Methods for digital image sharpening with noise amplification avoidance

1. A method for sharpening an original digital image comprising:

generating a smoothed image from the original image;

generating a residual image from the smoothed image and the original image by generating the ratio between the original image and the smoothed image, then for positive values of the difference between the rational residual image and one, multiplying the difference between the residual image values and one by a factor and adding the product to one, and for negative values of the difference between the rational residual image and one, multiplying the difference between the inverse residual image values and one by the factor, adding the product to one and dividing one by the sum;

generating a sharpened smoothed image; and

combining the residual image and the sharpened smoothed image to produce a sharpened version of the original image.

2. The method of claim 1 wherein generating a smoothed image comprises low-pass filtering the original image.

3. The method of claim 2 wherein generating a smoothed image comprises resizing the original image by downsampling the original image and then upsampling the downsampled image.

4. The method of claim 3 wherein resizing the original image by downsampling the original image and then upsampling the downsampled image comprises using one of bilinear and bicubic interpolation to produce a smoothed image having dimensions identical to that of the original image.

5. The method of claim 2 wherein generating a smoothed image comprises using at least one of spatial averaging, median, and wavelet-based filters.

6. The method of claim 2 wherein generating a smoothed image comprises performing a combination of resizing and low-pass filtering operations on the original image.

7. The method of claim 6 wherein performing a combination of resizing and low-pass filtering operations on the original image comprises performing first downsampling, then low-pass filtering, and then upsampling.

8. The method of claim 1 wherein generating a sharpened version of the smoothed image comprises applying unsharp masking to the smoothed image.

9. The method of claim 1 wherein generating a sharpened version of the smoothed image comprises applying Laplacian filtering to the smoothed image.

10. The method of claim 1 wherein generating a sharpened version of the smoothed image comprises applying sharpening order-statistic filters to the smoothed image.

11. The method of claim 1 wherein combining the residual image and the sharpened smoothed image to produce a sharpened version of the original image comprises multiplying the residual image and the sharpened smoothed image.

12. The method of claim 1 wherein combining the residual image and the sharpened smoothed image to produce a sharpened version of the original image comprises adding the residual image to the sharpened smoothed image.

13. The method of claim 1 wherein combining the residual image and the sharpened smoothed image to produce a sharpened version of the original image comprises dividing the sharpened smoothed image with the residual image.

14. The method of claim 1 wherein combining the residual image and the sharpened smoothed image to produce a sharpened version of the original image comprises subtracting the residual image from the sharpened smoothed image.

15. The method of claim 1 wherein combining the residual image and the sharpened smoothed image to produce a sharpened smoothed version of the original image comprises multiplying the residual image by a factor prior to combining the residual image and the sharpened smoothed image.

16. The method of claim 15 wherein the factor is less than one.

17. The method of claim 15 wherein the factor is more than one.