JPH0248944B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] A gradation conversion device that obtains an output image with a small number of colors from an input image with a large number of colors.
When determining colors in order from the largest number of pixels in the input image, an appropriate similar color range is set for each color in the vicinity of the determined color, and the colors that will be determined later are determined first. Avoid similar colors.

[Industrial application field]

The present invention relates to a color gradation conversion device in a color image processing system, and in particular to a color gradation conversion device for a color image processing system. The present invention relates to a color gradation conversion device for converting into a color image.

[Conventional technology]

The number of color images that can be displayed with current video techs is about 16 colors, and many personal computers are limited to about 8 colors.

For this reason, generally, in order to display a color image input using a color TV camera or the like on these devices, the input color image must be reduced to 16 or 8 colors.

FIG. 4 conceptually shows one method of such tone conversion. Figure a shows the hue space of the input image, and Figure b shows the hue space of the output image. Note that the illustrated example is a case of a three-dimensional color display method of R, G, and B.

In the hue space of figure a, A ₁ , A ₂ , ...Al,
B ₁ , B ₂ , ...B _n , C ₁ , C ₂ , ...C _o , ..., are
Each represents an individual color used in the input image, and multiple colors in each group of A, B, C are represented by a single color, denoted by α, β, γ, ... in Figure b. converted to color.

That is, A ₁ , A ₂ ,... included in group A
…, the l colors denoted by Al are 1 denoted by α
Similarly, B ₁ , B ₂ , B 2 of group B
m colors consisting of ..., B _n are reduced to one color β, and n colors consisting of C ₁ , C ₂ , ..., C _o of group C are reduced to one color γ. Degenerates.

Conventionally, when performing such conversion, there is a fixed conversion method that cuts off the lower bits of the input color, and
Check the number of pixels for each color in the input image, select a predetermined number of pixels sequentially from the one with the largest number of pixels, adopt each color as the color of the output image, and select the color in the input image that is the most similar among these colors. A dynamic conversion method for converting to color was used.

[Problem that the invention seeks to solve]

In conventional conversion to reduce the number of color gradations in a color image, if the lower bits are truncated,
The color correspondence between the input image and the output image becomes poor, and if you adopt a method that selects colors from the input image starting with the largest number of pixels, colors that occupy a large area among the pixels (such as the sky and the sea) ( In other words, the number of pixels is large)
In addition, if the color shading is only slightly different, many similar colors will be selected, and the number of other colors will be reduced accordingly, resulting in an output image that gives an impression different from the input image as a whole. There was a tendency to

Similarly, colors with low brightness are often visually almost indistinguishable, but if many such colors are selected, visually important colors may not be selected as much, resulting in an image with little variation. It was hot.

Furthermore, in order to determine the degree of similarity between colors, it is necessary to calculate the difference between two color vectors in the hue space, which poses a problem in that it requires a considerable amount of processing.

[Means for solving problems]

The present invention provides a simple means for quickly obtaining an output image with a tone close to that of an input image while reducing the number of color gradations.

Therefore, in the present invention, when determining the type of color to be used in the output image, the colors to be output are selected in the order of the number of pixels for each color in the input image. The selection is made from among colors that are not similar to the already selected colors to prevent multiple similar colors from being selected.

FIG. 1 shows the basic configuration of an apparatus according to the present invention.

In the figure, 1 is an input image data storage section, 2 is a color-based pixel number counting section, 3 is a color-based pixel number data storage section, 4 is a color selection section, 5 is a similar color prohibition section, and 6 is an output color data storage section. , 7 is a color conversion section, and 8 is an output image data storage section.

The input image data storage section 1 stores input image data with a high number of color gradations.

The color-specific pixel number counting section 2 counts the number of pixels for each color of the input image stored in the input image data storage section 1, and stores it in the color-based pixel number data storage section 3.

The color selection section 4 sequentially checks the number of pixels for each color stored in the pixel number data storage section 3 for each color, starting from the largest one, and stores it in the output color data storage section 6 if the color is selectable. Note that the number of output colors is set to n at most.

