JP2003288585A - Apparatus and method for color reduction processing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for color reduction processing for preventing important colors included in an image as an object of color reduction processing from being changed. <P>SOLUTION: A color reduction processing part 42 includes an important color information storage part 51 which stores information on a specified important color, a color palette generation part 52 which generates a color palette consisting of the specified number of representative colors preferentially including the specified important color on the basis of color information of the object of color reduction processing and the information stored in the important color information storage part 51, and a color substitution processing part 53 which substitutes color included in the color palette for color of each pixel of the image as the object of color reduction processing. For the color reduction processing, the important color of the image as the object of color reduction processing is specified and then the important color of the object image is prevented from easily being deleted from the color palette based upon appearance frequencies to perform the color reduction processing wherein deterioration in image quality is suppressed as much as possible. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color-reduction processing apparatus for converting a color of each pixel of an image to be subjected to color-reduction processing into a color included in a predetermined color palette so as to convert an image having a smaller number of colors than the original image. The present invention relates to a color reduction processing method.

[0002]

2. Description of the Related Art In recent years, an image of digital data taken by a digital camera or the like is transferred to an image distribution server using a mobile phone or a personal computer, or an image prepared by an image distributor for various image data for distribution. An image distribution system that operates a distribution server and distributes the images accumulated in these image distribution servers to mobile terminals such as mobile phones and personal digital assistants (PDAs) has been put into practical use.

In this type of image distribution system, there are strong restrictions on the memory capacity and display capacity of the mobile terminal, and since the memory capacity and display capacity differ depending on the type of mobile terminal, the image distribution server distributes images to the mobile terminal. When doing, depending on the memory capacity and display capacity of the distribution destination mobile terminal,
A process of reducing the data amount by reducing the number of images and the number of colors of the image is performed. For example, if the number of pixels is 130 × 130 pixels,
After performing image reduction processing (thinning-out processing) for reducing the number of pixels to a predetermined number such as 90 × 90 pixels, color reduction for further reducing the number of colors included in the image to a predetermined number of colors of 256 colors, 128 colors, 16 colors I am trying to process it.

Conventionally, as this type of color reduction processing method, a fixed limited color display method (fixed palette color reduction method) and an adaptive limited color display method (free palette color reduction method) are known.

The fixed limited color display method uses one color palette consisting of a predetermined number of representative colors that are uniformly selected from colors that can be reproduced by a display device, and the color of each pixel of the image to be subjected to the color reduction processing is This is a subtractive color processing method that replaces the closest color included in the color palette. With this method, it is not necessary to create a color palette for each image subject to color reduction processing.
Although the color reduction process can be performed at high speed, if the colors in the image are biased, such as in a face image or autumnal image, only some of the colors included in the color palette are used and the remaining There is a waste that the color of is not used at all. For example, even though a color palette composed of 256 representative colors is used, only 50 colors are used. In such a case, the color image representation after the color reduction processing has low quality.

The adaptive limited color display method creates a color palette consisting of a predetermined number of representative colors suitable for expressing the image to be subjected to the color-reduction processing, based on the color information of the image to be subjected to the color-reduction processing. This is a subtractive color processing method for replacing the color of each pixel of an image with the closest color included in the color palette. This method requires a color palette to be created for each image to be subjected to color reduction processing, so the processing speed is slower than the fixed limited color display method, but a color palette suitable for each image to be subjected to color reduction processing is used. High-quality color image expression is possible.

[0007]

In the adaptive limited color display method described above, the frequency of appearance of each color in the image to be subjected to the color-reduction processing is determined by statistical processing using a histogram or the like,
A color palette is created by giving priority to a color having a higher appearance frequency for each image to be subjected to color reduction processing and selecting a predetermined number of representative colors.

For this reason, for example, in the case of an image in which a person is small in the background of autumn leaves or green mountains,
Even though the person is the main subject, the frequency of appearance of the colors included in the background is much higher than that of the colors included in the person. Therefore, the colors included in the background are included more than the colors included in the person. A color palette will be created. When the color reduction process is performed using this color palette, the degree of image quality deterioration is small because the image of the background part is smoothly expressed with a large number of colors, but the image of the important person part has the background Since the image is expressed by a small number of colors with a tint, the degree of deterioration of the image quality becomes large.
In particular, when a large color reduction process is performed, an important color such as a person's skin color may be deleted from the color palette, and the color may significantly change from the original image. For example, in the case of an image in which a person is small in the background of autumnal leaves, the color of the person becomes a reddish color as a result of the color reduction processing.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to prevent important colors in an image to be subjected to color reduction processing from being deleted from the color palette when performing color reduction processing, and to improve image quality. An object of the present invention is to provide a color reduction processing apparatus and a color reduction processing method capable of performing color reduction processing while suppressing deterioration as much as possible.

