JPH06301773A - Method and device for color reduction picture processing - Google Patents

Abstract

PURPOSE:To provide a color reduction picture processing technique capable of automatically performing conversion maintaining high picture quality to the source pictures of especially animation and illustration, etc., and also substan tially improving the efficiency of a picture conversion processing. CONSTITUTION:This color reduction picture processor for selecting plural palette colors optimum for expressing the source pictures, selecting colors to be converted for respective picture elements from the palette colors, performing color reduction picture conversion and preparing color reduction picture data is constituted of the extraction part 1 of outlines, the identification part 2 of a non-gradation part and a gradation part, the selection part 3 of the palette colors and a conversion part 4 for preparing the collar reduction picture data. Then, an input part 5 for fetching the souce pictures is connected to the input side and further, an output part 6 for outputting color reduction pictures is connected to an output side. Then, a contour extraction processing, a solid part and solid color extraction processing, a palette color selection processing and a color reduction picture conversion processing are successively performed to the source pictures of the animation and the illustration, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color-reduced image processing technique, and particularly in an image to be processed (hereinafter referred to as an original image) in which characters and the like are solidly drawn by non-gradation such as animation and illustration. The present invention relates to a technique effectively applied to a color-reduced image processing method and apparatus capable of clearly expressing a solidly drawn portion in this original image.

[0002]

2. Description of the Related Art Generally, a color-reduced image processing technique, like a color map display widely used for a display device such as a personal computer, can display only about 8 to 256 colors at a time because of a memory capacity limitation. Due to the limitation, each pixel has 8 bits of RGB (2 ⁸ × 2 ⁸
It is used as one of the methods for converting so that a high-gradation image can be displayed due to a density change of about (× 2 ⁸ = about 16 million colors).

For example, in a color map display, by adopting a look-up table method against the restriction of the memory capacity, a high degree of freedom is given to the selection of the type of color used for display, and it can be used in a color scanner or video digitizing. The captured image of about 16 million colors is subjected to color reduction processing to be displayed in 16 colors and is used in personal computers, game machines and the like.

This color reduction processing is basically divided into two steps of "palette color selection" and "color reduction image conversion". Pellet color selection is a representative of several color reduction colors suitable for expressing the original image. This is a process for selecting colors. For this palette color selection algorithm, for example, a number of methods such as a frequency method, a color space linear division method, and a median division method have been devised. It is selected as the basis.

Further, the color-reduced image conversion after selecting the palette color is a process of selecting a color to be converted for each pixel corresponding to one pixel from the palette color and creating the color-reduced image data. For example, for each pixel. There are a method of searching the nearest color from the palette color and using that color as the color after conversion, a method of expressing a gradation of more than the subtractive color number such as the average error minimum method and the systematic dither method.

[0006] Note that techniques related to this are described in, for example, references such as "Image Analysis Handbook" P475-P582, Function Edition "Conversion", etc., published by The University of Tokyo Press, January 17, 1991. Has been done.

[0007]

However, in the prior art as described above, all of the palette color selection algorithms are selected based on the statistics of the color distribution of the original image. No consideration has been given to which specific color should be identified and which color of which part should be emphasized, and an appropriate color may not be selected as the palette color depending on the original image.

That is, for natural images such as photographs, the policy of emphasizing hues with a large distribution is certainly an effective and easy method to implement, and when this method can select almost good palette colors. However, there is a big problem when processing a solid original image such as an illustration or animation.

For example, considering the case where the original image shown in FIG. 6 is subjected to color reduction processing, this image is actually about 1600 in which one cut of animation is captured by video digitizing.
It is an image of all colors, and the background of this image is drawn with gradation due to changes in density, and the person is drawn as a solid cel image with no gradation.

In this case, the color distribution in this original image is actually a three-dimensional RGB histogram,
In order to make the problem easy to understand, a one-dimensional histogram as shown in FIG. 9 is used, and the number of color reduction colors after conversion in the color reduction image processing is assumed to be three.

