Method and apparatus for sharpening an image by scaling elements of a frequency-domain representation

• US 6,421,468 B1
• Filed: 01/06/1999
• Issued: 07/16/2002
• Est. Priority Date: 01/06/1999
• Status: Expired due to Term

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Bhaskaran, Vasudev, Ratnakar, Viresh
• Primary Examiner(s): Lee, Thomas D.
• Assistant Examiner(s): BRINICH, STEPHEN M
• Application Number:
  US09/226,370
• Time in Patent Office: 1,287 Days
• Field of Search: 382/254-255, 382/260-269, 382/190-191
• US Class Current: 382/254
• CPC Class Codes: G06T 2207/20192 Edge enhancement; Edge preservation
• CPC Class Codes: G06T 3/4084 in the transform domain, e.g. fast Fourier transform [FFT] domain scaling
• CPC Class Codes: G06T 5/10 using non-spatial domain filtering
• CPC Class Codes: G06T 5/73 Deblurring; Sharpening
• CPC Class Codes: H04N 1/4092 Edge or detail enhancement
• CPC Class Codes: H04N 5/208 for compensating for attenuation of high frequency components, e.g. crispening, aperture distortion correction

Abstract

An original image is sharpened by obtaining a first frequency-domain representation of the original image, selecting one or more elements from this first representation based on one more criteria such as element magnitude and frequency, scaling the selected elements according to one or more scale factors, and forming a second frequency-domain representation by combining the scaled selected elements with the unselected elements of the first representation. A sharpened reproduction of the original image may be generated by applying an inverse transform to the second frequency-domain representation. A technique for deriving the value of the one or more scale factors is also discussed.

Claims

1. A method for enhancing a reproduction of an image that comprises performing the acts of:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more, but less than all, elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

generating a second signal conveying the second frequency-domain representation.

2. A method according to claim 1 that comprises applying an inverse orthogonal transform to the second signal to generate an output signal conveying the enhanced reproduction of the image in a spatial-domain.

3. A method according to claim 1 wherein the selected elements are scaled by a plurality of scale factors that vary as a function of frequency.

4. A method for enhancing a reproduction of an image that comprises performing the acts of:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

generating a second signal conveying the second frequency-domain representation;

wherein at least some elements of the first frequency-domain representation convey a measure of intensity and the selected elements correspond to frequencies of those elements that have an intensity greater than an intensity threshold.

5. A method according to claim 4 wherein the intensity threshold varies as a function of frequency.

6. A method for enhancing a reproduction of an image that comprises performing the acts of:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

generating a second signal conveying the second frequency-domain representation;

wherein the selected elements represent multiple colors in the image.

7. A method according to claim 6 that comprises deriving a third frequency-domain representation from the first signal, wherein the selected elements in the first frequency-domain representation correspond to elements in the third frequency-domain representation that have a value greater than a threshold.

8. A method according to claim 7 wherein the threshold varies as a function of frequency.

9. A method for enhancing a reproduction of an image that comprises performing the acts of:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected;

generating a second signal conveying the second frequency-domain representation; and

applying an entropy decoder to the first signal to obtain the first frequency-domain representation, wherein the first signal conveys the image in a form having a lower entropy than the first frequency-domain representation that is obtained therefrom.

10. An apparatus for generating an output signal representing an enhanced image that comprises:

means for receiving a first signal and for obtaining therefrom a first frequency-domain representation of the image;

means for selecting one or more, but less than all, elements of the first frequency-domain representation according to one or more criteria;

means for scaling the selected elements by a scale factor;

means for forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

means for generating a second signal conveying the second frequency-domain representation.

11. An apparatus according to claim 10 that comprises means for applying am inverse orthogonal transform to the second signal to generate an output signal conveying the enhanced reproduction of the image in a spatial-domain.

12. An apparatus according to claim 10 wherein the selected elements are scaled by a plurality of scale factors that vary as a function of frequency.

13. An apparatus for generating an output signal representing an enhanced image that comprises:

means for receiving a first signal and for obtaining therefrom a first frequency-domain representation of the image;

means for selecting one or more elements of the first frequency-domain representation according to one or more criteria;

means for scaling the selected elements by a scale factor;

means for forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

means for generating a second signal conveying the second frequency-domain representation;

wherein at least some elements of the first frequency-domain representation convey a measure of intensity and the selected elements correspond to frequencies of those elements that have an intensity greater than an intensity threshold.

14. An apparatus according to claim 13 wherein the intensity threshold varies as a function of frequency.

15. An apparatus for generating an output signal representing an enhanced image that comprises:

means for receiving a first signal and for obtaining therefrom a first frequency-domain representation of the image;

means for selecting one or more elements of the first frequency-domain representation according to one or more criteria;

means for scaling the selected elements by a scale factor;

means for forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

means for generating a second signal conveying the second frequency-domain representation;

wherein the selected elements represent multiple colors in the image.

16. An apparatus according to claim 15 that comprises means for deriving a third frequency-domain representation from the first signal, wherein the selected elements in the first frequency-domain representation correspond to elements in the third frequency-domain representation that have a value greater than a threshold.

17. An apparatus according to claim 16 wherein the threshold varies as a function of frequency.

18. An apparatus for generating an output signal representing an enhanced image that comprises:

means for receiving a first signal and for obtaining therefrom a first frequency-domain representation of the image;

means for selecting one or more elements of the first frequency-domain representation according to one or more criteria;

means for scaling the selected elements by a scale factor;

means for forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected;

means for generating a second signal conveying the second frequency-domain representation; and

means for applying an entropy decoder to the first signal to obtain the first frequency-domain representation, wherein the first signal conveys the image in a form having a lower entropy than the first frequency-domain representation that is obtained therefrom.

