JPH04152386A - Expressing device for limited color of color image - Google Patents

Abstract

PURPOSE:To execute the limited color display at a high speed by providing a representative color allocating means for allocating a representative color of the shortest distance to an original picture element. CONSTITUTION:One-dimensional histogram data generated by a histogram generating means 12 is supplied to a 256 area dividing means 13. This dividing means 13 divides the whole color space into the limited number of areas, for instance, 256 areas, based on a distribution of a histogram, and in this case, the space in which the distribution is dense, and the space in which the distribution is sparse are divided minutely and roughly, respectively. This division information is supplied to a representative color determining means 14, and in this representative color determining means 14, an average value of each area is calculated, and a representative color of each area is determined. The average value in this case is the number on a one-dimensional color space, but is converted to values of R, G and B of a three-dimensional color space.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a limited color expression device for a color image that expresses a digitized natural color image using a limited number of representative colors.

[Prior Art] A color image is usually represented by three components: R (red), G (green), and B (blue), and digitized image data is often composed of 8 bits for each component. When displaying such digital color images on a display device,
As shown in FIG. 6, the three components of R, G, and B in one pixel are each represented by 8 bits in the image memory 1, and each component is converted into analog by D/A converters 2a, 2b, and 2c, and then transferred to the CRT. In a full-color display device that supplies display 3, one pixel is represented by 24 bits, so it is possible to display 2''?16.77 million colors at the same time, but each pixel requires 24 bits of memory. Therefore, the required capacity of the image memory 1 becomes enormous.

On the other hand, a color display device used in a personal computer, workstation, etc. is provided with an 8-bit image memory 4 for each pixel, as shown in FIG. The R, G, B values (
8 bits each) and then convert each component to a D/A converter 6a.
.

6b and 6c respectively convert the signals into analogs and supply them to the CRT display 7.

In this device, the output from the color map is 24 pixels per pixel.
Since it is expressed in bits, it is possible to select a color from 16.77 million colors, but the capacity for one pixel of the image memory 4 is 8.
Since it is a bit, the number of colors that can be displayed simultaneously is 256. However, since this device requires only a small amount of image memory, it is desirable to be able to display natural images with this limited number of colors without causing any discomfort.

For this reason, in the past, a histogram was created to examine the color distribution of the original image. For color areas with few colors, the color space is divided sparsely to obtain the same number of color areas as the limited number of colors, then a representative color is selected within each area as the expression color, and then a representative color is assigned to each original pixel. It is known to allocate and display limited colors.

As for the method of assigning the representative color, there is a method that calculates the Euclidean distance between the original pixel and all the representative colors and assigns the representative color with the shortest distance, and a method that assigns the original pixel to the representative color of the area to which it belongs. It is being

[Problem to be solved by the invention] However, with the method of calculating the Euclidean distance between the original pixel and all representative colors and assigning the representative color, it is difficult to obtain the best image quality or if the original image size is 512
If the number of representative colors is 256 in x512 pixels, then 512x
There was a problem in that 512×256 calculations had to be performed, resulting in an enormous amount of calculation, making it difficult to display limited colors at high speed.

In addition, in the method of assigning the original pixel to the representative color of the area to which it belongs, if the correspondence between the representative color and the area is created at the time of region division, it is only necessary to perform table conversion at the time of assignment, which allows for high-speed assignment to the representative color. However, depending on how the areas are divided, it may not be possible to allocate a suitable representative color. For example, FIG. 8 shows an example of a one-dimensional histogram of the original image.
The representative color B of the area A should be assigned, but ideally the representative color A of the nearby area A should be assigned.

Therefore, the present invention provides a limited color expression device for color images that enables high-speed limited color display by reducing the amount of calculation when assigning representative colors, and that can always assign representative colors suitable for original pixels. This is what we are trying to provide.

[Means for Solving the Problems] The present invention provides a color distribution creation means for creating a color distribution in the color space of a digitized natural color image, and a method based on the color distribution created by the color distribution creation means. A region dividing means divides the color space into a limited number of regions, a representative color determining means determines a representative color of each region divided by the region dividing means, and a region dividing means determines which region each original pixel of the color original image belongs to. For each original pixel, calculate the Euclidean distance between the original pixel and the representative color of the area to which the original pixel belongs and the representative colors of the two areas adjacent to this area, and select the representative color with the shortest distance. A representative color allocating means for allocating original pixels is provided.

