Image compression method and apparatus, and image coding method and apparatus

• US 20030198381A1
• Filed: 04/17/2003
• Published: 10/23/2003
• Est. Priority Date: 04/17/2002
• Status: Active Grant

• Current Assignee: Canon Ayutthaya Limited (Canon Inc.)
• Original Assignee: Canon Ayutthaya Limited (Canon Inc.)
• Inventors: Yamazaki, Takeshi, Tanaka, Tetsuomi
• Granted Patent: US 7,158,669 B2
• Time in Patent Office: Days
• US Class Current: 382/166
• CPC Class Codes: H04N 1/41 Bandwidth or redundancy reduction by scanning H04N1/17 ; methods or arrangements for coding, decoding, compressing or decompressing digital video signals H04N19/00
• CPC Class Codes: H04N 1/642 Adapting to different types of images, e.g. characters, graphs, black and white image portions
• CPC Class Codes: H04N 1/644 using a reduced set of representative colours, e.g. each representing a particular range in a colour space

Abstract

There are provided an image coding method and apparatus and image compression method and apparatus which can properly compress a color document image with a high compression efficiency. First of all, a color reduction processing unit (102) converts each pixel of a color document image into an index assigned in correspondence with a color value, and generates color information (103) containing the number of pixels for each indexed color and an index color image (104). A color information sort unit (105) and the like determine, for example, a color value corresponding to an index of the maximum number of pixels as a background color. An order determination unit (113) determines an order. A binary image generation compression unit (109) generates a partial binary image for each index and compresses the binary image according to the order. A data integrating unit (111) generates compressed image (112) by integrating background data (108) and the compressed data of the partial binary image.

Claims

1. An image compression method of compressing a color image, comprising:

an index conversion step of converting each pixel of the color image into an index assigned in correspondence with a color value and generating color information containing the number of pixels for each indexed color and an index image obtained by converting each of the pixels into an index;

a background determination step of determining a color value corresponding to a predetermined index in the index image as a background color of the color image;

an order determination step of determining, on the basis of the color information and the background color, an order in which the color image is compressed for each index;

a binary image generating step of generating a binary image for each index from the index image;

a compression step of compressing the binary image according to the order; and

a generating step of generating compressed image data by integrating background data containing a size of the color image and a color value of the background color and compressed data of the binary image for each of the indexes.

2. The method according to claim 1, wherein

in the background color determination step, a color value corresponding to an index of the maximum number of pixels is determined as a background color of the color image, and

in the order determination step, the binary image generating step, and the compression step, determination of an order with respect to the index and generation and compression of a binary image are not executed.

3. The method according to claim 1, wherein in the index conversion step, one index is made to correspond to color values in a predetermined range to reduce the number of bits of the color image, thereby generating an index image.

4. The method according to claim 1, wherein in the index conversion step, a color gravity and a color distribution range for each indexed color of the color image are counted as the color information.

5. The method according to claim 1, further comprising an identical color integration step of integrating pieces of color information, of the color information, which have the color gravities close to each other.

6. The method according to claim 1, further comprising an intermediate color reduction step of reducing intermediate colors by integrating pieces of color information, of the color information, which have luminance/color differences close to each other.

7. The method according to claim 5, wherein in the identical color integration step, sorting is performed according to a product of the number of pixels for each color in the color image and a factor determined in advance in accordance with a position of the color in a color space, and identical color determination is performed with reference to a sorting order.

8. The method according to claim 5, wherein in the identical color integration step, when identical colors are to be integrated into a color at a higher sorting rank, the counted number of pixels, the color distribution range, and the gravity of color values are re-calculated.

9. The method according to claim 6, wherein in the intermediate color reduction step, intermediate colors are reduced with reference to a sorting order.

10. The method according to claim 6, wherein in the intermediate color reduction step, when similar colors are to be integrated into a color at a higher sorting rank, the counted number of pixels and the color distribution range are re-calculated, and only the color gravity is not re-calculated.

11. The method according to claim 1, wherein in the background color determination step, a gravity of color values of a color positioned at a highest sorting rank is extracted as the background color.

12. The method according to claim 7, wherein in the intermediate color reduction step, a plurality of predetermined thresholds are held, gravities of color values are converted into luminance/color difference signals, and colors within a threshold range are integrated/reduced according to a sorting order.