[0010]

A color reduction processing apparatus of the present invention comprises a color palette creating means for creating a color palette having a predetermined number of representative colors, and the color of each pixel of an image subjected to color reduction processing using the color palette. A color replacement processing means for replacing any of the colors included in the color palette with any one of the colors included in the color palette, and an important color information storage means for storing information of an important color designated as additional information of the image, and the color palette creating means, A color palette that preferentially includes the important color is created based on the color information of the image to be subjected to the color reduction processing and the important color information stored in the important color information storage means.

Further, in the color-reduction processing apparatus of the present invention, the color palette creating means further uses, as the additional information, weighting information in which a weighting coefficient is set for each area in the image to be subjected to the color-reduction processing, and determines the important color. It is characterized by creating a color palette that is preferentially included.

The color reduction processing method of the present invention is based on an important color information storage step of storing information of important colors designated as additional information of an image, color information of an image to be subjected to color reduction processing, and the stored important color information. And a color palette creating step of creating a color palette that preferentially includes the important color as a color palette having a predetermined number of representative colors, and using the color palette to set the color of each pixel of the image to be subjected to color reduction processing to the color palette. And a color replacement processing step for replacing with any of the colors included in.

In the color-reduction processing apparatus and the color-reduction processing method of the present invention configured as described above, the important color is preferentially included based on the color information of the image to be subjected to the color-reduction processing and the information of the designated important color. A color palette including a predetermined number of representative colors is created, and color replacement processing is performed to replace the color of each pixel of the image to be subjected to the color reduction processing with the color included in the created color palette. Therefore, when performing the color reduction processing, by designating the important color of the image to be subjected to the color reduction processing, it is possible to easily delete the important color of the image to be subjected to the color reduction processing from the color palette based on the appearance frequency. It is possible to prevent the important color from largely changing due to the color reduction, and it is possible to perform the color reduction processing in which the deterioration of the image quality is suppressed as much as possible.

Further, in the color-reduction processing apparatus and the color-reduction processing method of the present invention, for example, the image to be subjected to the color-reduction processing is divided into a plurality of areas, and the weight of the central portion of the image is increased. The weighting coefficient is set to preferentially include colors often included in areas with large weights.
A color palette composed of a predetermined number of representative colors may be created.

By doing so, a predetermined number of color palettes containing the colors included in the areas of higher importance during the color reduction processing are created preferentially, so that the color palettes included in the areas of high importance of the image to be subjected to the color reduction processing are created. Prevent the colors that are displayed from being easily deleted from the color palette based on their appearance frequency,
It is possible to preferentially leave important colors so that they do not change due to color reduction, and it is possible to perform color reduction processing that suppresses deterioration of image quality as much as possible.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration example of an image distribution system using a color reduction processing device according to an embodiment of the present invention.

The image distribution system 1 of this embodiment is
Personal computer (Distributor terminal device, hereinafter PC
10), a mobile phone (destination terminal device) 20 as a mobile terminal, and an image distribution server (image conversion / distribution device) 30 are interconnected via a communication network such as the Internet. The image of digital data such as the image taken in step 3 is transferred to the image delivery server 30 using the PC 10, and the image stored in the image delivery server 30 is delivered to the mobile phone 20.

In the PC 10, the image distribution server 2 together with the information of the important color designating the image to be distributed to the mobile phone 30 is displayed.
An application program (hereinafter referred to as an image distribution application) for transferring to 0 is installed. A user of the PC 10, such as an image distributor, uses the image distribution application to transfer the image from the digital camera 2 to the P
An image desired to be distributed is selected from a large number of images such as an image captured in C10 or an image created using image processing software, and an important color is designated for each selected image. At that time, 1 is set as the important color in each image for distribution.
You can specify a color or multiple colors. PC10
When an image to be distributed using the image distribution application is selected and an important color is specified and then a transfer instruction is issued, the selected image and information on the specified important color are sent to the image distribution server 20. Forward. The information of the designated important color is transferred to the image distribution server 20 as additional information of the image to be distributed.