The first problem is that the palette color selection algorithm shown in FIG.
Fig. 7 shows the result of color-reduced image conversion using these palette colors by selecting the colors A, G, and I that have a high frequency according to the histogram in Fig. 7. It is very dirty because it is used and expressed.

The degree of this stain varies depending on the method of color-reduced image conversion, but the main cause is that C color is not adopted as the palette color in selecting the palette color, that is, C color is the statistics of color distribution. It should not have been adopted from, but should have a very high priority in the sense of the image.

In particular, in color reduction processing such as animation, it is more important in image processing to express a solid portion in front of a character or the like rather than to enrich the gradation expression of the background. is there.

As the second problem, for example, assuming that the colors A, C, and G are selected as the palette colors by solving the first problem, the subtractive color image conversion is performed using the palette colors. The result is shown in FIG. 8, which is much better than that in FIG. 7, but it is annoying that dots of different colors are scattered on the face and hair.

The cause of this is that even in a portion represented by a solid image in the original image before being captured, color irregularity originally present therein and noise at the time of capturing are included, and that portion of the original image captured as a discrete value. Is not limited to a constant value, but may have a predetermined width.

Therefore, the conventional techniques have the above-mentioned problems, and in order to obtain a high quality image conversion processing result, a work such as manually adjusting the palette color or correcting the converted image is performed. Especially when performing a large amount of image conversion such as animation, this correction work not only requires a huge number of man-hours but also depends on unstable factors such as the sensitivity and skill of the operator. Then, there arises a problem that it becomes difficult to maintain consistent quality for all images.

Therefore, an object of the present invention is to make it possible to automatically perform a conversion while maintaining a high image quality, especially for an original image such as an animation or an illustration, and to greatly improve the efficiency when a large amount of image conversion is performed. An object of the present invention is to provide a color-reduced image processing method and apparatus that can be improved.

It should be noted that the present invention is not a substitute for the conventional technique, but applies the conventional technique to add auxiliary processing, and can be added to any of the many conventional techniques. is there.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0020]

Among the inventions disclosed in the present application, a brief description will be given to the outline of typical ones.
It is as follows.

That is, the subtractive color image processing method and apparatus of the present invention selects a palette color of a plurality of colors most suitable for expressing the original image, and selects a color to be converted for each pixel from the palette colors. A subtractive color image processing device for creating reduced color image data by subtractive color image conversion, wherein an extraction unit for extracting a contour line of an original image and a non-gradation part and a gradation part are automatically provided for each part divided by the contour line. Identification section, a selection section that selects a palette color by emphasizing the color of the non-gradation part, and a reduced color image data is created using the selected palette color, and at the same time the non-gradation of the original image The portion is provided with a conversion unit that performs subtractive color image conversion so that the processing result is expressed in a single color.

[0022]

According to the color-reduced image processing method and apparatus described above, in the color-reduced image processing of the original image, the contour line extraction unit,
A non-gradation part and a gradation part are identified, a palette color selection part, and a conversion part for creating reduced color image data are provided, so that the non-gradation part and the gradation part are automatically detected in the original image. When the palette color is selected with emphasis on the color of the non-gradation part and the reduced color image data is created using this selected palette color, the processing result of the non-gradation part becomes a single color. Subtractive image conversion can be performed as represented.

As a result, the original image can be automatically converted while maintaining high image quality, and no manual correction work is required. Therefore, the efficiency of a large amount of image conversion is greatly improved. Can be made. In particular, with respect to an original image in which a character or the like is drawn in a non-tone solid image such as an animation or an illustration, the solid image portion in this original image can be clearly expressed.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic block diagram showing a color-reduced image processing apparatus according to an embodiment of the present invention, and FIG. 2 is a flow chart showing a color-reduced image processing procedure in the color-reduced image processing apparatus according to this embodiment.
3 to 5 are explanatory views showing images at respective processing stages in the subtractive color image processing apparatus of this embodiment.

