Image processing method and image processing apparatus

• US 5,936,684 A
• Filed: 10/29/1997
• Issued: 08/10/1999
• Est. Priority Date: 10/29/1996
• Status: Expired due to Term

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Murayama, Yasuhiko
• Primary Examiner(s): Faile, Andrew I.
• Assistant Examiner(s): BAHGI, HABTE K
• Application Number:
  US08/960,292
• Time in Patent Office: 650 Days
• Field of Search: 348/625, 348/628, 348/629, 348/630, 348/631, 348/671, 348/672, 348/673, 382/172, 382/270, 382/272, 382/274, 358/451, 358/455, 358/461, 358/466
• US Class Current: 348/673
• CPC Class Codes: H04N 5/20 Circuitry for controlling amplitude response
• CPC Class Codes: H04N 5/21 Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
• CPC Class Codes: H04N 5/66 Transforming electric information into light information scanning details H04N3/00

Abstract

A method and apparatus are provided to allow threshold values needed for the desired number of gradations to be set through a simple process when converting image data with numerous values into a low number of gradations. The desired number of gradations is achieved without shading being noticeable even for images having shading. The cumulative frequency distribution of the pixels for each brightness is determined from image data having a brightness range with designated gradations. The average brightness and standard deviation of the portion that is primarily background is determined from this cumulative frequency distribution. The first threshold value is determined on the basis of this average brightness and standard deviation, the second through n- 1st threshold values corresponding to the meaningful information are determined on the basis of this first threshold value. An n value conversion process is accomplished using these threshold values. Furthermore, this n value conversion process accomplishes n value conversion through a dither method for background pixels in order to reduce the effects of shading.

Claims

1. An image processing method wherein image data, having a brightness range in designated gradations and including at least a background and meaningful information in the background, is converted to image data in n gradations (n>

2), with n differing from said designated gradations, the method comprising the steps of;

determining the cumulative frequency distribution of the pixels for each brightness from the image data having said brightness range in the designated gradations;

determining the average brightness and standard deviation of the part wherein the background is predominant from the cumulative frequency distribution;

determining a first threshold value that is an indicator of the boundary between the background and said meaningful information on the basis of said average brightness and standard deviation;

determining second through n-1st threshold values in an area corresponding to said meaningful information on the basis of said first threshold value; and

performing an n value conversion process on said image data using said first through n-1st threshold values.

3. The image processing method according to claim 1, further comprising determining said first threshold value by determining the average brightness and standard deviation of the part wherein the background is predominant from said cumulative frequency distribution, and subtracting from said average brightness the value obtained by multiplying said standard deviation by a designated coefficient.

4. The image processing method according to claim 1, wherein the step of determining second through n-1st threshold values in the area corresponding to said meaningful information on the basis of said first threshold value includes determining the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information, dividing this total of the cumulative frequencies by n-1 and setting respective threshold values as the brightness value for each cumulative frequency obtained by dividing by n-1.

5. The image processing method according to claim 4, wherein the step of dividing the total of said cumulative frequencies by n-1 includes dividing the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information into n-1 equal parts.

6. The image processing method according to claim 4, wherein the step of dividing the total of said cumulative frequencies by n-1 includes dividing the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information into n-1 unequal parts with weighting.

8. The image processing method according to claim 1, wherein the meaningful information in said background is line drawing information.

17. The image processing method according to claim 8, wherein the meaningful information in said background is at least one of characters or a diagram.

2. An image processing method wherein image data, having a brightness range in designated gradations and including at least a background and meaningful information existing in the background, is converted to image data in n gradations (n>

2), with n differing from said designated gradations, the method comprising the steps of;

determining the cumulative frequency distribution of the pixels for each brightness from the image data having said brightness range in the designated gradations;

determining the average brightness and standard deviation of the part wherein the background is predominant from the cumulative frequency distribution;

determining a first threshold value that is an indicator of the boundary between the background and said meaningful information on the basis of the average brightness and standard deviation;

determining second through n-1st threshold values corresponding to said meaningful information on the basis of the first threshold value; and

determining whether there are effects from shading on the basis of a brightness value of the pixel being processed;

determining for pixels for which it is determined that there are effects from shading, whether that pixel is in the background or the meaningful information,

performing, when a background pixel, an n value conversion process through a gradation process having two-dimensional gradations and,

performing, when the pixel is one including the meaningful information, an n value conversion process using said first through n-1st threshold values.

