In one embodiment, the method includes obtaining quantization information related to at least one of a first block and an adjacent second block, comparing the quantization information with a predefined constant value, and performing a filtering process based on a result from the comparing step. The filtering process includes calculating a value of at least one pixel in the first block based on pixel values of at least two pixels in the first block, and calculating a value of at least one pixel in the second block based on pixel values of at least two pixels in the second block.