The mobile phone 20 has a communication unit 21 for communicating with other mobile phones and servers on the Internet via a base station (not shown), a display unit 22 for displaying characters and images, a dial button, etc. An operating unit 23 including various operators, and a terminal information storage unit 24 that stores information such as the communication speed of the mobile phone 20 and the display capability (the number of pixels and the number of expressible colors) of the display unit 22 are configured. Has been done. When the mobile phone 20 communicates with another mobile phone or a server on the Internet, the mobile phone 20 reads the terminal information from the terminal information storage unit 24 and notifies the communication destination.

The image distribution server 30 stores an image (original image) received from the PC 10 as a pre-distribution image storage unit 31.
And a terminal information storage unit 32 that stores the terminal information received from the mobile phone 30 and an image read from the pre-delivery image storage unit 31 based on the terminal information read from the terminal information storage unit 32. And a distribution image generation unit 33 that performs a color reduction process and then a compression process to generate a distribution image, and a transfer unit 34 that transfers the distribution image generated by the distribution image generation unit 33 to the mobile phone 20. Is equipped with.

As shown in FIG. 2, the distribution image generation unit 33 includes a pixel number reduction processing unit 41 for reducing the number of images, a color reduction processing unit 42 for performing color reduction, and a compression processing unit for performing reversible or irreversible data compression. 43 and a data capacity determination unit 44 that determines whether or not the data capacity of the output image is less than or equal to a predetermined value.

The pixel number reduction processing unit 41 sets the number of pixels of the image (original image) read from the pre-delivery image storage unit 31 to 130.
Image reduction processing (decimation processing) is performed to reduce the number of pixels to a predetermined number, such as × 130 pixels or 90 × 90 pixels. that time,
The reduced number of pixels (target value) is determined according to the number of pixels included in the terminal information read from the terminal information storage unit 33.

The color reduction processing section 42 is a pixel number reduction processing section 41.
By replacing the color of each pixel of the image (image to be subjected to color reduction processing) after processing in step 1 with the closest color among the colors included in the color palette created for each image, the number of colors included in the image is 256 colors. , 128 colors, 16 colors, and the like, to reduce the number of colors to a predetermined number.

The compression processing section 43 performs compression processing on the image (reduced color image) processed by the color reduction processing section 42 using a known compression method such as GIF or PNG. The data capacity determination unit 44 determines whether or not the data capacity of the image after data compression is equal to or less than the data capacity that can be processed in the communication path to the delivery destination or the portable terminal of the delivery destination and can be transmitted and displayed. The image after being compressed here is transferred to the mobile phone 20 through the transfer unit 34 after the data capacity is determined by the data capacity determination unit 44.

Since the amount of data that can be transferred varies depending on the type of mobile phone and the communication carrier company, there is a possibility that the data cannot be transferred or the reception is rejected due to the excess of the data capacity. When the data capacity determination unit 44 determines that the data capacity is exceeded, the image data is again processed by the color reduction processing unit 42.
In (1), further color reduction processing is performed, and the compression processing unit 43 performs recompression processing.

As shown in FIG. 3, the color reduction processing section 42 includes an important color information storage section (important color information storage means) 51 for storing important color information and a color palette creating section (color A palette creating unit) 52 and a color replacement processing unit (color replacement processing unit) 53 that replaces the color of each pixel in the original image with a color palette.

The important color information storage section 51 stores important color information sent from the PC 10 as additional information of the image distributed. The color palette creation unit 52 is designated by the user of the PC 10 based on the important color information read from the important color information storage unit 51 and the color information of the image to be subjected to the color reduction processing given from the pixel number reduction processing unit 41. Create a color palette that preferentially includes important colors.

The color replacement processing section 53 includes a pixel number reduction processing section 4
By replacing the color of each pixel of the image to be subjected to the color reduction processing given from No. 1 with the closest color included in the color palette created by the color palette creating unit 52, the image to be subjected to the color reduction processing is included in the color palette. Convert to a reduced color image consisting of. This color-reduced image is compressed by the compression processing unit 43 and then transferred to the mobile phone 20.

