Reduction of moiré in screened images using hierarchical edge detection and adaptive-length averaging filters

• US 6,233,060 B1
• Filed: 09/23/1998
• Issued: 05/15/2001
• Est. Priority Date: 09/23/1998
• Status: Expired due to Fees

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Shu, Joseph, Bhattacharjya, Anoop, Lin, Tsung-Nan
• Primary Examiner(s): ROGERS, SCOTT A
• Application Number:
  US09/159,401
• Time in Patent Office: 965 Days
• Field of Search: 382/199, 382/260, 382/261, 382/264, 382/275, 382/224, 358/454, 358/453, 358/533, 358/458, 358/463
• US Class Current: 358/1.9
• CPC Class Codes: G06T 5/20 using local operators
• CPC Class Codes: H04N 1/40075 Descreening, i.e. converting a halftone signal into a corresponding continuous-tone signal; Rescreening, i.e. combined descreening and halftoning

Abstract

In an image reproduction system, an image processor analyzes a hierarchy of image representations of different resolutions to detect edges. In regions having edges that are detected in all hierarchical representations, little or no filtering is applied to the image. In regions having no edges or edges that are detected in only the highest-resolution representation, greater amounts of filtering are applied to the image. In regions having edges detected in two or more but not all hierarchical representations, an intermediate amount of filtering is applied to the image. In preferred embodiments, two-dimensional averaging filters of varying size are applied to the image. The size of the averaging filter is selected according to the number of hierarchical representations in which an edge is detected.

Claims

1. A method for reproducing a screened image with reduced moiré

, wherein the method comprises;

analyzing a plurality of representations of the screened image to detect edges, each representation having a respective resolution,

performing edge detection analysis in each resolution independent of the other resolutions, wherein, if edges are detected in corresponding regions of the representations at all resolutions, that region is classified as containing a first type of edge, if an edge is only detected in a particular region at the first resolution, that region is classified as containing a second type of edge, if edges are only detected in corresponding regions at the first resolution and a second resolution that region is classified as containing a third type of edge,

generating a control signal for each region of the screened image, wherein the control signal is generated for a respective region based on the type of edge detected in that respective region, and

generating an output signal reproducing the screened image with reduced moiré

by filtering the screened image, wherein a respective region is filtered by an amount that varies according the control signal for the respective region.

2. A method according to claim 1 wherein the respective region is filtered by an averaging filter having a size that varies according to the control signal for the respective region.

3. A method according to claim 1 wherein a respective region is analyzed independently of other regions to generate the control signal, and a respective region is filtered independently of other regions to generate a portion of the output signal.

4. A method according to claim 1 wherein the screened image comprises dots in areas having a plurality of screen frequencies.

5. A method according to claim 1 wherein the screened image comprises dots in a plurality of colors.

6. An apparatus for receiving a first input signal conveying a screened image and, in response thereto, generating an output signal reproducing the image in a form having reduced moiré

, wherein the apparatus comprises;

a plurality of filters, each filter having a size, a filter output, and a filter input receiving the first input signal,

a plurality of input terminals, each input terminal receiving a plurality of second input signals representing the image in respective resolutions,

a plurality of edge detectors, each of the edge detectors having a detector input coupled to a respective input terminal and having a detector output,

a controller having a control output and a plurality of control inputs, each control input coupled to the detector output of a respective edge detector, the controller being adapted to perform edge detection analysis in each resolution independent of the other resolutions based on the edge detector outputs, wherein, if edges are detected in corresponding regions of the representations at all resolutions, that region is classified as containing a first type of edge, if an edge is only detected in a particular region at the first resolution, that region is classified as containing a second type of edge, if edges are only detected in corresponding regions at the first resolution and a second resolution that region is classified as containing a third type of edge,

a selector having a selector-control input, a plurality of selector inputs and a selector output, the selector-control input coupled to the control output of the controller and a respective selector input coupled to the filter output of a respective filter, and

an output terminal coupled to the selector output to convey the output signal.

