Enhancing image quality in an image system

Abstract

A technique for improving image compression by pre-processing the image frames. In particular, methods for de-interlacing and noise reduction using combinations of median filters, applied both spatially and temporally, with and without motion analysis, are described.

What is claimed is: 
     
       1. A method for enhancing image quality in an image encoding system, including:
 applying a temporal median filter to corresponding pixel values of a previous digital video image, a current digital video image, and a next digital video image to create a noise-reduced digital video image; 
 comparing the difference between each corresponding pixel value of each noise-reduced digital video image and each corresponding current digital video image to a threshold value to generate a difference value; and 
 selecting, for each final pixel value for the noise-reduced digital video image, a corresponding pixel value from the current digital video image if the difference value is within a first threshold comparison range, and a corresponding pixel value from the noise-reduced digital video image if the difference value is within a second threshold comparison range. 
 
     
     
       2. A method for enhancing image quality in an image encoding system, including:
 applying a temporal median filter to corresponding pixel values of a previous digital video image, a current digital video image, and a next digital video image to create a noise-reduced digital video image; 
 comparing the difference between each corresponding pixel value of each noise-reduced digital video image and each corresponding current digital video image to a threshold value to generate a difference value; and 
 selecting, for each final pixel value for the noise-reduced digital video image, a corresponding pixel value from the current digital video image if the difference value is within a first threshold comparison range, and a corresponding pixel value from the noise-reduced digital video image if the difference value is within a second threshold comparison range, 
 wherein the threshold value is selected from the range of approximately 0.1 to approximately 0.3. 
 
     
     
       3. A method for enhancing image quality in an image encoding system, including creating a noise-reduced digital video image comprising a linear weighted sum of five terms:
 a current digital video image; 
 an average of horizontal and vertical medians of the current digital video image; 
 a thresholded temporal median; 
 an average of horizontal and vertical medians of the thresholded temporal median; and 
 a median of the thresholded temporal median and horizontal and vertical medians of the current digital video image, 
 wherein the weights of the five terms are approximately 50%, 15%, 10%, 10%, and 15%, respectively. 
 
     
     
       4. A method for enhancing image quality in an image encoding system, including creating a noise-reduced digital video image comprising a linear weighted sum of five terms:
 a current digital video image; 
 an average of horizontal and vertical medians of the current digital video image; 
 a thresholded temporal median; 
 an average of horizontal and vertical medians of the thresholded temporal median; and 
 a median of the thresholded temporal median and horizontal and vertical medians of the current digital video image, 
 wherein the weights of the five terms are approximately 35%, 20%, 22.5%, 10%, and 12.5%, respectively. 
 
     
     
       5. A method for enhancing image quality in an image encoding system, including:
 creating a noise-reduced digital video image comprising a linear weighted sum of five terms:
 a current digital video image; 
 an average of horizontal and vertical medians of the current digital video image; 
 a thresholded temporal median; 
 an average of horizontal and vertical medians of the thresholded temporal median; and 
 a median of the thresholded temporal median and horizontal and vertical medians of the current digital video image; 
 
 determining a motion vector for each n×n pixel region of the current digital video image with respect to at least one previous digital video image and at least one subsequent digital video image; 
 applying a center weighted temporal filter to each n×n pixel region of the current digital video image and corresponding motion-vector offset n×n pixel regions of the at least one previous digital video image and at least one subsequent digital video image to create a motion-compensated image; and 
 adding the motion-compensated image to the noise-reduced digital video image. 
 
     
     
       6. A method for enhancing image quality in an image encoding system, including:
 determining a motion vector for each n×n pixel region of a current digital video image with respect to at least one previous digital video image and at least one subsequent digital video image; and 
 applying a center weighted temporal filter to each n×n pixel region of the current digital video image and corresponding motion-vector offset n×n pixel regions of the at least one previous digital video image and at least one subsequent digital video image to create a motion-compensated image, 
 wherein each digital video image is a three-field-frame de-interlaced image. 
 
     
     
       7. A method for enhancing image quality in an image encoding system, including:
 determining a motion vector for each n×n pixel region of a current digital video image with respect to at least one previous digital video image and at least one subsequent digital video image; and 
 applying a center weighted temporal filter to each n×n pixel region of the current digital video image and corresponding motion-vector offset n×n pixel regions of the at least one previous digital video image and at least one subsequent digital video image to create a motion-compensated image, 
 wherein each digital video image is a thresholded three-field-frame de-interlaced image. 
 
     
     
       8. A method for enhancing image quality in an image encoding system, including:
 determining a motion vector for each n×n pixel region of a current digital video image with respect to at least one previous digital video image and at least one subsequent digital video image; and 
 applying a center weighted temporal filter to each n×n pixel region of the current digital video image and corresponding motion-vector offset n×n pixel regions of the at least one previous digital video image and at least one subsequent digital video image to create a motion-compensated image, 
 wherein the center weighted temporal filter is a three-image temporal filter having weights for each of such images of approximately 25%, 50%, and 25%, respectively. 
 
     
     
       9. A method for enhancing image quality in an image encoding system, including:
 determining a motion vector for each n×n a pixel region of a current digital video image with respect to at least one previous digital video image and at least one subsequent digital video image; and 
 applying a center weighted temporal filter to each n×n the pixel region of the current digital video image and corresponding motion-vector offset n×n pixel regions region of the at least one previous digital video image and at least one subsequent digital video image to create a motion-compensated image, 
 wherein the center weighted temporal filter is a five-image temporal filter having weights for each of such images of approximately 10%, 20%, 40%, 20%, and 10%, respectively. 
 
     
     
       10. A method for enhancing image quality in an image encoding system, including:
 applying a normal down filter to an image to create a first intermediate image; 
 applying a Gaussian up filter to the first intermediate image to create a second intermediate image; and 
 adding a weighted fraction of the second intermediate image to a selected image to create an image having reduced high frequency noise. 
 
     
     
       11. The method of  claim 10 , wherein the weighted fraction is between approximately 5% and 10% of the second intermediate image. 
     
     
       12. A method for enhancing image quality in an image system, the method comprising:
 applying a first filter to adjacent pixel values of a video image to generate a filtered luminance value for motion compensation with sub-pixel displacement, the first filter including a first negative lobe, second negative lobe, and a positive lobe disposed between the first and second negative lobes;   wherein an absolute amplitude of each of the first and second negative lobes is less than an absolute amplitude of the positive lobe.   
     
     
       13. The method of claim 12 further comprising applying a second filter to adjacent pixel values of the video image to generate a filtered chrominance value. 
     
     
       14. The method of claim 13, wherein the second filter comprises only four values. 
     
     
       15. The method of claim 13, wherein the second filter comprises a third negative lobe, fourth negative lobe, and a second positive lobe disposed between the third and fourth negative lobes. 
     
     
       16. The method of claim 12, wherein the adjacent pixel values correspond to adjacent pixels vertically aligned. 
     
     
       17. The method of claim 12, wherein the adjacent pixel values correspond to adjacent pixels horizontally aligned. 
     
     
       18. A method for enhancing image quality in an image system, the method comprising:
 applying a filter to adjacent pixel values of a video image to generate a filtered chrominance value for motion compensation with sub-pixel displacement, the filter including a first negative lobe, second negative lobe, and a positive lobe disposed between the first and second negative lobes;   wherein an absolute amplitude of each of the first and second negative lobes is less than an absolute amplitude of the positive lobe.