FIG. 4 shows an operation procedure of the color palette creating section 52. The color palette creation unit 52 first
The color of each pixel forming the color-reduction processing target image supplied from the pixel number reduction processing unit 41 is checked, and the appearance frequency of each color in the color-reduction processing target image is obtained by statistical processing using a histogram or the like (step S1). . Next, the important color information is read from the important color information storage unit 51 (step S2). Then, after the important color is selected as the representative color of the color palette (step S3), it is confirmed whether the representative color of the color palette is sufficient with only the important color (step S3).
4) If not enough (Yes in step S4), a color having a higher appearance frequency is preferentially selected as the remaining representative color (step S5). In this way, a color palette (free palette) is created for each image to be subjected to the color-reduction processing, which is composed of representative colors that always include the important color specified by the distribution source and preferentially include the color with high appearance frequency (step S6). ).

As described above, the color palette creating section 52
In order to create a color palette consisting of a predetermined number of representative colors that include important colors preferentially, based on the color information of the image to be subjected to color reduction processing and the information of the designated important colors, It is possible to prevent the important color of the image from being easily deleted from the color palette based on the appearance frequency.

The color palette created by the color palette creating section 52 is used, and the color replacement processing section 53 replaces the color of each pixel of the image to be subjected to the color reduction processing with the color included in the color palette. The color reduction processing can be performed without losing the color.

Therefore, if the image to be distributed is an image in which a person is small in the background of autumn leaves or green mountains, the user of the PC 10 uses the image distribution application to display the image. By specifying the colors included in the person of (such as skin color and clothes color) as important colors,
It is possible to prevent the color included in the person in the image from being easily deleted from the color palette, and prevent the person's color from being damaged as much as possible even after the color reduction processing. In the case of an image in which a person appears small against a background of autumn leaves, there is a problem that the color of the person becomes a reddish color with the conventional color reduction processing method as a result of the color reduction processing. According to the color-reduction processing method of the present invention described above, such a problem is solved.

The function of the function of the color reduction processing section 42 can be realized by installing a program for executing color reduction processing by the color reduction processing method according to the present invention into a general-purpose server computer and operating it. That is,
Based on the color information of the color-reduction processing target image and the information of the designated important color, a step of creating a color palette consisting of a predetermined number of representative colors preferentially including the important color, and each of the color-reduction processing target images By installing and executing a program including a step of replacing the color of the pixel with the color included in the color palette, the server computer having the function of the color reduction processing unit 42 can be realized.

As shown in the operation flow of FIG. 4, this program examines the color of each pixel forming the image to be subjected to the color-reduction processing, and performs statistical processing using a histogram or the like for each color in the image to be subjected to the color-reduction processing. There is a step of determining the appearance frequency and a step of judging whether or not the representative color of the color palette is sufficient for only the important color, and if it is not sufficient, preferentially selecting a color having a higher appearance frequency as the remaining representative color. Is desirable.

In the above embodiment, the important color information designated by the distribution source user is transferred from the PC 10 to the image distribution server 20 together with each image as additional information for each image to be distributed. Instead of using the additional information for each image, it may be transferred as a separate file linked to each image.

Further, the specified important color information may be transferred as information for each distribution source. In this case, the distribution source user can specify the color that he or she wants to retain as an important color regardless of the image and send the information to the image distribution server 20 once, so that the trouble of specifying the important color for each image can be saved. be able to.

Further, as shown in FIG. 5, the image to be subjected to the color-reduction processing is divided into a plurality of areas (5 × 5 areas in this example), and a weighting coefficient corresponding to the degree of importance is set for each area.
The color palette creating unit 52 may create a color palette that includes a predetermined number of representative colors that preferentially include colors that are often included in areas with large weighting factors. That is, weighting information is set together with important color information as additional information of an image, and a color palette in which important colors are prioritized is created based on these pieces of information.

By doing so, a predetermined number of color palettes are created that preferentially include colors included in areas of higher importance (in the example of FIG. 5, areas closer to the center of the image). It is possible to prevent color that is included in a highly important area of the processing target image from being easily deleted from the color palette based only on the frequency of appearance, and perform color reduction processing that suppresses deterioration of image quality as much as possible. .