7. The image processing method according to claim 2, wherein the gradation process having two-dimensional gradations uses the dither method.

9. An image processing apparatus wherein image data, having a brightness range in designated gradations and including at least a background and meaningful information in the background, is converted to image data in n gradations (n>

2), with n differing from said designated gradations, comprising;

a parameter computation means for determining the cumulative frequency distribution of the pixels for each brightness from the image data having said brightness range in the designated gradations, and for determining the average brightness and standard deviation of the part wherein the background is predominant from the cumulative frequency distribution;

a threshold value computation means for determining a first threshold value, that is an indicator of the boundary between the background and said meaningful information, on the basis of said average brightness and standard deviation of the part wherein the background is predominant as determined by said parameter computation means, and for determining second through n-1st threshold values corresponding to said meaningful information on the basis of said first threshold value; and

an n value conversion process means for performing an n value conversion process on said image data using the first through n-1st threshold values determined by said threshold value computation means.

11. The image processing apparatus according to claim 9, wherein said first threshold value is determined by said threshold value computation means on the basis of the average brightness and standard deviation of the part wherein the background is predominant from the cumulative frequency distribution, and the threshold value computation means subtracts from said average brightness a value obtained by multiplying the standard deviation by a designated coefficient.

12. The image processing apparatus according to claim 9, wherein determining the second through n-1st threshold values on the basis of said first threshold value, determined by said threshold value computation means, determines the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information, divides this total of the cumulative frequencies by n-1 and sets respective threshold values as the brightness value for each cumulative frequency obtained by dividing by n-1.

13. The image processing apparatus according to claim 12, wherein dividing the total of said cumulative frequencies by n-1 includes dividing the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information into n-1 equal parts.

14. The image processing apparatus according to claim 12, wherein dividing the total of said cumulative frequencies by n-1 includes dividing the total of the cumulative frequencies for each brightness in the brightness range corresponding to the meaningful information into n-1 unequal parts with weighting.

16. The image processing apparatus according to claim 9, wherein the meaningful information in said background is line drawing information.

18. The image processing apparatus according to claim 16, wherein the meaningful information in said background is at least one of characters or a diagram.

10. An image processing apparatus, wherein image data, having a brightness range in designated gradations and including at least a background and meaningful information in the background, is converted to image data in n gradations (n>

2) with n differing from said designated gradations, comprising;

a parameter computation means for determining the cumulative frequency distribution of the pixels for each brightness from the image data having said brightness range in the designated gradations, and for determining the average brightness and standard deviation of the part wherein the background is predominant from the cumulative frequency distribution;

a threshold value computation means for determining a first threshold value, that is an indicator of the boundary between the background and said meaningful information, on the basis of said average brightness and standard deviation of the part where the background is predominant, as determined by said parameter computation means, and for determining second through n-1st threshold values corresponding to said meaningful information on the basis of said first threshold value; and

an n value conversion process means for reducing the effects of shading, the n value conversion process means making a determination as to whether there are effects from shading on the basis of a brightness value of the pixel being processed, the n value conversion process means, for pixels for which it is determined that there are effects from shading, making a determination as to whether that pixel is in the background or the meaningful information, the n value conversion process means performing an n value conversion process through a gradation process having two-dimensional gradations when a background pixel, and the n value conversion process means performing an n value conversion process using said first through n-1st threshold values when the pixel is one including the meaningful information.

15. The image processing apparatus according to claim 10, wherein the gradation process having two-dimensional gradations is a gradation process using the dither method.