Apparatus for Filtering an Image Obtained by Block Based Image Decompression
Block-wise compressed image data is decompressed and the decompressed data is subjected to a cascade of low pas filtering operations, in order to remove block edge artefacts and/or ringing artefacts. Selected ones of said low pass filtering operations in the cascade are skipped for selected pixel locations. The decision to skip is based on the size of the variation for the selected pixel location. An increasing number of filtering operations in the cascade is skipped with increasing size of the variations.
1 . An image handling apparatus for handling block-wise compressed image data, the apparatus comprising an input for receiving a decompressed image, a filtering circuit ( 14 a - b , 16 a - b , 18 a - b ) coupled to the input and arranged to perform at least two spatial filtering operations at least partly in cascade with each other on the block-wise decompressed image data, each of the filtering operations effecting low-pass filtering at least along a same common image direction, the filtering circuit ( 14 a - b , 16 a - b , 18 a - b ) being arranged to perform tests of pixel location dependent variation in pixel values determined by the decompressed image data, and to locally skip selected ones of said low pass filtering operations for selected pixel locations in the cascade, each selected one of said low pass filtering operations being locally skipped dependent on whether a respective one of the test detects that the variation for the selected pixel location exceeds a respective minimum criterion.
2 . An image handling apparatus according to claim 1 , arranged to perform, as part of the tests for at least one of the low pass filtering operation, a first comparison of a first threshold with a size of a difference between pixel values of the selected pixel location and a neighboring pixel location of the selected pixel location and one or more second comparisons of a second threshold with spreads, or a combined spread, among pixel values of pixel locations in regions extending in mutually opposite directions from the selected pixel location and the neighboring location respectively, the at least one of the low pass filtering operation being skipped if said size and/or said spread or spreads exceed the first and second threshold respectively.
3 . An image handling apparatus according to claim 2 , wherein each of the regions comprises at least three pixel locations.
4 . An image handling apparatus according to claim 2 , wherein each of the regions comprises no more than three pixel locations.
5 . An image handling apparatus according to claim 1 , wherein different ones of the low pass filtering operations comprise a first one dimensional filtering operation and a second one dimensional filtering operation respectively, both along said common image direction.
6 . An image handling apparatus according to claim 1 , wherein the filtering circuit ( 14 a - b , 16 a - b , 18 a - b ) is arranged to perform a common test for a pair of neighboring pixel locations, the common test testing for a size of a difference between pixel values for the pair of locations and/or pixel location dependent spread determined by the decompressed image data in regions extending in mutually opposite directions from the selected pixel location and the neighboring location respectively, and to skip a selected one of the low pass filtering operations in the cascade for both locations of the pair on the basis of said common test.
7 . An image handling apparatus according to claim 6 , wherein the low pass filtering operations comprise a further low pass filtering operation that follows the selected one of the low pass filtering operations in the cascade, the further low pass filtering operation having as inputs input values of said selected one of the low pass filtering operations for pixel locations on mutually opposite sides of said pair and filtered pixel values produced by said selected one of the low pass filtering operations for said pair if the selected one of the low pass filtering operations is not skipped.
8 . A method of decompressing block-wise compressed image data, the method comprising:
performing at least two spatial filtering operations at least partly in cascade on block-wise decompressed image data, each of the filtering operations effecting low-pass filtering at least along a same common image direction,
performing tests of pixel location dependent variation in pixel values determined by the decompressed image data,
skipping selected ones of said low pass filtering operations in the cascade for selected pixel locations, each selected one of said low pass filtering operations being skipped dependent on whether a respective one of the test detects that the variation for the selected pixel location exceeds a respective minimum criterion.
9 . A method according to claim 9 , comprising, as part of performing the tests for at least one of the low pass filtering operation, performing:
a first comparison of a first threshold with a size of a difference between pixel values of the selected pixel location and a neighboring pixel location of the selected pixel location and,
one or more second comparisons of a second threshold with spreads, or a combined spread, among pixel values of pixel locations in regions extending in mutually opposite directions from the selected pixel location and the neighboring location respectively,
the at least one of the low pass filtering operation being skipped if said size and/or said spread or spreads exceed the first and second threshold respectively.
10 . A method according to claim 9 , wherein different ones of the low pass filtering operations comprise a first one dimensional filtering operation and a second one dimensional filtering operation respectively, both along a said common image direction.
11 . A method according to claim 9 , comprising:
performing a common test for a pair of neighboring pixel locations, testing for a size of a difference between pixel values for the pair of locations and/or pixel location dependent spread determined by the decompressed image data in regions extending in mutually opposite directions from the selected pixel location and the neighboring location respectively,
skipping a selected one of the low pass filtering operations in the cascade for both locations of a pair on the basis of said common test.
12 . A method according to claim 11 , wherein the low pass filtering operations comprise a further low pass filtering operation that follows the selected one of the low pass filtering operations in the cascade, the further low pass filtering operation having as inputs input values of said selected one of the low pass filtering operations for pixel locations on mutually opposite sides of said pair and filtered pixel values produced by said selected one of the low pass filtering operations for said pair if the selected one of the low pass filtering operations is not skipped.
13 . A computer program product comprising a program with instructions which, when executed by a programmable processing circuit, cause the programmable processing circuit to:
perform at least two spatial filtering operations at least partly in cascade on block-wise decompressed image data, each of the filtering operations effecting low-pass filtering at least along a common image direction,
perform tests of pixel location dependent variation in pixel values determined by the decompressed image data,
skip selected ones of said low pass filtering operations in the cascade for selected pixel locations, each selected one of said low pass filtering operations being skipped dependent on whether a respective one of the test detects that the variation for the selected pixel location exceeds a respective minimum criterion.