Efficient rounding for deblocking

An image decoding apparatus for decoding an encoded image on a per block basis, the image decoding apparatus comprising:

• one or more processors;
• and
• storage connected to the one or more processors,
• wherein, when decoding encoded image, the one or more processors execute the steps of:
• decoding the encoded image to generate and store a reconstructed image in the storage;
• judging whether a strong or a weak filter is to be applied to a boundary between a current block included in the reconstructed image held in the storage and a neighboring block adjacent to the current block;
• calculating a linear combination of current samples in the current block and adjacent samples in the neighboring block, the current samples and the adjacent samples forming a line of samples;
• shifting the linear combination right by a predetermined number of bits;
• and
• clipping the shifted linear combination to generate a filtered sample of the filtered current block,
• wherein, in the clipping, (i) the shifted linear combination is clipped within a first range controlled by a first clipping parameter when the strong filter is to be applied and (ii) the shifted linear combination is clipped within a second range controlled by a second clipping parameter, the first clipping parameter and the second clipping parameter being dependent on a strength of the boundary and the first range and the second range having a different range from each other, and
• the first clipping parameter and the second clipping parameter are respectively determined using a quantization parameter.