Cross-component adaptive loop filter for chroma
A method of filtering an image is disclosed, the method comprising: receiving filter coefficients and first component sample values corresponding to samples surrounding a reference sample, and inputting said filter coefficients and first component sample values into a cross component filter to produce a cross component filter output; wherein said cross-component filter uses a restricted number of bits to represent said filter coefficients and/or first component sample values to produce said filter output.
1 . A method of filtering an image portion using Cross-Component Adaptive Loop Filter, the method comprising:
acquiring filter coefficients and first component sample values corresponding to samples surrounding a reference sample; and
using said filter coefficients and first component sample values for the Cross-Component Adaptive Loop Filter to produce an output of the Cross-Component Adaptive Loop Filter for a second component,
wherein the method comprises restricting a possible value of each filter coefficient,
wherein possible values of said filter coefficients are restricted to a range represented by values of powers of two defined by a restricted number of bits,
wherein the possible value of each filter coefficient is restricted to any one of zero and values of powers of two represented by the restricted number of bits,
wherein said restricted number of bits is lower than a number of bits defined by bit depth, and
the method further comprising combining the output of the Cross-Component Adaptive Loop Filter with an output of a filter corresponding to a different component.
2 . The method according to claim 1 , wherein a restricted number of bits to represent each filter coefficient and/or first component sample value is so that the filter output can be represented on 16 or fewer bits.
3 . The method according to claim 1 , wherein said first component is Luma and the different component is a Chroma component.
4 . The method according to claim 1 , wherein restricting a possible value comprises determining an absolute value for a filter coefficient from said restricted number of bits used to represent said filter coefficient.
5 . The method according to claim 1 , wherein said restricting comprises using a reduced fixed-point decimal precision of the filter coefficient value.
6 . The method according to claim 5 , wherein the reduced fixed-point decimal precision is 7 bits.
7 . The method according to claim 1 , wherein a filter operation comprises a multiplication of sample values and a filter coefficient, and said multiplication is implemented via a bitwise shift and/or combination with another filter coefficient so as to replace said multiplication operation by bitwise shift and addition operations.
8 . The method according to claim 1 , further comprising:
determining if Cross-Component Adaptive Loop Filter (CCALF) is indicated for a given component; and
decoding said filter coefficients from a bitstream.
9 . The method according to claim 1 , further comprising:
determining if Cross-Component Adaptive Loop Filter (CCALF) is indicated for a given component; and
encoding said filter coefficients into a bitstream.
10 . The method according to claim 8 , wherein said determining if CCALF is indicated for a given component comprises decoding a flag.
11 . The method according to claim 9 , wherein said determining if CCALF is indicated for a given component comprises encoding a flag into said slice header.
12 . The method according to claim 9 , wherein said CCALF coefficients are directly encoded in a parameter set.
13 . The method according to claim 12 , wherein the parameter set is an Adaptive Parameter Set (APS).
14 . A device for filtering an image portion using Cross-Component Adaptive Loop Filter, comprising:
means for acquiring filter coefficients and first component sample values corresponding to samples surrounding a reference sample; and
means for using said filter coefficients and first component sample values for the Cross-Component Adaptive Loop Filter to produce an output of the Cross-Component Adaptive Loop Filter for a second component,
wherein the device is configured to restrict a possible value of each filter coefficient,
wherein possible values of said filter coefficients are restricted to a range represented by values of powers of two defined by a restricted number of bits,
wherein the possible value of each filter coefficient is restricted to any one of zero and values of powers of two represented by the restricted number of bits,
wherein said restricted number of bits is lower than a number of bits defined by bit depth, and
the device being further configured to combine the output of the Cross-Component Adaptive Loop Filter with an output of a filter corresponding to a different component.
15 . A non-transitory computer readable storage medium storing a program for causing a computer to execute a method of filtering an image portion using Cross-Component Adaptive Loop Filter, the method comprising:
acquiring filter coefficients and first component sample values corresponding to samples surrounding a reference sample; and
using said filter coefficients and first component sample values for the Cross-Component Adaptive Loop Filter to produce an output of the Cross-Component Adaptive Loop Filter for a second component,
wherein the method comprises restricting a possible value of each filter coefficient,
wherein possible values of said filter coefficients are restricted to a range represented by values of powers of two defined by a restricted number of bits,
wherein the possible value of each filter coefficient is restricted to any one of zero and values of powers of two represented by the restricted number of bits,
wherein said restricted number of bits is lower than a number of bits defined by bit depth, and
the method further comprising combining the output of the Cross-Component Adaptive Loop Filter with an output of a filter corresponding to a different component.