13. The method according to claim 12, wherein a value obtained by converting the gravity of color values into a luminance/color difference signal is re-calculated as a temporary color gravity every time integration/reduction is performed.

14. The method according to claim 12, wherein the plurality of thresholds are changed in value depending on whether priority is given to a compression ratio or image quality.

15. The method according to claim 1, wherein in the compression step, the binary image is MMR-compressed.

16. The method according to claim 1, wherein an order is determined with reference to distances to the color extracted in the background color extraction step in the color space.

17. The method according to claim 1, wherein

image compression can be executed in a plurality of image compression modes,

in a first compression mode, the index conversion step, the background color determination step, the order determination step, the binary image generating step, the compression step, and the generating step are executed with respect to a color image as a coding target, and

in a second compression mode, an area identification step of identifying a character area and a non-character area of the color image, a second compression step of compressing an image of the character area according to a binary image algorithm, and a third compression step of compressing an image of the non-character according to a multilevel image algorithm are executed with respect to a color image as a coding target.

18. An image compression method of compressing a color image, comprising:

an index conversion step of converting each pixel of the color image into an index assigned in correspondence with a color value and generating color information containing the number of pixels for each indexed color and an index image obtained by converting each of the pixels into an index;

a background determination step of determining a color value corresponding to a predetermined index in the index image as a background color of the color image;

an order determination step of determining an index order on the basis of the color information and the background color;

a binary image generating step of generating a binary image for each index from the index image;

a compression step of compressing the binary image according to the index order; and

a generating step of generating compressed image data by integrating background data containing a size of the color image and a color value of the background color and compressed data of the binary image for each of the indexes.

19. An image compression method of compressing a color image, comprising:

a sorting step of storing each pixel constituting the color image to one of a predetermined number of specific colors;

a specifying step of specifying a color of the specific colors which corresponds to a background color;

a ranking step of ranking the respective specific colors in a relationship in terms of overwriting with reference to a luminance value of the background color in accordance with luminance values of the specific colors except for the background color;

a generating step of generating, for each of the specific colors, a concatenated pixel group constituted by a pixel corresponding to the specific color and neighboring pixels of the pixel;

a replacing step of, for a concatenated pixel group corresponding to a predetermined specific color, when neighboring pixels contained in the concatenated pixel group include a pixel of a specific color ranked higher than the predetermined specific color in the relationship in terms of overwriting, replacing the neighboring pixel with the predetermined specific color; and

a compression step of compressing the concatenated pixel group obtained in the replacing step.

20. An image compression apparatus for compressing a color image, comprising:

index conversion means for converting each pixel of the color image into an index assigned in correspondence with a color value and generating color information containing the number of pixels for each indexed color and an index image obtained by converting each of the pixels into an index;

background determination means for determining a color value corresponding to a predetermined index in the index image as a background color of the color image;

order determination means for determining, on the basis of the color information and the background color, an order in which the color image is compressed for each index;

binary image generating means for generating a binary image for each index from the index image;

compression means for compressing the binary image according to the order; and

generating means for generating compressed image data by integrating background data containing a size of the color image and a color value of the background color and compressed data of the binary image for each of the indexes.

21. A program for causing a computer to execute

an index conversion procedure of converting each pixel of the color image into an index assigned in correspondence with a color value and generating color information containing the number of pixels for each indexed color and an index image obtained by converting each of the pixels into an index;

a background determination procedure of determining a color value corresponding to a predetermined index in the index image as a background color of the color image;

an order determination procedure of determining, on the basis of the color information and the background color, an order in which the color image is compressed for each index;

a binary image generating procedure of generating a binary image for each index from the index image;

a compression procedure of compressing the binary image according to the order; and

a generating procedure of generating compressed image data by integrating background data containing a size of the color image and a color value of the background color and compressed data of the binary image for each of the indexes.

