System and method for a robust de-screening filter
View Patent ↗A method and apparatus for de-screening a halftone image, therefore allowing the recovery of an approximation of an original image, is disclosed. A method according to one embodiment first performs a screen conversion filter upon a scanned representation of the halftone image to produce an intermediate image. This method then performs a line smoothing filter upon the intermediate image to produce an approximation to the original image. In an alternate embodiment, the method performs a single convolution filter upon a scanned representation of the halftone image to produce an approximation to the original image. In this embodiment, the single convolution filter is equal to the resulting convolution of first performing a screen conversion filter and then performing a line smoothing filter.
1. A method for de-screening a halftone image, comprising:
performing screen conversion filtering to convert dots of a scanned representation of said halftone image to produce parallel lines of an intermediate image; and
performing line smoothing filtering upon said intermediate image to produce an output image.
2. The method of claim 1 , wherein said screen conversion filter utilizes a 3 by 3 coefficient matrix.
3. The method of claim 2 , wherein said coefficient matrix is diagonal along the lower right to upper left direction.
4. The method of claim 3 , wherein coefficients c (−1, 1) =c (1, −1) =1, and coefficient c (0, 0) =2.
5. The method of claim 2 , wherein said coefficient matrix is diagonal along the lower left to upper right direction.
6. The method of claim 3 , wherein coefficients c (−1, −1) =c (1, 1) =1, and coefficient c (0, 0) =2.
7. The method of claim 1 , wherein said line smoothing filter utilizes a 3 by 3 coefficient matrix.
8. The method of claim 7 , wherein said coefficient matrix is diagonal along the lower right to upper left direction.
9. The method of claim 8 , wherein coefficients c (−1, 1) =c (1, −1) =1, and coefficient c (0, 0) =2.
10. The method of claim 9 , wherein said coefficient matrix is diagonal along the lower left to upper right direction.
11. The method of claim 10 , wherein coefficients c (−1, −1) =c (1, 1) =1, and coefficient c (0, 0) =2.
12. The method of claim 1 , wherein said screen conversion filter passes low-frequencies, passes high-frequencies along a diagonal line from lower left to upper right, and attenuates high-frequencies away from said diagonal line.
13. The method of claim 1 , wherein said screen conversion filter passes low-frequencies, passes high-frequencies along a diagonal line from lower right to upper left, and attenuates high-frequencies away from said diagonal line.
14. The method of claim 1 , wherein said line smoothing filter passes low-frequencies, passes high-frequencies along a diagonal line from lower left to upper right, and attenuates high-frequencies away from said diagonal line.
15. The method of claim 1 , wherein said line smoothing filter passes low-frequencies, passes high-frequencies along a diagonal line from lower right to upper left, and attenuates high-frequencies away from said diagonal line.
16. A method for de-screening a halftone image, comprising:
performing single convolution filtering upon a scanned representation of said halftone image to produce an output image, wherein a result of said single convolution filter equals the combined results obtained from a convolution of a separate screen conversion filter and a line smoothing filter, wherein said single convolution filter passes low-frequencies, passes high-frequencies at a central area, and attenuates high-frequencies along a horizontal axis and a vertical axis.
17. A computer-readable medium having stored thereon sequences of instructions, the sequences of instructions including instructions which, when executed by a processor, causes the processor to perform various processing, the sequences of instructions comprising:
a first sequence to perform screen conversion filtering to convert dots of a scanned representation of said halftone image to produce parallel lines of an intermediate image; and
a second sequence to perform line smoothing filtering upon said intermediate image to produce an output image.
18. A computer-readable medium having stored thereon sequences of instructions, the sequences of instructions including instructions which, when executed by a processor, causes the processor to perform various processing, the sequences of instructions comprising:
a first sequence to perform single convolution filtering upon a scanned representation of a halftone image to produce an output image, wherein said single convolution filter equals the resulting convolution of a screen conversion filter and a line smoothing filter, wherein said single convolution filter passes low-frequencies, passes high-frequencies at a central area, and attenuates high-frequencies along a horizontal axis and a vertical axis.
19. A system for de-screening a halftone image, comprising:
a memory to store an input image from a halftone print; and
a processor to perform a screen conversion filter to convert dots of said input image and create parallel lines of an intermediate image, and to perform a line smoothing filter upon said intermediate image and create an output image.
20. A method for de-screening a halftone image, comprising:
converting dots of said halftone image into parallel lines of an intermediate image; and
smoothing said parallel lines of said intermediate image into a final image.
21. The method of claim 20 , wherein said parallel lines are at an approximately 45 degree angle with respect to a side of said intermediate image.