The similar color prohibition section 5 sets a similar color prohibition range predetermined for each color in the vicinity of the colors sequentially selected by the color selection section 4, and prevents colors from being selected from that range thereafter. The similar color prohibited range is notified to the color-by-color pixel number data storage section 3 and is set.

Therefore, if the color of the number of pixels for each color to be selected next is included in the similar colors set by the similar color prohibition section 5, the color selection section 4 selects the next color without selecting that color. Select the color according to the number of pixels of each color.

The output color data storage section 6 stores a predetermined number of output colors (up to n) selected by the color selection section 4.

The color conversion unit 7 converts the color of the input image stored in the input image data storage unit 1 based on the output color stored in the output color data storage unit 6.

The conversion method calculates the distance between each color in the input image and each output color, selects the output color with the shortest distance, and replaces it with the color of the input image.
Here, a method is used to determine the distance (similarity) between colors based on the differences between the elements that make up the colors.

The output image data storage unit 8 stores the output image resulting from the conversion in this manner.

[Effect]

FIG. 2 shows an example of the pixel number data for each color stored in the pixel number data storage unit 3 for each color in FIG. 1, and shows the appearance frequency of colors in the input image on a one-dimensional color spectrum. It is something that That is, the horizontal axis represents the color gradation (hue), and the vertical axis represents the number of pixels for each color. Also, , indicates the number of pixels for each of the top three colors, and H ₁ , H ₂ , and H ₃ indicate the corresponding colors.

The color selection unit 4 first selects the color H ₁ having the maximum number of pixels for each color.
is selected and stored in the output color data storage section 6. At this time, the similar color prohibition unit 5 selects the selected output color H ₁
A similar color prohibition range, indicated by a diagonal line, is set around the .

For this reason, the color selection unit 4 cannot select the color H ₂ with the next largest number of pixels per color, selects the color H ₃ with the third largest number of pixels per color, and stores it in the output color data storage unit 6. Store.

In this way, a fixed number of output colors H ₁ , H ₃ ,...
When ... is determined, the color conversion unit 7 converts each of the many colors included in the input image in the input image data storage unit 1 into a fixed number of output colors H ₁ , H ₃ , etc. Convert to For example, pixels of colors H ₁ and H ₂ in the input image are converted to the same color H ₁ .

〔Example〕

FIG. 3 is a configuration diagram of an apparatus according to an embodiment of the present invention.

In the figure, 31 is an image input device, 32 is a color gradation conversion device, 33 is an input buffer, 34 is a preprocessing section, 35 is an input image data memory, 36 is a color-based pixel number counting section, and 37 is a color-based pixel number data memory, 3
8 is a color selection section, 39 is an output color data memory, 40
41 is a prohibited range determining section, 41 is a color-specific prohibited range table, 42 is a pixel number data rewriting section, 43 is a color converting section, 44 is an output image data memory, and 45 is an image output section.

The image input device 31 is a color TV camera or a color scanner, and converts a color document or other arbitrary image into color image data in which each of the color elements R, G, and B is expressed in 8 bits, and outputs the converted image data.

The color gradation conversion device 32 is a device configured according to the present invention, and is a device configured according to the present invention.
24-bit fine gradation color image data is converted into, for example, eight colors of coarse gradation color image data and output.

In the color gradation conversion device 32, the input color image data is temporarily stored in an input buffer 33.

In order to reduce the amount of gradation conversion processing, the preprocessing unit 34 cuts off the middle and lower 4 bits of each 8 bits of R, G, and B of the input data, compresses and converts them into color image data of a total of 12 bits (4096 colors), and converts the input image into the input image. The data is stored in the data memory 35.

The color-by-color pixel number counting section 36 reads out the color image data from the input image data memory 35 pixel by pixel, counts the number of pixels for each color gradation (4096 colors), and stores it in the color-by-color pixel number data memory 37. .

The color selection unit 38 has a pixel count data memory 37 for each color.
The maximum number of pixels for each color stored in is detected, and the detected color is stored in the output color data memory 39.

Next, the prohibited range determining unit 40 determines from the color prohibited range table 41 a range of similar colors that should be prohibited from being selected from now on for the color with the maximum number of pixels for each color detected by the color selecting unit 38, and rewrites the pixel number data. 42.

