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

FIG. 2 is a conversion characteristic diagram of input density and output density in the above embodiment.

FIG. 3 is a block diagram showing a configuration of a second embodiment of the present invention.

FIG. 4 is a density conversion characteristic diagram of input density and output density.

FIG. 5 is a block diagram showing a configuration of a third embodiment of the present invention.

FIG. 6 is a density conversion characteristic diagram of a dither image.

FIG. 7 is a conversion characteristic diagram of input density and output density in a conventional example.

[Description of Reference Signs] 1 image input unit 2 conversion characteristic storage unit 3 gradation conversion unit 4 image output unit 5 density conversion unit 7 dither processing unit

Claims (3)

[Claims] 1. An image processing apparatus for inputting a multi-valued digital image, converting the gradation thereof, and outputting the gradation image. The gradation conversion of the inputted image is carried out at a gradation level number larger than the gradation level number of the inputted image. An image processing apparatus, which outputs the image as an output. 2. An image processing apparatus for inputting a multi-valued digital image, converting the density of the multi-valued digital image, and outputting the multi-valued digital image by converting the density of the input image at a density level greater than the density level of the input image. An image processing device characterized by the above. 3. An image processing apparatus for inputting a multi-valued digital image, converting the density thereof and outputting the same, wherein the density of the dither image outputted by dithering the inputted image is larger than the number of gradation levels of the inputted image. An image processing apparatus which outputs at the number of gradation levels.