1. An image decoding method comprising: obtaining a first quantization parameter for a first plurality of macroblocks from a bitstream, where the first plurality of macroblocks are not located on a boundary of a picture; calculating a second quantization parameter for a second plurality of macroblocks in a predefined manner, where the second plurality of macroblocks are located on the boundary of the picture; and inverse-quantizing the first plurality of macroblocks of the picture using the first quantization parameter and the second plurality of macroblocks of the picture using the second quantization parameter, respectively, wherein the second quantization parameter is not obtained directly from the bitstream.