Adaptive interpolation filters for video coding

1. A method for encoding images in a digital video sequence for providing encoded data indicative of the video sequence, wherein an interpolation filter is used in motion compensated prediction, said method comprising:

selecting a filter type for the interpolation filter based on symmetry properties of the images,

calculating coefficient values for the interpolation filter based on the filter type, and

providing information in the encoded data indicating the filter type, the calculated coefficient values, and a tap length of the interpolation filter, wherein the tap length of the interpolation filter is indicative of a number of the coefficient values of the interpolation filter.

2. The method of claim 1 , wherein the digital video sequence comprises a sequence of video frames, and the coefficient values are calculated also based on a prediction signal representative of a difference between a video frame and a reference image.

3. The method of claim 1 , wherein the symmetry properties of the images include a statistical characteristic such that the statistical characteristic of each image is symmetric.

4. The method of claim 1 , wherein the symmetry properties of the images include a plurality of different symmetry properties.

5. The method of claim 1 , wherein each video frame is partitioned in a plurality of sections, at least one section having a different symmetry property from other sections, and wherein said interpolation filter comprises a filter for said at least one section and one or more different interpolation filters for the other sections.

6. A non-transitory computer readable storage medium having a software application stored thereon for use in encoding the digital video sequence comprising a sequence of video frames for providing the encoded data indicative of the video sequence, said software application comprising programming codes that, when executed, are capable of causing a processor to carry out the method of claim 1 .

7. An apparatus comprising:

a processor; and

memory including computer program code, the memory and the computer program configured to, with the processor, cause the apparatus at least to

select a filter type for an interpolation filter based on symmetry properties of images in a digital video sequence for providing encoded data indicative of the video sequence wherein the interpolation filter is for use in motion compensated prediction for encoding the images in the encoded data indicative of the video sequence,

calculate coefficient values for the interpolation filter based on the filter type, and

provide information in the encoded data indicating the filter type, the calculated coefficient values, and a tap length of the interpolation filter, wherein the tap length of the interpolation filter is indicative of a number of the coefficient values of the interpolation filter.

8. The apparatus of claim 7 , wherein the coefficient values are calculated also based on a prediction signal representative of a difference between a video frame in the video sequence and a reference image.

9. The apparatus of claim 7 , wherein the symmetry properties of the images include a statistical characteristic such that the statistical characteristic of each image is symmetric.

10. The apparatus of claim 7 , wherein the symmetry properties of the images include a plurality of different symmetry properties.

11. The apparatus of claim 7 , wherein each video frame in the video sequence is partitioned in a plurality of sections, at least one section having a different symmetry property from other sections, and wherein said interpolation filter comprises a filter for said at least one section and one or more different interpolation filters for the other sections.

12. A method for decoding encoded video data indicative of a digital video sequence comprising a sequence of video frames, each frame of the video sequence comprising a plurality of pixel values, said method comprising:

retrieving from the encoded video data a filter type, a set of coefficient values of an interpolation filter and an indication of a tap length of said at least one interpolation filter, the tap length of the interpolation filter indicative of a number of coefficients of the interpolation filter; and

constructing the interpolation filter based on the filter type, the set of coefficient values and the tap length, and

reconstructing the pixel values in a frame of the video sequence based on the constructed interpolation filter and the encoded video data.

13. A non-transitory computer readable storage medium having a software application stored thereon for use in decoding the encoded digital video sequence comprising a sequence of video frames for providing the encoded video data indicative of the video sequence, said software application comprising programming codes that, when executed, are capable of causing a processor to carry out the method of claim 12 .

14. An apparatus comprising:

a processor; and

memory including computer program code, the memory and the computer program code configured to, with the processor, cause the apparatus at least to

retrieve from encoded video data a filter type, a set of coefficient values of an interpolation filter and an indication of a tap length of the interpolation filter; and

construct the interpolation filter based on the filter type, the set of coefficient values of the interpolation filter and the tap length, wherein the tap length of the interpolation filter is indicative of a number of coefficients of the interpolation filter, wherein the encoded video data is indicative of a digital video sequence comprising a sequence of video frames, each frame of the video sequence comprising a plurality of pixel values, and wherein the memory and the computer program are also configured to cause the apparatus to construct the pixel values in a frame of the video sequence based on the constructed interpolation filter and the encoded video data.

15. A video coding system comprising:

an encoder for encoding images in a digital video sequence having a sequence of video frames for providing encoded video data indicative of the video sequence, and

a decoder for decoding the encoded video data, wherein

the encoder comprises:

a selection module for selecting a filter type for an interpolation filter based on symmetry properties of images in the digital video sequence for providing the encoded video data indicative of the video sequence wherein the interpolation filter is for use in motion compensated prediction for encoding the images in the encoded data indicative of the video sequence, wherein said least one interpolation filter has a tap length, wherein the tap length of the interpolation filter is indicative of a number of coefficients of the interpolation filter, and wherein said apparatus is configured to provide information indicating the tap length in the encoded data, wherein the interpolation filter is representable by a plurality of coefficient values,

a module for calculating the coefficient values based on the filter type, and

a multiplexing module for providing information in the encoded data indicating the filter type, the calculated coefficients values, and the tap length in the encoded video data; and

the decoder comprises:

a demultiplexing module for retrieving from the encoded video data, the filter type, the set of coefficient values of the interpolation filter and the tap length of the interpolation filter, and

a filter construction module for constructing the interpolation filter based on the filter type, the set of coefficient values and the tap length.

16. An electronic device having a video coding system of claim 15 .

17. The electronic device of claim 16 , comprising a mobile terminal.

18. The video coding system of claim 15 , wherein the encoded video data is indicative of the digital video sequence comprising a sequence of video frames, each frame of the digital video sequence comprising a plurality of pixel values, and wherein the decoder is configured to construct the pixel values in a frame of the digital video sequence based on the constructed interpolation filter and the encoded video data.

19. A mobile terminal comprising the video coding system of claim 18 .

20. An electronic device comprising a video coding system of claim 18 .