Color image compressing method and device

• US 20050100212A1
• Filed: 07/01/2004
• Published: 05/12/2005
• Est. Priority Date: 11/11/2003
• Status: Active Grant

• Current Assignee: Fujitsu Frontech Limited (Fujitsu Limited), Fujitsu Limited
• Original Assignee: Fujitsu Frontech Limited (Fujitsu Limited), Fujitsu Limited
• Inventors: Suzuki, Naoko, Katsumata, Yutaka, Eguchi, Shinichi, Kanamoto, Kouichi
• Granted Patent: US 7,729,025 B2
• Time in Patent Office: Days
• US Class Current: 382/164
• CPC Class Codes: G06V 30/413 Classification of content, e.g. text, photographs or tables
• CPC Class Codes: H04N 1/642 Adapting to different types of images, e.g. characters, graphs, black and white image portions

Abstract

It is an object of the present invention to improve the compression ratio of a color image and to clearly display the outlines of characters and the like. A hue cluster classifying/unifying unit reduces the number of hue values of each pixel in a color image, based on a hue histogram, allocates the number-reduced hue value to each pixel and classifies pixels with the same hue value into one cluster. Furthermore, the unit unifies clusters whose hue values are below a predetermined value. The unit also traces the outline of a cluster whose size is below a reference value and determines that a cluster that has a lot of change points belongs to a character area. An encoding unit determines the characteristic of each cluster, based on both an area determined by an area determining unit and whether the cluster belongs to a ruled line area or a character area, and encodes pixels in each cluster by a coding method suitable for the characteristic of the cluster.

Claims

1. A compression method for compressing color images read by an optical sensor, comprising:

counting the number of appearances of a hue value of each pixel of a color image, generating a hue histogram and storing the hue histogram in a storage unit;

reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying the pixels of the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the character of the cluster.

3. The color image compressing method according to claim 1, wherein

it is determined whether the characteristic of the relevant cluster is a ruled line area or a character area, based on the features of an outline of the cluster, and

each of a cluster determined to belong to the ruled line area and a cluster determined to belong to the character area is encoded by a coding method suitable for the characteristic of the respective area.

5. The color image compressing method according to claim 1, wherein

a color image is divided into a plurality of areas, both a distributed amount of color values and a distributed amount of hue are calculated and it is determined which each area is, a photograph area, a ruled line/character area or a background area, based on both the distributed amount of color values and distributed amount of hue, and

it is determined which the area is, a ruled line area or a character area, by extracting the features of the outline of the cluster.

8. The color image compressing method According to claim 1, wherein

if the distributed amount of color values exceeds a first reference value and the distributed amount of hue is below a second reference value, it is determined that the relevant area is a ruled line area or a character area, and

if the distributed amount of color values is below the first reference value, it is determined that the relevant area is a background/plane area.

9. The color image compressing method According to claim 1, wherein

if the distributed amount of color values exceeds a first reference value and the distributed amount of hue exceeds a second reference value, it is determined that the relevant area is a photograph area, and

if the distributed amount of color values exceeds the first reference value and the distributed amount of hue is below the second reference value, it is determined that the relevant area is a ruled line or a character area.

10. The color image compressing method According to claim 1, wherein

if the distributed amount of color values exceeds a first reference value and the distributed amount of hue is below a second reference value, it is determined whether a size of the relevant area exceeds a reference value, then, if the size exceeds the reference value, it is determined that the area is a ruled line area, and if the size is below the reference value, an outline of the area is extracted, and then if the outline hardly changes, it is determined that the area is a ruled line area and if the outline changes much, it is determined that the area is a character area.

11. The color image compressing method According to claim 1, wherein

HSL conversion is applied to RGB data of each pixel in each area of a color image;

a color hue histogram is generated by counting number of appearances of a value of each hue H of each color pixel;

a monochrome hue histogram is generated by counting number of appearances of a value of each luminance L of each color pixel;

both a color hue peak list and a monochrome hue peak list are generated by detecting a predetermined number of higher-ranked hue values in each of the color hue histogram and monochrome histogram;

a typical hue value list of the entire color image is generated by overlapping the color hue histogram and monochrome hue histogram of each area;

a color list is generated by allocating a corresponding hue value of the typical hue value list to each color pixel in the color image, counting number of appearances of the hue value allocated to each color pixel and selecting a predetermined number of hue values whose number of appearances is highly ranked;

a monochrome list is generated by allocating a corresponding hue value of the typical hue value list to each monochrome pixel in a color image, counting number of appearances of the hue value allocated to each monochrome pixel and selecting a predetermined number of hue values whose number of appearances is highly ranked; and

one cluster is generated by allocating a corresponding hue value in the color list to each color pixel in the color image, allocating a corresponding hue value in the monochrome list to each monochrome pixel in the color image and unifying adjacent pixels with the same hue value into one cluster and all the areas of the color image are classified into a plurality of clusters.

12. The color image compressing method According to claim 1, wherein

pixel data of a ruled line area is compressed by chain coding, and pixel data of a character area is compressed by Huffman coding.

13. The color image compressing method According to claim 3, wherein

pixel data of a ruled line area is compressed by chain coding, pixel data of a character area is compressed by Huffman coding, outline data of a background area is compressed by chain coding and luminance information of the background area is compressed by JPEG coding.

