Video coding and decoding

1 . A method of decoding an image portion, the method comprising:

determining a threshold value from a header associated with the image portion,

wherein a decoding criterion is based on said threshold value, and

wherein determining the threshold value further comprises determining an index, the index corresponding to the threshold value from among a set of threshold values; and

decoding said image portion based on said decoding criterion.

2 . The method of claim 1 , further comprising decoding at least one syntax element in the header which is capable of indicating the presence of at least one threshold value in that header.

3 . The method of claim 2 , wherein the syntax element is also capable of indicating that no threshold value is transmitted and that the decoding process uses an alternative method.

4 . The method according to claim 1 , comprising skipping decoding of the corresponding syntax elements in hierarchically lower headers in the case where the syntax element indicates the presence of the threshold value.

5 . The method according to claim 1 , wherein the decoding comprises a decision to perform motion vector refinement based on the criterion.

6 . The method of claim 1 , comprising receiving the set of threshold values.

7 . The method of claim 6 , wherein the set of threshold values are transmitted in a hierarchically higher level than the header.

8 . The method of claim 6 , comprising decoding a variable indicating the number of threshold values.

9 . The method of claim 6 , wherein the set of threshold values is transmitted in a parameter set at a level relating to a plurality of image portions.

10 . The method of claim 9 , wherein the parameter set at a level relating to a plurality of image portions is a sequence parameter set.

11 . The method of claim 1 , wherein a threshold value not included in the set of threshold values is decoded from the header based on a value of the index.

12 . The method of claim 11 , wherein the value of the index is a maximum value.

13 . The method of claim 11 , further comprising adding the threshold value decoded from the header to the set of threshold values and associating this with the index.

14 . A method of encoding a header associated with a plurality of image portions, the method comprising:

determining a threshold value for each image portion, said threshold value to be used to determine a decoding criterion for said image portion;

generating an index of a threshold value from among a set of threshold values;

wherein said index is ordered based on a frequency of occurrence of the threshold values; and

encoding the index into the header.

15 . The method of claim 14 , wherein the header is a sequence parameter set (SPS).

16 . An apparatus for decoding image data from a bitstream, the apparatus comprising:

a determiner which determines a threshold value from a header associated with the image portion,

wherein a decoding criterion is based on said threshold value, and

wherein determining the threshold value further comprises determining an index, the index corresponding to the threshold value from among a set of threshold values; and

a decoding unit which decodes said image portion based on said decoding criterion.

17 . An apparatus for encoding image data into a bitstream, the apparatus comprising:

a determiner which determines a threshold value for each image portion, said threshold value to be used to determine a decoding criterion for said image portion;

a generator which generates an index of a threshold values from among a set of threshold values;

wherein said index is ordered based on a frequency of occurrence of the threshold values; and

an encoding unit which encodes the index into the header.

18 . A non-transitory computer-readable storage medium carrying a computer program comprising instructions adapted to perform, when executed in a computing device, a method of decoding an image portion, the method comprising:

determining a threshold value from a header associated with the image portion,

wherein a decoding criterion is based on said threshold value, and

wherein determining the threshold value further comprises determining an index, the index corresponding to the threshold value from among a set of threshold values; and

decoding said image portion based on said decoding criterion.

19 . A non-transitory computer-readable storage medium storing a computer program comprising instructions adapted to perform, when executed in a computing device, a method of encoding an image portion, the method comprising:

determining a threshold value for each image portion, said threshold value to be used to determine a decoding criterion for said image portion;

generating an index of a threshold value from among a set of threshold values;

wherein said index is ordered based on a frequency of occurrence of the threshold values; and

encoding the index into the header.