First, the structure of the color-reduced image processing apparatus of this embodiment will be described with reference to FIG.

The subtractive color image processing apparatus of the present embodiment selects, for example, a plurality of palette colors that are optimal for expressing the original image, selects a color to be converted for each pixel from the palette colors, and performs the subtractive color image. A subtractive color image processing device that performs conversion and creates reduced color image data for an original image such as an animation or an illustration. A contour line extraction unit 1, a non-graded portion / graded portion discrimination unit 2, and a palette color. Selection unit 3 and a conversion unit 4 for creating reduced color image data, an input unit 5 for inputting an original image is connected to the input side, and an output unit 6 for outputting the reduced color image is connected to the output side. .

The extraction unit 1 is a unit for extracting the contour line of the original image, and the contour information is output to the identification unit 2 by the input of the high-gradation original image information of about 8 bits for each RGB from the input unit 5. The extraction unit 1 performs contour line extraction processing on the original image to divide the original image into a plurality of parts.

The discriminating section 2 is a section for automatically discriminating a solid portion and a gradation portion by non-gradation for each of the portions divided by the contour line, and the original image information from the input section 5 and the extracting section 1 Solid information and solid color information are output to the selection unit 3 by inputting contour information from the, and the identification unit 2 performs solid portion and solid color extraction processing to determine whether each individual portion is solid. To be judged.

The selection section 3 is a section for selecting a palette color by emphasizing the color of the solid portion due to non-gradation, and by inputting the original image information from the input section 5 and the solid color information from the identification section 2, the palette is selected. The information is output to the conversion unit 4 and the selection unit 3
In, the palette color with an emphasis on solid color is selected.

The conversion unit 4 creates reduced color image data using the selected palette color, and at the same time, reduces the reduced color image so that the processing result is expressed in a single color for the solid portion due to the non-gradation of the original image. The original image information from the input unit 5, the solid portion information from the identification unit 2, the solid color information, and the palette information from the selection unit 3 are input, and the subtractive color image information is output to the output unit 6. In the conversion unit 4, a normal color-reduced image conversion is performed on a portion other than the solid portion, while the solid portion is filled with a solid color to obtain a beautiful reduced-color image in which the solid portion is emphasized.

The color-reduced image processing apparatus configured as described above uses, for example, an existing personal computer and the like, and executes a predetermined color-reduced image processing algorithm from extraction of an outline of an original image to generation of color-reduced image data. As a result, the original image input through the input unit 5 is converted through the subtractive color image processing device, and the converted subtractive color image is output and displayed at the output unit 6.

In this case, a color scanner for capturing images such as animations and illustrations, a laser disc player for capturing images from a laser disc, or the like is used as the input unit 5, while the output unit 6 is simply A television monitor that displays a color-reduced image, a color video printer that prints out, and a display such as a game machine are used.

Next, the operation of this embodiment will be described based on the color-reduced image processing flow of FIG.

In this case, as the original image to be subjected to the color reduction processing, the original image shown in FIG. 6 described in the section of the problem to be solved by the invention in the prior art is used, and the background of this original image is the density change. Is drawn with gradation, and the person is drawn as a solid cel image with non-gradation.

Further, the number of reduced colors in the reduced color image processing is usually set to the number of colors matched to the device specifications of the output section 6, but in the present embodiment, the invention is not required in the prior art in order to simplify the explanation. Similar to the case described in the section of the problem to be solved, the colors are reduced to three colors.

First, the original image is taken in through the input unit 5 and this original image information is stored in a file. Then, the extraction unit 1 reads the original image information from the file,
Contour extraction processing is performed on this original image information, and the processing result becomes an image as shown in FIG. Then, the original image is divided into a plurality of parts based on the contour line, and the obtained contour information is stored in a file (step 201).

Further, the identifying section 2 reads out the original image information and the contour information from the respective files, and performs a solid portion and solid color extraction processing on the original image information and the contour information. Then, it is judged whether each part divided by the contour line is a solid part or a part drawn with gradation, and the result is stored in a file as solid part information and solid color information. (Step 202).

