A flag parameter in a digital image decoding is calculated. For a macroblock consisting of M×N blocks, a first operation is performed on M block along a first edge to obtain M first parameters, and a second operation is performed on N blocks along a second edge to obtain N second parameters. The first and second parameters are stored into corresponding locations in a first and a second buffer array. Then a flag parameter corresponding to a given block is calculated according to corresponding values stored in the first and second buffer arrays. Calculation for all of the M×N blocks is performed in the order that neighboring left and upper blocks next to the give block is processed prior to the given block.