2. A compression method for compressing color images read by an optical sensor, comprising:

dividing a color image into a plurality of areas and determining a characteristic of each area, based on both a distributed amount of a color value and a distributed amount of a hue value of a pixel in each area;

counting number of appearances of a hue value of each pixel in a color image, generating a hue histogram and storing the histogram in a storage unit;

reducing a number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying pixels in the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the character of the cluster.

4. The color image compressing method according to claim 2, wherein

it is determined whether the characteristic of the relevant cluster is a ruled line area or a character area, based on the features of an outline of the cluster, and

each of a cluster determined to belong to the ruled line area and a cluster determined to belong to the character area is encoded by a coding method suitable for the characteristic of the respective area.

6. The color image compressing method according to claim 2, wherein

a color image is divided into a plurality of areas, both a distributed amount of color values and a distributed amount of hue are calculated and it is determined which each area is, a photograph area, a ruled line/character area or a background area, based on both the distributed amount of color values and distributed amount of hue, and

it is determined which the area is, a ruled line area or a character area, by extracting the features of the outline of the cluster.

7. A color image compressing device for compressing a color image read by an optical sensor, comprising:

a hue histogram generating unit counting number of appearances of a hue value of each pixel in a color image;

a cluster classifying unit reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying the pixels of the entire color image into clusters, based on the newly allocated hue value; and

an encoding unit determining the characteristic of each cluster and encoding pixels in each cluster suitable for a characteristic of the cluster.

15. The color image compressing device according to claim 7, comprising:

a hue histogram generating unit applying HSL conversion to RGB data of each pixel in each area of a color image, generating a color hue histogram by counting number of appearances of a value of each hue H of each color pixel and generating a monochrome hue histogram by counting number of appearances of a value of each luminance L of each color pixel;

a hue peak list generating unit generating a color hue peak list and a monochrome hue peak list by detecting a predetermined number of higher-ranked hue values in each of the color hue histogram and monochrome histogram, respectively;

a typical hue value list generating unit generating a typical hue value list of the entire color image by overlapping the color hue histogram and monochrome hue histogram of each area;

a color and a monochrome list generating unit generating a color list by allocating a corresponding hue value of the typical hue value list to each color pixel in the color image, counting number of appearances of the hue value allocated to each color pixel and selecting a predetermined number of hue values whose number of appearances is highly ranked and generating a monochrome list by allocating a corresponding hue value of the typical hue value list to each monochrome pixel in a color image, counting number of appearances of the hue value allocated to each monochrome pixel and selecting a predetermined number of hue values whose number of appearances is highly ranked; and

a cluster classifying unit generating one cluster by allocating a corresponding hue value in the color list to each color pixel in the color image, allocating a corresponding hue value in the monochrome list to each monochrome pixel in the color image and unifying adjacent pixels with the same hue value into one cluster and all the areas of the color image into a plurality of clusters.

14. A color image compressing device for compressing a color image read by an optical sensor, comprising:

an area determining unit dividing a color image into a plurality of areas and determining a characteristic of each area, based on both a distributed amount of color value and distributed amount of hue of pixels of each area;

a hue histogram generating unit counting number of appearances of a hue value of each pixel in a color image and generating a hue histogram;

a cluster classifying unit reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying the pixels of the entire color image into clusters, based on the newly allocated hue value; and

an encoding unit determining the characteristic of each cluster and encoding pixels in each cluster suitable for a characteristic of the cluster.

16. A computer data signal realized by a carrier signal, for providing a program for compressing a color image read by an optical sensor, said program comprising:

counting number of appearances of a hue value of each pixel in the color image and generating a hue histogram;

reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying pixels of the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable the characteristic the cluster.

17. A computer data signal realized by a carrier signal, for providing a program for compressing a color image read by an optical sensor, said program comprising:

dividing a color image into a plurality of areas and determining the relevant area, based on both a distributed amount of color values and a distributed amount of hue of each area;

counting number of appearances of a hue value of each pixel in a color image and generating a hue histogram;

reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying pixels of the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the characteristic of the cluster.

18. A computer-readable storage medium on which is recorded a program for enabling a computer to compress a color image read by an optical sensor, said process comprising:

counting number of appearances of a hue value of each pixel in the color image and generating a hue histogram;

reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying pixels of the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the characteristic of the cluster.

19. A computer-readable storage medium on which is recorded a program for enabling a computer to compress a color image read by an optical sensor, said process comprising:

dividing a color image into a plurality of areas and determining the relevant area, based on both a distributed amount of color values and a distributed amount of hue of each area;

counting number of appearances of a hue value of each pixel in a color image and generating a hue histogram;

reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying pixels of the entire color image into clusters, based on the newly allocated hue value; and

determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the characteristic of the cluster.

20. A color image compressing device for compressing a color image read by an optical sensor, comprising:

hue histogram generating means for counting number of appearances of a hue value of each pixel in a color image;

cluster classifying means for reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying the pixels of the entire color image into clusters, based on the newly allocated hue value; and

encoding means for determining the characteristic of each cluster and encoding pixels in each cluster suitable for a characteristic of the cluster.