In this case, for each part, the total number of pixels contained therein, the color value of each image in the part, that is, 8 bits of RGB for each pixel (R: 0 to 255,
G: 0 to 255, B: 0 to 255), the average value and the dispersion value of the gradation values are obtained. For example, the smaller the color value, the darker the color value, and the larger the color value, the brighter the color.

Then, a portion whose area by the total number of pixels is equal to or larger than a fixed value and whose variance value is equal to or smaller than the fixed value is recognized as a solid portion. In this case, even if it is recognized as a gradation portion other than solid, if the difference between the average values of the respective colors is less than a certain value, it is recognized as the solid portion of the same color.
Then, the average value of the colors of the part recognized as the solid part is set as the solid color.

Here, the minimum number of pixels used for solid recognition, the threshold value of the variance value, and the value of the color difference recognized as a solid portion of the same color are the processing environment such as the accuracy of capturing the original image and the original image. Since it is affected by the sharpness, etc., it is determined by adjusting so that recognition can be performed according to the purpose.

The result of this solid portion and solid color extraction processing becomes an image as shown in FIG. 4, and the hair portion and the face portion are recognized as solid, and at the same time, the average values A and C of the colors of those portions are recognized. Is recognized as a solid color. In addition, the background, eyes, and mouth are recognized as non-solid gradation parts depending on the variance value and area conditions.

Subsequently, the selection unit 3 reads the original image information and the solid color information from the respective files, and, for example, by using the median division method, the palette in which the solid color is emphasized with respect to the original image information and the solid color information. Select a color. Then, three colors including the solid color and other colors other than the solid color are selected, and the selected three colors are stored in the file as palette information (step 203).

The outline of this median division method will be described. Considering a rectangular parallelepiped circumscribing the color distribution of the original image in the RGB space, the total number of pixels of the original image is defined by the axes parallel to the longest side of the rectangular parallelepiped among the three RGB axes. This is a method in which a rectangular parallelepiped is subdivided by repeatedly dividing the rectangular parallelepipeds, and finally a rectangular parallelepiped having the same number of pixels with a reduced color number is generated, and the average value of the color distribution in each rectangular parallelepiped is used as a palette color.

In this case, the palette color can be selected as follows by applying the conventional palette color selection method.

First, the statistical processing of the color distribution of the original image is performed as in the usual palette color selection method. Furthermore, the following corrections are made to the statistics of the color distribution. First, the frequency of the solid color is set to 0, and the frequencies around the appropriate range of the solid color are also reduced in the order closer to the solid color. The purpose of this correction is to prevent a color overlapping with a solid color from being selected in the next palette color selection.

Then, from the corrected color distribution, the number of pellet colors is selected by the usual palette color selection method by subtracting the number of solid colors extracted from the number of reduced colors. In this way, three colors including the selected color and the extracted solid color are selected as the palette color.

For example, if the histogram shown in FIG. 9 described in the section of the problem to be solved by the invention in the prior art is used as the histogram of the color distribution in the original image, the color A of the hair and the color C of the face are used. A color is selected as a solid color, and when the remaining one color is selected from the histogram obtained by subtracting the distributions of A color and C color, G color is adopted. Therefore, the first problem of the related art is solved, and the palette color is A
It is determined to be three colors of color, C color, and G color.

Further, in the conversion unit 4, the original image information,
Solid part information, solid color information, and palette information are read out from the respective files, and a subtractive color image conversion process is performed on these information using, for example, the average error minimum method,
The result is stored in a file as reduced color image information (step 204).

An outline of the average value error minimum method will be described. In order to reduce the error between the density of the original image and the density of the output image as an average, the threshold value of the pixel of interest is set to a peripheral value. This is a method of performing a subtractive color image conversion by determining with a weighted average of pixel errors.