7. An apparatus according to claim 6 wherein the filters are averaging filters having a respective size that is an integral power of two.

8. An apparatus according to claim 7 wherein each input terminal is coupled to the filter output of a respective averaging filter.

9. An apparatus according to claim 6 wherein each input terminal is coupled to the output of a respective resolution-reduction filter having an input receiving the input signal and having a respective low pass frequency characteristic.

10. An apparatus according to claim 6 comprising memory coupled to the detector inputs and the filter inputs, wherein the edge detectors analyze portions of the second input signals that each represent a corresponding region of the image, a respective filter is applied to a portion of the first input signal that represents a respective region of the image, and wherein the memory has a capacity that is sufficient to store the portions of the second input signals representing the corresponding region and the portion of the first input signal representing the respective region but is insufficient to store enough of the second input signals and the first input signal to represent the entire image.

11. An apparatus according to claim 6 wherein the plurality of edge detectors analyze a respective region of the image independently of other regions to detect edges, and the plurality of filters are applied to a respective region of the image independently of other regions to generate a portion of the output signal.

12. An apparatus according to claim 6 wherein the first input signal conveys dots representing the image with a plurality of screen frequencies.

13. An apparatus according to claim 6 wherein the first input signal conveys dots representing the image in a plurality of colors.

14. An apparatus for reproducing a screened image with reduced moiré

, wherein the apparatus comprises;

means for analyzing a plurality of representations of the screened image to detect edges, each representation having a respective resolution,

means for performing edge detection analysis in each resolution independent of the other resolutions, wherein, if edges are detected in corresponding regions of the representations at all resolutions, that region is classified as containing a first type of edge, if an edge is only detected in a particular region at the first resolution, that region is classified as containing a second type of edge, if edges are only detected in corresponding regions at the first resolution and a second resolution that region is classified as containing a third type of edge,

means for generating a control signal for each region of the screened image, wherein the control signal is generated for a respective region based on the type of edge detected in that respective region, and

means for generating an output signal reproducing the screened image with reduced moiré

by filtering the screened image, wherein a respective region is filtered by an amount that varies according the control signal for the respective region.

15. An apparatus according to claim 14 wherein the respective region is filtered by an averaging filter having a size that varies according to the control signal for the respective region.

16. An apparatus according to claim 14 wherein a respective region is analyzed independently of other regions to generate the control signal, and a respective region is filtered independently of other regions to generate a portion of the output signal.

17. An apparatus according to claim 14 wherein the screened image comprises dots in areas having a plurality of screen frequencies.

18. An apparatus according to claim 14 wherein the screened image comprises dots in a plurality of colors.

19. A medium readable by a machine embodying a program of instructions for execution by said machine to perform a method for reproducing a screened image with reduced moiré

, wherein said method comprises;

analyzing a plurality of representations of the screened image to detect edges, each representation having a respective resolution,

performing edge detection analysis in each resolution independent of the other resolutions, wherein, if edges are detected in corresponding regions of the representations at all resolutions, that region is classified as containing a first type of edge, if an edge is only detected in a particular region at the first resolution, that region is classified as containing a second type of edge, if edges are only detected in corresponding regions at the first resolution and a second resolution that region is classified as containing a third type of edge,

generating a control signal for each region of the screened image, wherein the control signal is generated for a respective region based on the type of edge detected in that respective region, and

generating an output signal reproducing the screened image with reduced moiré

by filtering the screened image, wherein a respective region is filtered by an amount that varies according the control signal for the respective region.

20. A medium according to claim 19 wherein the respective region is filtered by an averaging filter having a size that varies according to the control signal for the respective region.

21. A medium according to claim 19 wherein a respective region is analyzed independently of other regions to generate the control signal, and a respective region is filtered independently of other regions to generate a portion of the output signal.

22. A medium according to claim 19 wherein the screened image comprises dots in areas having a plurality of screen frequencies.

23. A medium according to claim 19 wherein the screened image comprises dots in a plurality of colors.