The pixel number data rewriting unit 42 stores the color-specific pixel number data of the colors included in the prohibited range notified from the prohibited range determining unit 40 in the color-based pixel number data memory 37.
(Specifically, the pixel value may be set to 0).

In addition, the color-based prohibited range table 41 contains 4 bits each.
The respective values of color elements R, G, and B (0
-F) are registered as prohibited ranges. For example, when RGB=000, each color element of R, G, and B is ±3
Similarly, RGB=444 indicates that colors within the range of ±2 are prohibited for each color element. In this example, by setting the prohibited range larger for the smaller RGB value, the lower the brightness of the color, the harder it is to select a similar color. but,
The prohibited range can be arbitrarily specified for each individual color.

After that, the color selection unit 38 again detects the maximum number of pixels for each color for the number of pixels for each color stored in the pixel number data for each color data memory 37, that is, the remaining data after deleting the pixel number data within the prohibited range. Then, the color is stored in the output color data memory 39. Then, the prohibited range determination unit 40 again similarly refers to the color-based prohibited range table 41 to determine the prohibited range for that color, and deletes the pixel number data from the color-based pixel number data memory 37.

In this way, loop processing is performed until a predetermined number of output colors, in this case eight colors, are stored in the output color data memory 39.

Each time the color selection unit 38 selects one output color from the color-specific pixel count data memory 37, the color-specific pixel count data within the prohibited range set for that color and its vicinity are deleted from the memory. The output color selected will be in no way similar to any of the output colors already selected.

The color conversion unit 43 performs conversion to replace the color of each pixel of the input image data stored in the input image data memory 35 with the closest color among the predetermined number of output colors stored in the output color data memory 39. Note that the degree of similarity between colors is originally expressed as the difference between two vectors in the three-dimensional space of R, G, and B, and requires complex calculations. , B. This is easily determined by looking at the difference amount between each of them.

The image data as a result of conversion by the color conversion unit 43 is
The image data is stored in the output image data memory 44, and is further outputted from the image output section 45 to a floppy disk or the like as required.

〔Effect of the invention〕

According to the present invention, when the number of colors that can be used in an output image is limited, it is possible to obtain an image with an overall tone close to the original image without using many similar colors or colors with low brightness. Furthermore, when determining usable colors, there is no need to calculate the distance between colors, making the process simple and quick.

Furthermore, by narrowing or widening the prohibited range of a specific color in the color-based prohibited range table, you can, for example, make the entire image look reddish or limit the blue color to only one color. ,
This can be easily achieved.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the principle configuration of the present invention, Fig. 2 is a diagram showing an example of pixel number data for each color, Fig. 3 is a diagram showing the configuration of one embodiment of the present invention, and Fig. 4 is a diagram showing gradation. FIG. 2 is a diagram illustrating the concept of conversion. In FIG. 1, 1: input image data storage unit;
2: Color-based pixel number counting section, 3: Color-based pixel number data storage section, 4: Color selection section, 5: Similar color prohibition section, 6: Output color data storage section, 7: Color conversion section, 8: Output image Data storage.

Claims (1)

[Claims] 1. In a color gradation conversion device that inputs an image with a large number of colors, converts it into an image with a small number of colors, and outputs the image, the number of pixels of the same color included in the digitized input image a color-based pixel number counting section 2 that counts the number of pixels; and a color-based pixel number data storage section 3 that stores the color-based pixel number data counted by the color-based pixel number counting section 2.
, a color selection section 4 that selects the largest color among the pixel number data for each color stored in the pixel number data for each color stored in the pixel number data for each color storage section 3; A similar color prohibition section 5 is provided for setting a similar color prohibition range of a prespecified width and invalidating the color-specific pixel count data of the similar color prohibition range stored in the color-specific pixel count data storage section 3. , until the number of colors of the predetermined output image is reached.
A color gradation conversion device for a color image, characterized in that the similar color prohibition unit 5 is operated repeatedly, and the color of an input image is converted based on the obtained color. 2. When the brightness of the color selected by the color selection unit 4 is low, the similar color prohibition range is expanded to prevent the selection of many colors with low brightness that are difficult to visually distinguish. A color gradation conversion device for a color image according to claim 1.