Method, an apparatus and a computer program product for implementing gradual decoding refresh

A method for encoding, comprising:

• receiving a sequence of pictures to be encoded;
• defining a refresh period for coded pictures, the refresh period being from a random access point picture, inclusive, to a recovery point picture, exclusive, wherein a decoded version of the recovery point picture and subsequent decoded pictures in output order are correct in content when starting decoding from the random access point picture;
• encoding a picture within the refresh period, the picture comprising a refreshed area and an unrefreshed area, wherein the encoding is constrained in a manner that a decoded version of the refreshed area is correct in content when starting decoding from the random access point picture;
• determining a value by which a size of the refreshed area increases, per picture, during the refresh period;
• selecting, based on the determined value, a number of sample columns or rows outside a conformance cropping window by which offset values of the conformance cropping window are increased to adjust a size of the conformance cropping window so that a boundary between the refreshed area and the unrefreshed area of the picture is aligned with a coding tree unit boundary, wherein a coding tree unit is characterized in one or both of:
• one or more prediction processes are disabled across the coding tree unit boundary;
• or
• the coding tree unit is a root unit for partitioning into coding blocks or transform blocks;
• and
• indicating the selected number of sample columns or rows outside the conformance cropping window in or along the bitstream.

A non-transitory computer-readable medium comprising program instructions stored thereon for performing at least the following:

• receiving a sequence of pictures to be encoded;
• defining a refresh period for coded pictures, the refresh period being from a random access point picture, inclusive, to a recovery point picture, exclusive, wherein a decoded version of the recovery point picture and subsequent decoded pictures in output order are correct in content when starting decoding from the random access point picture;
• encoding a picture within the refresh period, the picture comprising a refreshed area and an unrefreshed area, wherein the encoding is constrained in a manner that a decoded version of the refreshed area is correct in content when starting decoding from the random access point picture;
• determining a value by which a size of the refreshed area increases, per picture, during the refresh period;
• selecting, based on the determined value, a number of sample columns or rows outside a conformance cropping window by which offset values of the conformance cropping window are increased to adjust a size of the conformance cropping window so that a boundary between the refreshed area and the unrefreshed area of the picture is aligned with a coding tree unit boundary, wherein a coding tree unit is characterized in one or both of:
• one or more prediction processes are disabled across the coding tree unit boundary;
• or
• the coding tree unit is a root unit for partitioning into coding blocks or transform blocks;
• and
• indicating the selected number of sample columns or rows outside the conformance cropping window in or along the bitstream.