[Operation] In the present invention having such a configuration, a color distribution in the color space of a digitized natural color image is created, and the color space is divided into a limited number of regions based on this color distribution. Determine the representative color of each area. Then, it is determined which area each original pixel of the color original image belongs to, and for each original pixel, the Euclidean diagram of the representative color of the area to which the original pixel belongs, the representative color of the two areas adjacent to this area, and the original pixel is determined. The distance is calculated and the representative color with the shortest distance is assigned to the original pixel.

[Example] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

Color spaces are usually 3 such as R, G, B, Lm, a*, b*
Is it a space of dimensions, e.g. R,G?

The space of B is expanded into a one-dimensional space in advance. That is, the color space of the upper 5 bits of R, G, and B, each represented by 8 bits, is developed one-dimensionally.

In this case, the total combination of R, G, and B is 215-32.
There are 768 pairs. Therefore, a conversion table for the correspondence between the one-dimensional space and the three-dimensional space is created by assigning numbers from 0 to 32,767 to the combinations of RlG and B. The Beano curve is used as a method for expanding into a one-dimensional space.

Specifically, as shown in FIG. 1, a natural color image is read by a color image scanner 11 and output as digital image data of 8 bits each for R, G, and B.

First, the first reading is performed at high speed with the resolution reduced and digital image data is output.

This digital image data is taken in by a histogram creation means 12 serving as a color distribution creation means, and a histogram of the original image is created on a one-dimensional color space by validating only the upper five bits of each color of RlG and B.

Although the data output from the scanner 11 at this time is three-dimensional data, it can be easily converted into one-dimensional data using the conversion table created previously.

The one-dimensional histogram data thus created by the histogram creation means 12 is supplied to the 256 area division means 13.

The area dividing means 13 divides the entire color space into a limited number of areas, for example 256 areas, based on the distribution of the histogram. At this time, the dense parts of the distribution are divided into smaller parts,
Roughly divide areas where the distribution is sparse.

This division information is supplied to the representative color determining means 14, and the average value of each area is calculated in this representative color determining means 14.
A representative color for each area is determined. The average value at this time is a number on a one-dimensional color space, but R, G, B in a three-dimensional color space
is converted to the value of

Also, at this time, the conversion tables shown in FIGS. 3 and 4 are created. In FIG. 3, one-dimensional color numbers from 0 to 32,767 are grouped and assigned to area numbers "0" to r255J, and brown code numbers "0" to r255J are assigned. In FIG. 4, R, G, and B values in a three-dimensional color space are assigned to each color code number "0" to r255J.

The contents of the conversion table shown in FIG. 4 are then registered in the color map 15.

Further, a natural color image at the original resolution is read by the color image scanner 11 and R, G is read.

Each of B is output as 8-bit digital image data. Then, this digital image data is assigned to representative color assignment means 1.
6 is set to import.

As shown in FIG. 2, the representative color allocation means 16 is comprised of an area determination means 21, a distance calculation means 22, and a shortest distance assignment means 23, and the area determination means 21 converts the conversion table shown in FIG. It is determined to which divided area the original pixel belongs by referring to . Then, the distance calculation means 22 calculates the Euclidean distance between the original pixel and three representative colors: the representative color of the area to which the original pixel belongs and the representative colors of two areas adjacent to this area. That is, the Euclidean distance is D, the original pixels are (r, g, b), and the representative colors are (r',
', b'), then D2- (rr') 2
+ (gg')2+(bb')2.

Furthermore, the closest representative color calculated by the shortest distance representative color assignment means 23 is assigned to the original pixel.

Once a representative color is assigned to each original pixel in this way, the data is supplied to the image memory 7 and stored therein.

The representative color data of each original pixel from the image memory 7 is supplied to the color map 1, and the color map 15
Based on the conversion table shown in Figure 4, the color code numbers corresponding to the representative colors of each original pixel are R, G, and R in the three-dimensional color space.
It is converted into a value of B and output to the display unit.