22. A computer-readable recording medium storing a program defined in claim 21.

23. An image coding apparatus for coding an image, comprising:

color reduction processing means for generating a color-reduced image by performing color reduction processing of an original image;

color information generating means for generating color information concerning an area having a color in the color-reduced image for each color contained in the color-image;

partial image generating means for generating a color index image having a color in the color-reduced image for each color contained in the color-reduced image;

first updating means for updating and integrating pieces of color information of the pieces of color information generated by said color information generating means which exhibit colors relatively close to each other;

second updating means for referring to the color information updated by said first updating means and updating and integrating pieces of color information exhibiting luminance/color differences relatively close to each other;

compression means for, in accordance with integration by said first and second updating means, obtaining an OR image based on color index images as a corresponding integration destination and integration source, generating a binary image of the OR image, compressing the binary image, and generating binary image compressed data constituted by the compressed data and the color information which corresponds to the compressed data and updated by said second updating means; and

compressed data generating means for generating output data containing data representing a color and contained in the color information of the area having the largest number of pixels, and the binary image compressed data generated by said compression means.

24. The apparatus according to claim 23, wherein the color information contains data representing a color, data representing the number of pixels constituting an area having a color in the color-reduced image, data representing a position of the area in the color-reduced image.

25. The apparatus according to claim 23, wherein said partial image generating means further assigns an index to a generated color index image, and also assigns the same index to the corresponding color information, thereby associating the color index image with the color information.

26. The apparatus according to claim 24, further comprising sorting means for sorting the color information by referring to data obtained by further adjusting the data representing the number of pixels and contained in the color information,

said sorting means sorting the color information in decreasing order of the number of pixels.

27. The apparatus according to claim 24, wherein said first updating means refers to data representing a color and contained in first color information and data representing a color and contained in second color information, and if a difference between the respective data is not more than a predetermined value, obtains an average color of the respective colors, and updates the data representing the color and contained in the first color information and the data representing the color and contained in the second color information into data representing the average color.

28. The apparatus according to claim 24, wherein said first updating means refers to data representing a color and contained in first color information and data representing a color and contained in second color information, and if a difference between the respective data is not more than a predetermined value, adds data representing the number of pixels and contained in the first color information to data representing the number of pixels and contained in the second color information, and updates the data representing the number of pixels and contained in the first color information and the data representing the number of pixels and contained in the second color information into data representing the sum value.

29. The apparatus according to claim 24, wherein said first updating means refers to data representing a color and contained in first color information and data representing a color and contained in second color information, and if a difference between the respective data is not more than a predetermined value, refers to data representing a position and contained in the first color information and data representing a position and contained in the second color information, specifies data representing an uppermost, leftmost position and data representing a lowermost, rightmost position, and updating the data representing the position and contained in the first color information and the data representing the and contained in the second information into the specified data.

30. The apparatus according to claim 24, wherein said second updating means further comprises luminance/color difference data conversion means for converting data representing a color and contained in color information updated by said first updating means into luminance/color difference data.

31. The apparatus according to claim 23, wherein in accordance with integration by said first and second updating means, said compression means obtains an OR image based on color index images as a corresponding integration destination and integration source, generates a binary image of the OR image, and compresses the binary image.

33. A program causing an information processing apparatus to load the program to make the information processing apparatus function as an image coding apparatus defined in claim 23.

35. A computer-readable storage medium storing a program defined in claim 33.

32. An image coding method of coding an image, comprising:

a color reduction processing step for generating a color-reduced image by performing color reduction processing of an original image;

a color information generating step for generating color information concerning an area having a color in the color-reduced image for each color contained in the color-image;

a partial image generating step for generating a color index image having a color in the color-reduced image for each color contained in the color-reduced image;

a first updating step for updating and integrating pieces of color information of the pieces of color information generated in the color information generating step which exhibit colors relatively close to each other;

a second updating step for referring to the color information updated in the first updating step and updating and integrating pieces of color information exhibiting luminance/color differences relatively close to each other;

a compression step for, in accordance with integration in the first and second updating step, obtaining an OR image based on color index images as a corresponding integration destination and integration source, generating a binary image of the OR image, compressing the binary image, and generating binary image compressed data constituted by the compressed data and the color information which corresponds to the compressed data and updated in the second updating step; and

a compressed data generating step for generating output data containing data representing a color and contained in the color information of the area having the largest number of pixels, and the binary image compressed data generated in the compression step.

34. A program causing an information processing apparatus to load the program to make the information processing apparatus execute an image coding method defined in claim 32.