Entropy-inspired directional filtering for image coding

1. A method for encoding an image block, the method comprising:

initializing values at indices of an array of angle candidates to zero, wherein a number of the indices of the array is equal to a number of angle candidates available for encoding the image block, wherein each of the indices corresponds to one of the angle candidates;

for each current pixel of a plurality of pixels of the image block:

determining an intensity difference based on a difference in intensity values of neighboring pixels of the current pixel;

estimating an angle for the current pixel by calculating a function of the intensity difference of the current pixel; and

increasing a value at an index within the array of the angle candidate that corresponds to the estimated angle;

determining a main filtering direction of the image block based on the angle candidate having the largest value in the array;

filtering the image block according to the main filtering direction to remove one or more artifacts along an oblique edge associated with the image block; and

encoding the filtered image block to an encoded image.

2. The method of claim 1 , wherein determining an intensity difference based on a difference in intensity values of neighboring pixels of the current pixel comprises:

determining a first delta value representing a difference in intensity values between a left neighboring pixel of the current pixel and a right neighboring pixel of the current pixel;

determining a second delta value representing a difference in intensity values between an above neighboring pixel of the current pixel and a below neighboring pixel of the current pixel; and

determining the intensity difference for the current pixel based on the first delta value and the second delta value.

3. The method of claim 2 , wherein increasing the value at the index within the array of the angle candidate that corresponds to the estimated angle comprises:

calculating a function of an edge strength of the current pixel along the estimated angle to determine a floating point value for the current pixel; and

increasing the value at the index of the angle candidate based on the floating point value.

4. The method of claim 3 , wherein the function calculated for the edge strength of the current pixel is one of a logarithmic function or an exponential function.

5. The method of claim 1 , wherein increasing a value at an index of the angle candidate within the array for a current pixel comprises:

increasing the value at the index of the angle candidate in the array by a first increase value; and

increasing values at neighboring indices of the index of the angle candidate in the array by a second increase value which is less than the first increase value.

6. The method of claim 1 , wherein filtering the image block according to the main filtering direction to remove the one or more oblique edges associated with the image block comprises:

determining an average of the values at one or more of the indices of the array; and

selecting a filter type based on a ratio of the largest value to the average,

wherein the image block is filtered using a filter of the selected filter type.

7. The method of claim 1 , further comprising:

generating an angular map used to indicate the main filtering direction of the image block to a decoder for decoding the encoded image.

8. An apparatus for encoding an image block, the apparatus comprising:

a memory storing instructions and an array of angle candidates having a number of indices based on a number of angle candidates available for encoding the image block; and

a processor configured to execute the instructions to:

initialize values at the indices of the array of angle candidates to zero;

for each current pixel of a plurality of pixels of the image block:

determine an intensity difference based on a difference in intensity values of neighboring pixels of the current pixel;

estimate an angle for the current pixel by calculating a function of the intensity difference of the current pixel; and

increase a value at an index within the array of the angle candidate that corresponds to the estimated angle;

determine a main filtering direction of the image block based on the angle candidate having the largest value in the array;

filter the image block according to the main filtering direction to remove one or more artifacts along an oblique edge associated with the image block; and

encode the filtered image block to an encoded image.

9. The apparatus of claim 8 , wherein the instructions to determine the intensity difference based on the difference in intensity values of the neighboring pixels of the current pixel include instructions to:

determine a first delta value representing a difference in intensity values between a left neighboring pixel of the current pixel and a right neighboring pixel of the current pixel;

determine a second delta value representing a difference in intensity values between an above neighboring pixel of the current pixel and a below neighboring pixel of the current pixel; and

determine the intensity difference for the current pixel based on the first delta value and the second delta value.

10. The apparatus of claim 9 , wherein the instructions to increase the value at the index within the array of the angle candidate that corresponds to the estimated angle instructions to:

calculate a function of an edge strength of the current pixel along the estimated angle to determine a floating point value for the current pixel; and

increase the value at the index of the angle candidate based on the floating point value.

11. The apparatus of claim 10 , wherein the function calculated for the edge strength of the current pixel is one of a logarithmic function or an exponential function.

12. The apparatus of claim 8 , wherein the instructions to increase a value at an index of the angle candidate within the array for a current pixel include instructions to:

increase the value at the index of the angle candidate in the array by a first increase value; and

increase values at neighboring indices of the index of the angle candidate in the array by a second increase value which is less than the first increase value.

13. The apparatus of claim 8 , wherein the instructions to filter the image block according to the main filtering direction to remove the one or more oblique edges associated with the image block include instructions to:

determine an average of the values at one or more of the indices of the array; and

select a filter type based on a ratio of the largest value to the average,

wherein the image block is filtered using a filter of the selected filter type.

14. The apparatus of claim 8 , wherein the instructions include instructions to:

generate an angular map used to indicate the main filtering direction of the image block to a decoder for decoding the encoded image.

15. A method for encoding an image block, the method comprising:

determining respective intensity differences for at least some pixels of the image block based on neighboring pixels of respective ones of the at least some pixels;

estimating respective angles for each of the at least some pixels based on the respective intensity differences;

determining a main filtering direction of the image block based on the estimated angles;

filtering the image block according to the main filtering direction to remove one or more artifacts along an oblique edge associated with the image block; and

encoding the filtered image block to an encoded image.

16. The method of claim 15 , wherein determining an intensity difference for a current pixel of the at least some pixels based on neighboring pixels of the current pixel comprises:

determining a first delta value representing a difference in intensities between a left neighboring pixel of the current pixel and a right neighboring pixel of the current pixel;

determining a second delta value representing a difference in intensities between an above neighboring pixel of the current pixel and a below neighboring pixel of the current pixel; and

determining the intensity difference for the current pixel based on the first delta value and the second delta value.

17. The method of claim 15 , wherein an array of angle candidates used for encoding the image block has a number of indices based on a number of angle candidates available for encoding the image block, wherein each of the indices corresponds to one of the angle candidates, wherein the method further comprises:

for a current estimated angle of the estimated angles:

determining a current angle candidate of the number of angle candidates which corresponds to the current estimated angle; and

increasing a value at an index of the current angle candidate within the array.

18. The method of claim 17 , wherein determining a main filtering direction of the image block based on the estimated angles comprises:

selecting, as the main filtering direction, an angle candidate of number of angle candidates corresponding to an index of the array having a largest value.

19. The method of claim 17 , further comprising:

initializing values at the indices of the array of angle candidates to zero before encoding the image block; and

resetting the values at the indices of the array of angle candidates to zero after encoding the image block.

20. The method of claim 17 , wherein filtering the image block according to the main filtering direction to remove the one or more oblique edges associated with the image block comprises:

determining an average of values at one or more of the indices of the array of angle candidates; and

selecting a filter type based on a ratio of the largest value to the average,

wherein the image block is filtered using a filter of the selected filter type.