Method and apparatus for processing block coded image data to reduce boundary artifacts between adjacent image blocks

A method for processing block coded image data wherein each block comprises a plurality of pixels, the method comprising the steps of:

• selecting a local pixel from a first block of pixels, said local pixel being near a border between said first block of pixels and a second block of pixels,
• selecting at least one external pixel from said second block of pixels, said external pixel being near said border, and
• modifying said selected local pixel to reduce the difference between said first pixel value of said local pixel and said at least one external pixel, wherein modifying said selected pixel value comprises the steps of:
• selecting a low pass filter coefficient for each said selected local pixel value and for each said external pixel value,
• multiplying each said selected local pixel value and each said external pixel value by said corresponding selected low pass filter coefficient,
• summing the products of said multiplication step to form a low pass filtered pixel value,
• selecting an all pass filter coefficient for each said selected local pixel value and each said external pixel value,
• multiplying each said selected local pixel value and each said external pixel value by said corresponding selected all pass filter coefficient,
• summing the products of said multiplication step to form an all pass filtered pixel value, and
• combining said all pass filtered pixel value and said low pass filtered pixel value to form a smoothed pixel value.

A method for processing block coded image data wherein each block comprises a plurality of pixels, the method comprising the steps of:

• selecting a local pixel from a first block of pixels, said local pixel being near a border between said first block of pixels and a second block of pixels,
• selecting at least one external pixel from said second block of pixels, said external pixel being near said border, and
• modifying said selected local pixel to reduce the difference between said first pixel value of said local pixel and said at least one external pixel,
• detecting the approximate direction of edges, if any, in the vicinity of said border, and wherein selecting said at least one external pixel comprises the step of:
• selecting at least one external pixel which forms a line with said local pixel such that said line extends in said detected direction, wherein detecting said approximate direction comprises the step of:
• computing a first gradient value representative of the average difference between at least one pair of local border pixels in the vicinity of said border such that said at least one pair of pixels defines a line in a first direction,
• computing a second gradient value representative of the average difference between at least one pair of local border pixels in the vicinity of said border such that said at least one pair of pixels defines a line in a second direction, and
• selecting said first direction as said detected approximate direction if said first gradient is less than said second gradient.