In this embodiment with such a configuration, first, the resolution is reduced and the image is read by the color image scanner 11, and the image is supplied to the histogram creation means 12 as digital data of 8 bits each for R, G, and B. As a result, the histogram creation means 12 creates a histogram of the original image on the one-dimensional color space by validating the upper five bits of each color.

Subsequently, the entire color space is divided into 256 areas by the 256 area dividing means 13.
divided into regions. Furthermore, the representative color determining means 14 calculates the average value of each area and determines the representative color of each area. Then, two conversion tables shown in FIGS. 3 and 4 are created, and the conversion table shown in FIG. 4 is registered in the color map 15.

When the above processing is completed, the resolution of the color image scanner 11 is then set to the original resolution and the original image is read. The digital data from the color image scanner 11 at this time is taken into the representative color allocation means 16, which determines which area each original pixel belongs to by referring to the conversion table shown in FIG. The Euclidean distance between the original pixel and a total of three representative colors, that is, the representative color of the region to which it belongs and the representative colors of two regions adjacent to this region, is calculated. Then, based on the distance calculation result, the closest representative color is assigned to the original pixel.

Therefore, for example, for a certain original image, 1 as shown in FIG.
A dimensional histogram distribution is obtained, and a certain original pixel G belongs to area B, and the representative color of area B is 81. The representative colors of areas A and C adjacent to area B are A and C, and the original pixel G belongs to area B. If the position shown in the figure is in area B, even if original pixel G belongs to area B, it is judged that the distance is closer to representative color A of area A than to representative color B of area B, and is assigned the representative color A of area A.

In this way, the entire color space is divided into 256 areas in advance, and the representative color of each area is determined, and area numbers "0" to r
One-dimensional color numbers from ○ to 32.767 are divided into groups and assigned to 255J, and brown code numbers "0" to r2 are assigned.
Conversion table to which 55J is assigned and each color code number "0"
”~Create a conversion table that assigns R, G, and H values in the three-dimensional color space to r255J, and then refer to the conversion table to determine which area each original pixel of the original image that is read normally belongs to. Then, the Euclidean distance between the representative color of the area to which the original pixel belongs and the representative colors of the two areas adjacent to this area and the original pixel is calculated, and the representative color with the shortest distance is assigned to the original pixel. Therefore, the amount of calculation required when assigning representative colors is reduced, and high-speed limited color display is possible. Furthermore, when the distance between the representative color of the adjacent area and the representative color of the area to which the original pixel belongs is closer than that of the area to which the original pixel belongs, the representative color of the adjacent area can be assigned to the original pixel, so that the representative color is always suitable for the original pixel. can be assigned.

In addition, although the color space was expanded into one dimension in the above embodiment, it is needless to say that the color space is not necessarily limited to this.

[Effects of the Invention] As detailed above, according to the present invention, it is possible to reduce the amount of calculation when assigning a representative color, to enable high-speed limited color display, and to always select a representative color suitable for the original pixel. It is possible to provide a limited color expression device for color images that can be assigned.

[Brief explanation of the drawing]

1 to 5 show an embodiment of the present invention, in which FIG. 1 is a block diagram, FIG. 2 is a block diagram showing the configuration of the representative color allocation means in FIG. 1, and FIGS. Figure 4 shows the configuration of a conversion table, Figure 5 is a graph showing a one-dimensional histogram, Figures 6 and 7 are block diagrams showing conventional examples, and Figure 8 explains conventional representative color assignment processing. This is a graph showing a one-dimensional histogram for 12... Histogram creation means, 13... 256 area dividing means, 14... Representative color determining means, 16... Representative color allocation means. Applicant's agent Patent attorney Takehiko Suzue

Claims (1)

[Claims] A color distribution creation means for creating a color distribution in the color space of a digitized natural color image, and an area for dividing the color space into a limited number of regions based on the color distribution created by the color distribution creation means. a dividing means; a representative color determining means for determining a representative color of each area divided by the area dividing means; and a representative color determining means for determining a representative color of each area divided by the area dividing means; A representative color allocation means is provided which calculates the Euclidean distance between the representative color of the area to which the pixel belongs and the representative colors of two areas adjacent to this area and the original pixel, and allocates the representative color with the shortest distance to the original pixel. A limited color expression device for color images characterized by: