IMAGE PROCESSING APPARATUS, COMPRESSION METHOD, AND STORAGE MEDIUM FOR STORING PROGRAMS THEREOF

• US 20120014597A1
• Filed: 07/06/2011
• Published: 01/19/2012
• Est. Priority Date: 07/13/2010
• Status: Active Grant

• Current Assignee: Canon Ayutthaya Limited (Canon Inc.)
• Original Assignee: Canon Ayutthaya Limited (Canon Inc.)
• Inventors: Matsunaga, Daisuke
• Granted Patent: US 8,849,021 B2
• Time in Patent Office: Days
• US Class Current: 382/166
• 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

Image data is divided into blocks, each block has M×N pixels, the number of colors of each block undergoes reduction from M×N to each number of one to (M×N−1). Differences between pixel values for each block after color reduction, and pixel values for each block before first color reduction. A first table in which thresholds are associated with either the amount of data to be reduced or the amount of data to be remained, is acquired based on the acquired difference. A threshold at which the amount of data after compression drops to or below a target data amount is specified, based on the first table, and compressed data is generated based on image data after second color reduction using the specified threshold.

Claims

1. An image processing apparatus, comprising:

an input unit configured to input image data;

a division unit configured to divide the image data inputted by the input unit into blocks, each of the blocks has M×

N pixels (where M and N are integers greater than or equal to

     2);

a color-reduction unit configured to perform first reduction of the number of colors of each of the blocks from M×

N to each number of colors, the each number of colors is from one to (M×

N−

1);

an acquisition unit configured to acquire differences between pixel values for each of the blocks after the first color reduction by the color-reduction unit, and pixel values for each of the blocks before the first color reduction;

a generation unit configured to generate a first table in which a plurality of thresholds are associated with either the amount of data to be reduced or the amount of data to be remained, based on the differences acquired by the acquisition unit;

a specifying unit configured to specify a threshold at which the amount of data after compression of the image data drops to or below a target data amount, based on the first table; and

a compression unit configured to perform second reduction of the number of colors of each of the blocks based on the threshold specified by the specifying unit, and to generate compressed data based on the image data after the second color reduction.

2. The image processing apparatus according to claim 1, further comprising a first storage unit configured to store in a memory the compressed data generated by the compression unit.

3. The image processing apparatus according to claim 1,

wherein the compression unit, based on the color data for each pixel in each of the blocks after the second color reduction based on the specified threshold, generates, as the compressed data, a pattern flag indicating a layout pattern for color data included in each of the blocks after the second color reduction, and color data for numbers of colors included in each of the blocks after the second color reduction.

4. The image processing apparatus according to claim 3, further comprising a second table in which the pattern flag is associated with the layout pattern laid out within each of the block,

wherein the compression unit specifies a pattern flag indicating the layout pattern for color data included in each of the blocks after the second color reduction based on the second table and the color data for each pixel in each of the blocks after the second color reduction, and generates, as the compressed data, the color data for numbers of colors included in each of the blocks after the second color reduction and the specified pattern flag.

5. The image processing apparatus according to claim 4,

wherein the compression unit, based on the color data for each pixel in each of the blocks after the second color reduction, specifies the pattern flag indicating the layout pattern of color data included in each of the blocks after the second color reduction,

selects color data corresponding to a pixel at a pre-determined position of each of the block after the second color reduction as color data of a first color,

selects color data of a second or subsequent color corresponding to the layout pattern defined by the specified pattern flag if there are two or more colors included in each of the block after the second color reduction, and

generates as the compressed data, the specified pattern flag, the first color data, and the second or subsequent color data.

6. The image processing apparatus according to claim 5,

wherein, if each of the block is a block of 2×

2 pixels, the pixel at the pre-determined position is either a upper-left pixel or a lower-right pixel in each of the block.

7. The image processing apparatus according to claim 1,

wherein, if the first table is such that the plurality of thresholds are associated with the amount of data to be reduced,

the generation unit generates the first table by successively processing each of the blocks divided by the division unit, for each of the blocks to undergo said processing, not increasing a counter corresponding to a threshold index less than a first difference when the first color reduction is from (M×

N) colors to (M×

N−

1) colors, increasing by K (where K=1, 2, . . . M×

N−

2) a counter corresponding to a threshold index that is at least a K-th difference when the first color reduction is from (M×

N) colors to (M×

N−

K) colors and less than a (K+1)-th difference when the first color reduction is from (M×

N) colors to (M×

N−

K−

1) colors, and increasing by (M×

N−

1) a counter corresponding to a threshold index that is at least an (M×

N−

1)-th difference when the first color reduction is from (M×

N) colors to one color.

8. The image processing apparatus according to claim 1,

wherein, if the first table is such that the plurality of thresholds are associated with the amount of data to be remained,

the generation unit generates the first table by successively processing each of the blocks divided by the division unit, for each of the blocks to undergo said processing, increasing by (M×

N) a counter corresponding to a threshold index less than a first difference when the first color reduction is from (M×

N) colors to (M×

N−

1) colors, increasing by (M×

N−

K) (where K=1, 2, . . . M×

N−

2) a counter corresponding to a threshold index that is at least a K-th difference when the first color reduction is from (M×

N) colors to (M×

N−

K) colors and less than a (K+1)-th difference when the first color reduction is from (M×

N) colors to (M×

N−

K−

1) colors, and increasing by 1 a counter corresponding to a threshold index that is at least an (M×

N−

1)-th difference when the first color reduction is from (M×

N) colors to one color.

9. The image processing apparatus according to claim 1,

further comprising a tile division unit for dividing the image data inputted by the input unit into tiles,

wherein the division unit further divides the image data divided into the tiles by the tile division unit into the blocks.

10. A compression method, comprising:

an input step of inputting image data;

a division step of dividing the image data inputted by the input unit into blocks, each of the blocks has M×

N pixels (where M and N are integers greater than or equal to

     2);

a color reduction step of performing first reduction of the number of colors of each of the blocks divided from M×

N to each number of colors, the each number of colors is from one to (M×

N−

1);

an acquisition step of acquiring differences between pixel values for each of the blocks after the first color reduction in the color reduction step, and a pixel value for each of the blocks before the first color reduction;

a generation step of generating a first table in which a plurality of thresholds are associated with either the amount of data to be reduced or the amount of data to be remained, based on the differences acquired by the acquisition unit;

a specifying step of specifying a threshold at which the amount of data after compression of the image data drops to or below a target data amount, based on the first table; and

a compression step of performing second reduction of the number of colors of each of the blocks based on the threshold specified in the specifying step, and generating compressed data based on the image data after the second color reduction.

11. A computer-readable storage medium that stores programs for causing a computer to function as:

a division unit configured to divide image data into blocks, each of the blocks has M×

N pixels (where M and N are integers greater than or equal to

     2);

a color-reduction unit configured to perform first reduction of the number of colors of each of the blocks from M×

N to each number of colors, the each number of colors is from one to (M×

N−

1);

an acquisition unit configured to acquire differences between pixel values for each of the blocks after the first color reduction by the color-reduction unit, and a pixel values for each of the blocks before the first color reduction;

a generation unit configured to generate a first table in which a plurality of thresholds are associated with either the amount of data to be reduced or the amount of data to be remained, based on the differences acquired by the acquisition unit;

a specifying unit configured to specify a threshold at which the amount of data after compression of the image data drops to or below a target data amount, based on the first table; and

a compression unit configured to perform second reduction of the number of colors of each of the blocks based on the threshold specified by the specifying unit, and to generate compressed data based on the image data after the second color reduction.