When the color-reduced image that has been compressed by the compression processing unit 43 needs to be further subjected to color-reduction processing by the color-reduction processing unit 42, the color of each pixel of the color-reduced image is created by the color palette creation unit 52. When substituting the colors included in the color palette, it is desirable to replace the colors by weighting the colors that are closer to the designated important color, that is, the colors that are closer to the important color in the color space. By increasing the weight of a color closer to the important color, it is possible to prevent the important color from being easily deleted by the statistical processing using a histogram or the like.

Further, in the above embodiment, an example of the color reduction processing using the color palette by the free palette color reduction method has been described. However, when the color reduction processing is further performed after the color reduction as described above, the fixed palette is used. Also in the color reduction processing using the color palette by the color reduction method, when the number of colors in the color palette is reduced, the same operation and effect can be obtained by applying the configuration and the procedure of the present embodiment described above.

As described above, according to the color-reduction processing apparatus and the color-reduction processing method of this embodiment, a color palette consisting of a predetermined number of representative colors is created based on the color information of the image to be subjected to the color-reduction processing, and the color-reduction processing is performed. When performing the color-reduction processing to convert the color of each pixel of the target image into the image with fewer colors than the original image by replacing the color in the color palette with the color included in the color palette, By specifying it, it is possible to prevent the important color of the image to be subjected to color reduction processing from being easily deleted from the color palette based on the appearance frequency, and perform color reduction processing with the deterioration of image quality suppressed as much as possible. be able to.

[0042]

As described above, according to the present invention, it is possible to prevent the important color in the image to be subjected to the color-reduction processing from being deleted from the color palette when performing the color-reduction processing, and to suppress the deterioration of the image quality as much as possible. It is possible to perform color reduction processing.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration example of an image distribution system using a color reduction processing apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration example of a distribution image generation unit in the present embodiment.

FIG. 3 is a block diagram showing a configuration example of a color reduction processing unit in the present embodiment.

FIG. 4 is a flowchart showing an operation procedure of a color palette creating unit in the present embodiment.

FIG. 5 is a diagram showing an example of weighting factor setting for each area of an image to be subjected to color reduction processing in the present embodiment.

[Explanation of symbols]

1 Image Distribution System 2 Digital Camera 10 Personal Computer (PC) (Distribution Source Terminal Device) 20 Mobile Phone (Delivery Destination Terminal Device) 30 Image Distribution Server (Image Conversion Distribution Device) 22 Display 24 Terminal Information Storage 31 Pre-Distribution Image Storage unit 32 Terminal information storage unit 33 Delivery image generation unit 34 Transfer unit 41 Number of pixels reduction processing unit 42 Color reduction processing unit (color reduction processing device) 43 Compression processing unit 51 Important color information storage unit 52 Color palette creation unit 53 Color replacement processing unit

Continued front page    F-term (reference) 5B057 AA20 BA02 CA01 CA08 CA12                       CA16 CB01 CB08 CB12 CB16                       CC01 CE17 DA17 DB02 DB06                       DB09 DC23 DC25                 5C077 MM27 MP08 NP07 PP31 PQ12                       PQ18 PQ19 PQ22                 5C079 HA02 HA03 LA02 LB11 MA01                       MA10 MA11                 5C082 BA20 BA34 BB51 CA12 CB01                       DA71 MM10

Claims (3)

[Claims] 1. A color palette creating means for creating a color palette having a predetermined number of representative colors, and the color of each pixel of an image to be subjected to color reduction processing to any one of the colors included in the color palette using the color palette. Color replacement processing means for replacing, and important color information storage means for storing information of important colors designated as additional information of the image, wherein the color palette creating means includes the color information of the image to be subjected to the color reduction processing and the important color. A color-reduction processing apparatus, which creates a color palette containing the important color with priority based on the important color information stored in the information storage means. 2. The color palette creating means creates a color palette that preferentially includes the important color by further using, as the additional information, weighting information in which a weighting coefficient is set for each area in the image to be subjected to color reduction processing. The color reduction processing apparatus according to claim 1, wherein 3. A predetermined number of representatives based on an important color information storage step of storing information of important colors designated as additional information of an image, and color information of an image to be subjected to color reduction processing and the stored important color information. A color palette creating step of creating a color palette that preferentially includes the important color as a color palette having a color; and a color included in the color palette for each pixel of the image to be subjected to the color reduction processing using the color palette. And a color replacement processing step for replacing with.