Color image compressing method and device

• US 7,729,025 B2
• Filed: 07/01/2004
• Issued: 06/01/2010
• 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, Kanamoto, Kouichi, Katsumata, Yutaka, Eguchi, Shinichi
• Primary Examiner(s): Tran; Douglas Q
• Application Number:
  US10/881,524
• Publication Number:
  US 20050100212A1
• Time in Patent Office: 2,161 Days
• Field of Search: 358/539, 358/464, 358/522, 382/164, 382/202, 382/176, 382/170, 382/171, 382/242, 382/195, 382/190, 382/224, 382/173, 382/168
• US Class Current: 358/539
• 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 color image 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, upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

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

reducing the number of hue values of the entire color image to obtain a number-reduced hue value, 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 of the clusters and encoding pixels in each cluster of the clusters by a coding method suitable for the characteristic of each of the clusters.

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

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

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

3. 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 values are calculated and it is determined which of 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 values, and the outline of each of the clusters is traced and it is determined whether or not a number of change points at which the direction of traced outline changes exceeds a specific value, if the number of change points is below the specific value, it is determined that the area is a ruled line area and if the number of change points exceeds the specific value, it is determined that the area is a character area.

4. 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 values is below a second reference value, it is determined whether or not the height and width of the outermost circumscribed rectangle exceed respective reference values, then, if the height and width exceed the reference values, it is determined that the area is a ruled line area, and if the height and width are below the reference values, the outline of boundary with another cluster is traced, and it is determined whether or not a number of change points at which the direction of the traced outline changes exceeds a specific value, if the number of change points is below the specific value, it is determined that the area is a ruled line area and if the number of change points exceeds the specific value, it is determined that the area is a character area.

5. 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.

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

7. A color image compressing 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, upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

counting the number of appearances of the 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 to obtain a number-reduced hue value, 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 of the clusters and encoding pixels in each of the clusters by a coding method suitable for the character of each of the clusters, wherein

HSL conversion is applied to RGB data of pixels in other than a photograph;

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 the 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 the 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.

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

an area determining unit for 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 pixels of each area, and upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

a hue histogram generating unit for counting the number of appearances of a hue value of each pixel in a color image and generating a hue histogram of pixels in other than a photograph area;

a cluster classifying unit for reducing the number of hue values of the entire color image to obtain a number-reduced hue value, 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 to obtain a newly allocated hue value, based on the newly allocated hue value; and

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

9. The color image compressing device for compressing a color image read by an optical sensor, comprising:

an area determining unit for 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, upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

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

a cluster classifying unit for reducing the number of hue values of the entire color image, based on the hue histogram to obtain a number-reduced hue value, 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;

an encoding unit for determining a characteristic of each of the clusters and encoding pixels in each of the clusters suitable for the characteristic of each of the clusters;

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

a color and a monochrome list generating unit for 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 the 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 the 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;

wherein

the hue histogram generating unit applies HSL conversion to RGB data of pixels in other than a photograph area, generates a color hue histogram by counting the number of appearances of a value of each hue H of the pixels and generates a monochrome hue histogram by counting the number of appearances of a value of each luminance L of the pixels;

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

10. 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 value and a distributed amount of hue value of each area, upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

counting the number of appearances of the hue value of each pixel in a color image and generating a hue histogram of pixels in other than a photograph area;

reducing the number of hue values of the entire color image to obtain a number-reduced hue value, 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 of the clusters and encoding pixels in each of the clusters by a coding method suitable for the characteristic of each of the clusters.

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

dividing means for 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, and upon calculating the distributed amount of the color value and the distributed amount of the hue value of each area of a color image, if the distributed amount of the color value exceeds a first reference value and the distributed amount of the hue value exceeds a second reference value, it is determined that the area is a photograph area;

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

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

hue histogram generating means for counting a 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 to obtain a number-reduced hue value, 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 to obtain a newly allocated hue value, based on the newly allocated hue value; and

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