In this case, in order to solve the second problem of the related art, different processing is performed for the portion recognized as the solid portion and the non-solid portion having other gradations. In other words, non-solid areas other than solid areas are subjected to normal color-reduction image conversion, and solid areas corresponding to the solid areas are present in the palette colors. Therefore, each area is filled with one corresponding solid color to reduce the color. Let it be an image.

Finally, the color-reduced image information is read from the file, and the color-reduced image that is the final result of the color-reduced image processing is output through the output unit. As a result, as shown in FIG. 5, a solid portion in the original image can be displayed neatly, and in particular, the face of a person does not have a rough surface, which allows the person to look at the image in the foreground. It is possible to clearly display a person drawn in solid colors.

Therefore, according to the subtractive color image processing apparatus of the present embodiment, the conventional palette color selecting process by the selecting unit 3 and the conventional subtractive color image converting process by the converting unit 4 are applied, and the original image by the additional extracting unit 1 is applied. The outline extraction processing of the above and the identification processing of the non-gradation solid portion and the gradation portion by the identification unit 2 are added, so that the non-gradation portion of the person who is drawn solid is clearly displayed in a single color, and Since the gradation part of the background can be displayed with a change in density, it is possible to perform conversion while maintaining high image quality with respect to the original image.

Moreover, since these processes are automatically performed, the conventional manual correction work becomes unnecessary,
Even when a large amount of image conversion is performed, the work efficiency can be significantly improved.

Although the present invention made by the present inventor has been specifically described based on the embodiments, the present invention is not limited to the embodiments and various modifications can be made without departing from the scope of the invention. Needless to say.

For example, with respect to the subtractive color image processing apparatus of the present embodiment, the case where the palette color selecting process by the median value dividing method and the subtractive color image converting process by the average value error minimum method are further explained. The present invention is not limited to the above-mentioned embodiment, and a frequency method, a color space linear division method, or the like is used as the palette color selection processing, and another method such as the systematic dither method is used as the color-reduced image conversion processing. Is also widely applicable.

Although a case has been described in which three subtractive color numbers are selected as the palette colors, it is also possible to select four or more color numbers, and normally a subtractive color image processing apparatus is also possible. And the number of colors is selected according to the device specifications of the output unit connected to it.

Further, as the processing procedure of the color-reduced image conversion, the same is applied to the case where the normal color-reduced image conversion is performed on the entire area and then the mask processing for making the solid area one solid color without distinguishing the solid area. The effect of can be obtained.

In the above description, the invention mainly made by the present inventor is applied to a subtractive color image processing apparatus used for original images such as animations and illustrations, which is the field of use thereof, but the invention is not limited to this. The present invention is also widely applicable to other images such as landscape photographs instead of objects.

[0059]

The effects obtained by the typical ones of the inventions disclosed in the present application will be briefly described as follows.
It is as follows.

(1). In the original image, a portion represented by non-gradation and a portion drawn with gradation are automatically identified, and the color of the identified non-gradation portion is determined. The palette color is selected with emphasis, and the reduced color image data is created by using the selected palette color. At the same time, the non-gradation part of the original image is converted into a reduced color image so that the processing result is expressed as a single color. Therefore, it is possible to perform conversion while maintaining high image quality with respect to the original image.

(2) Since the above-mentioned (1) enables automatic processing without human intervention, it is not necessary to manually adjust the palette color and correct the image after the color-reduced image conversion as in the prior art. You can

(3) By the above (1), the color-reduced image processing can be automatically performed, so that the conversion processing effect can be greatly improved, and in particular, huge images such as animations and illustrations can be processed. In this case, the work man-hour can be significantly reduced.

(4) By the above (1), since the adjustment work of the palette color in the conventional subtractive color image processing and the image correction work after the conversion are unnecessary, it is possible to perform consistent operation without depending on the skill of the operator. It is possible to generate a processed image of the desired quality.