19. A medium readable by a device embodying a program of instructions for execution by said device to perform a method for generating an output signal representing an enhanced image, wherein the method comprises:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more, but less than all, elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation the are not selected; and

generating a second signal conveying the second frequency-domain representation.

20. A medium according to claim 19 that comprises applying an inverse orthogonal transform to the second signal to generate an output signal conveying the enhanced reproduction of the image in a spatial-domain.

21. A medium according to claim 19 wherein the selected elements are scaled by a plurality of scale factors that vary as a function of frequency.

22. A medium readable by a device embodying a program of instructions for execution by said device to perform a method for generating an output signal representing an enhanced image, wherein the method comprises:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

generating a second signal conveying the second frequency-domain representation;

wherein at least some elements of the first frequency-domain representation convey a measure of intensity and the selected elements correspond to frequencies of those elements that have an intensity greater than an intensity threshold.

23. A medium according to claim 22 wherein the intensity threshold varies as a function of frequency.

24. A medium readable by a device embodying a program of instructions for execution by said device to perform a method for generating an output signal representing an enhanced image, wherein the method comprises:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected; and

generating a second signal conveying the second frequency-domain representation;

wherein the selected elements represent multiple colors in the image.

25. A medium according to claim 24 that comprises deriving a third frequency-domain representation from the first signal, wherein the selected elements in the first frequency-domain representation correspond to elements in the third frequency-domain representation have a value greater than a threshold.

26. A medium according to claim 25 wherein the threshold varies as a function of frequency.

27. A medium readable by a device embodying a program of instructions for execution by said device to perform a method for generating an output signal representing an enhanced image, wherein the method comprises:

receiving a first signal and obtaining therefrom a first frequency-domain representation of the image;

selecting one or more elements of the first frequency-domain representation according to one or more criteria;

scaling the selected elements by a scale factor;

forming a second frequency-domain representation of the enhanced reproduction of the image by combining the scaled selected elements with elements of the first frequency-domain representation that are not selected;

generating a second signal conveying the second frequency-domain representation; and

applying an entropy decoder to the first signal to obtain the first frequency-domain representation, wherein the first signal conveys the image in a form having a lower entropy than the first frequency-domain representation that is obtained therefrom.

28. A image processing system for generating an enhanced reproduction of an original image that comprises:

an input device comprising first processing circuitry coupled to an input apparatus and coupled to a first terminal, wherein the first processing circuitry receives from the input apparatus an image signal representing the original image, transforms the image signal into a first frequency-domain representation of the original image, and generates at the first terminal a first signal that conveys the first frequency-domain representation; and

an output device comprising second processing circuitry coupled to the first terminal and coupled to an output apparatus, wherein the second processing circuitry receives the first signal from the first terminal, obtains therefrom the first frequency-domain representation, selects one or more, but less than all, elements in the first frequency-domain representation, forms a second frequency-domain representation of the original image by scaling the selected elements, transforms the second frequency-domain representation into a spatial-domain representation of the original image, and causes the output apparatus to present an enhanced reproduction of the original image in response to the spatial-domain representation.

29. An image processing system according to claim 28 wherein the second processing circuitry selects elements representing a frequency higher than a frequency threshold.

30. An image processing system for generating an enhanced reproduction of an original image that comprises:

an input device comprising first processing circuitry coupled to an input apparatus and coupled to a first terminal, wherein the first processing circuitry receives from the input apparatus an image signal representing the original image, transforms the image signal into a first frequency-domain representation of the original image, and generates at the first terminal a first signal that conveys the first frequency-domain representation; and

an output device comprising second processing circuitry coupled to the first terminal and coupled to an output apparatus, wherein the second processing circuitry receives the first signal from the first terminal, obtains therefrom the first frequency-domain representation, selects one or more elements in the first frequency-domain representation, forms a second frequency-domain representation of the original image by scaling the selected elements, transforms the second frequency-domain representation into a spatial-domain representation of the original image, and causes the output apparatus to present an enhanced reproduction of the original image in response to the spatial-domain representation;

wherein the first processing circuitry reduces the information capacity requirements of the first signal and the second processing circuitry obtains the first frequency-domain representation by expanding the capacity requirements of information conveyed in the first signal.

31. An image processing system for generating an enhanced reproduction of an original image that comprises:

an input device comprising first processing circuitry coupled to an input apparatus and coupled to a first terminal, wherein the first processing circuitry receives from the input apparatus an image signal representing the original image, transforms the image signal into a first frequency-domain representation of the original image, and generates at the first terminal a first signal that conveys the first frequency-domain representation; and

an output device comprising second processing circuitry coupled to the first terminal and coupled to an output apparatus, wherein the second processing circuitry receives the first signal from the first terminal, obtains therefrom the first frequency-domain representation, selects one or more elements in the first frequency-domain representation, forms a second frequency-domain representation of the original image by scaling the selected elements, transforms the second frequency-domain representation into a spatial-domain representation of the original image, and causes the output apparatus to present an enhanced reproduction of the original image in response to the spatial-domain representation;

wherein the second processing circuitry selects elements having a magnitude greater than a magnitude threshold.