Image processing apparatus, image processing method, and computer program product

• US 7,881,553 B2
• Filed: 12/06/2006
• Issued: 02/01/2011
• Est. Priority Date: 12/06/2005
• Status: Expired due to Fees

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Ishii, Hiroshi
• Primary Examiner(s): Bali; Vikkram
• Assistant Examiner(s): ENTEZARI, MICHELLE M
• Application Number:
  US11/636,688
• Publication Number:
  US 20070127837A1
• Time in Patent Office: 1,518 Days
• Field of Search: 382/270
• US Class Current: 382/270
• CPC Class Codes: G06V 10/28 Quantising the image, e.g. histogram thresholding for discrimination between background and foreground patterns

Abstract

An image processing apparatus includes a threshold selecting unit that selects a threshold based on an input gray-scale value of the target pixel. When a degree of flexibility is defined as representing a degree of the number of pixels that can take different output values as a result of threshold processing on the pixels with the same input gray-scale value, the threshold selecting unit selects a threshold from at least two types of thresholds associated with pixel positions and having the same period and different degrees of flexibility.

Claims

1. An image processing apparatus that receives image information and determines an output gray-scale value of the image information, the image processing apparatus comprising:

a corrected input-value calculating unit to calculate a corrected input value by assigning a predetermined weight to an error value near a target pixel of the image information and adding the error value to an input gray-scale value;

a threshold selecting unit to select a threshold based on an input gray-scale value of at least one of the target pixel of the image information and a pixel near the target pixel;

an output gray-scale value determining unit to determine an output gray-scale value based on the corrected input value calculated by the corrected input-value calculating unit and the threshold selected by the threshold selecting unit; and

an error calculating unit to calculate a difference between the output gray-scale value determined by the output gray-scale value determining unit and the corrected input value as an error value, and transmit the calculated error value to the corrected input-value calculating unit, wherein

the threshold selecting unit selects one of at least two types of threshold tables based on the input gray-scale value of at least one of the target pixel and the pixel near the target pixel and selects a threshold from selected one of the threshold tables, and wherein

the threshold tables have a same period and different degrees of flexibility, where the degree of flexibility is defined as representing a number of pixels that can take different output values as a result of threshold processing on an input image formed only with a predetermined gray-scale value.

2. The image processing apparatus according to claim 1, wherein in a low gray-scale image region of an input image, the threshold selecting unit selects one of the threshold tables with a high degree of flexibility for an input gray-scale value with a lower gray-scale level and selects one of the threshold tables with a low degree of flexibility for an input gray-scale value with a higher gray-scale level.

3. The image processing apparatus according to claim 1, wherein in a high gray-scale image region of an input image, the threshold selecting unit selects one of the threshold tables with a high degree of flexibility for an input gray-scale value with a higher gray-scale level and selects one of the threshold tables with a low degree of flexibility for an input gray-scale value with a lower gray-scale level.

4. The image processing apparatus according to claim 1, wherein when a pixel that can take different output values for same input pixel value is referred to as a flexible pixel, near an input gray-scale value where switching is made between the threshold table with a high degree of flexibility and a threshold table with a low degree of flexibility, the threshold selecting unit selects a threshold from a threshold table serving as a threshold table with a high degree of flexibility containing, in addition to flexible pixels in the threshold table with the low degree of flexibility, flexible pixels at pixel positions near pixel positions of the flexible pixels in the threshold table with the low degree of flexibility.

5. The image processing apparatus according to claim 1, wherein the threshold selecting unit selects a threshold from threshold tables with an approximately same average quantization error.

6. The image processing apparatus according to claim 5, wherein the threshold selecting unit selects a threshold from threshold tables with the average quantization error being approximately 0.

7. An image processing method in an image processing apparatus, the image processing method comprising:

first calculating including calculating a corrected input value by assigning a predetermined weight to an error value near a target pixel of received image information and adding the error value to an input gray-scale value;

selecting a threshold based on an input gray-scale value of at least one of the target pixel of the image information and a pixel near the target pixel;

determining an output gray-scale value based on the corrected input value calculated at the calculating and the threshold selected; and

second calculating including calculating a difference between the output gray-scale value determined at the determining and the corrected input value as an error value, wherein the error value is used at the first calculating, wherein

the selecting includes selecting one of at least two types of threshold tables based on the input gray-scale value of at least one of the target pixel and the pixel near the target pixel and selecting a threshold from selected one of the threshold tables, wherein

the threshold tables have a same period and different degrees of flexibility, where the degree of flexibility is defined as representing a number of pixels that can take different output values as a result of threshold processing on an input image formed only with a predetermined gray-scale value.

8. The image processing method according to claim 7, wherein in a low gray-scale image region of an input image, the selecting includes selecting one of the threshold tables with a high degree of flexibility for an input gray-scale value with a lower gray-scale level and selecting one of at the threshold tables with a low degree of flexibility for an input gray-scale value with a higher gray-scale level.

9. The image processing method according to claim 7, wherein in a high gray-scale image region of an input image, the selecting includes selecting one of at the threshold tables with a high degree of flexibility for an input gray-scale value with a higher gray-scale level and selecting one of at the threshold tables with a low degree of flexibility for an input gray-scale value with a lower gray-scale level.

10. A non-transitory computer-readable recording medium having recorded thereon a computer program that causes a computer to perform the image processing method according to claim 7.