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

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.

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.

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.

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.

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.

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.