(5) Due to the above (1) to (4), especially with respect to original images such as animations and illustrations, it is possible to clearly express a person drawn with a solid color that is most noticeable when a person views the image. However, it is possible to obtain a subtractive color image processing technique capable of automating conversion while maintaining high image quality and improving efficiency of image conversion processing.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram illustrating a subtractive color image processing apparatus that is an embodiment of the present invention.

FIG. 2 is a flowchart showing a reduced color image processing procedure in the reduced color image processing apparatus of the present embodiment.

FIG. 3 is an explanatory diagram showing an image at each processing stage in the subtractive color image processing apparatus of this embodiment.

FIG. 4 is an explanatory diagram showing an image at each processing stage in the subtractive color image processing device of the present embodiment.

FIG. 5 is an explanatory diagram showing an image at each processing stage in the subtractive color image processing apparatus of this embodiment.

FIG. 6 is an explanatory diagram showing an original image in a subtractive color image processing apparatus which is an example of a conventional technique.

FIG. 7 is an explanatory diagram showing an image at each processing stage in a color-reduction image processing apparatus that is an example of a conventional technique.

FIG. 8 is an explanatory diagram showing an image at each processing stage in the subtractive color image processing apparatus which is an example of the conventional technique.

FIG. 9 is a characteristic diagram showing a histogram of color distribution in an original image in a subtractive color image processing apparatus which is an example of a conventional technique.

[Explanation of symbols]

 1 Extraction Section 2 Identification Section 3 Selection Section 4 Conversion Section 5 Input Section 6 Output Section

Claims (6)

[Claims] 1. A multicolor palette color optimum for expressing an image to be processed is selected, a color to be converted for each pixel is selected from the palette colors, and subtractive color image data is created by subtractive color image conversion. A subtractive color image processing method, wherein a portion represented by non-gradation and a portion drawn with gradation are automatically identified in the image to be processed, and the identified non-gradation portion The palette color is selected with an emphasis on the color, and the reduced color image data is created using the selected palette color, and at the same time, the processing result is expressed as a single color for the non-gradation portion of the processing target image. A subtractive color image processing method characterized by performing a subtractive color image conversion as described above. 2. A process of extracting a contour line from the processing target image when distinguishing between a portion expressed in non-gradation and a portion drawn in gradation in the processing target image. The subtractive color image processing method according to claim 1, wherein the non-gradation portion and the gradation portion are distinguished for each portion divided by the contour line. 3. When distinguishing a non-gradation part and a gradation part for each part divided by the contour line, the area within the division by the contour line is larger than a predetermined value, and the division by the contour line is performed. When the variance value of the color inside is smaller than a predetermined value, it is discriminated as a non-gradation portion, and the color of the non-gradation portion is set as the color of the average value of the colors in the section by the contour line. Two
The reduced color image processing method described. 4. When pallet color selection is performed by placing importance on the color of the identified non-gradation portion, a statistic of the color distribution of the processing target image is calculated, and the color of the non-gradation portion and the color of the non-gradation portion are calculated. 2. The color-reduced image processing according to claim 1, wherein a predetermined amount is subtracted from a statistic of a color distribution corresponding to a color adjacent to the color in the order of neighborhood, and the palette color is selected based on the statistic after the subtraction process. Method. 5. The color-reduced image processing method according to claim 1, wherein the image to be processed is an image including a non-gradated portion such as an animation or an illustration. 6. A reduced color image data is created by selecting a palette color of a plurality of colors most suitable for expressing an image to be processed, selecting a color to be converted for each pixel from the palette colors, and performing a reduced color image conversion. A subtractive color image processing device, comprising: an extraction unit that extracts a contour line of the image to be processed; and an identification unit that automatically distinguishes a non-gradation portion and a gradation portion for each portion divided by the contour line. , A selecting unit for selecting a palette color with an emphasis on the color of the non-gradation portion, and creating reduced color image data using the selected palette color, and at the same time, regarding the non-gradation portion of the processing target image, A subtractive color image processing apparatus, comprising: a conversion unit that performs a subtractive color image conversion so that a processing result is expressed in a single color.