A video decoding method for motion compensation performed under an inter-frame prediction to decode a target picture, the method comprising computer executable steps executed by a processor of a video decoder to implement: decoding a residual and a motion vector received from an encoder; referencing to the motion vector to retrieve reference samples from a reference picture stored in a reference picture memory, wherein the reference picture stored in the reference picture memory and the reference samples retrieved from the reference picture are both represented with a first bit depth; performing a scaling-up operation and a first fractional sample interpolation in a first direction on the retrieved reference samples to generate a first set of fractional samples represented with a second bit depth to which the first bit depth is scaled up by a scaling-up factor, wherein the second bit depth is constant and set equal to a number of bits available to represent the fractional sample, and the scaling-up factor is set equal to the second bit depth minus the first bit depth and is variable to keep the second bit depth constant and independent from a change of the first bit depth; performing a second fractional sample interpolation on the first set of fractional samples in a second direction to generate a second set of fractional samples represented with the second bit depth; deriving a prediction sample from the second set of fractional samples, wherein the prediction sample is represented with the second bit depth; scaling down and clipping the prediction sample to generate a prediction picture scaled down from the second bit depth to the first bit depth; and adding the prediction picture and the residual to reconstruct the target picture represented with the first bit depth. decoding a residual and a motion vector received from an encoder; referencing to the motion vector to retrieve reference samples from a reference picture stored in a reference picture memory, wherein the reference picture stored in the reference picture memory and the reference samples retrieved from the reference picture are both represented with a first bit depth; performing a scaling-up operation and a first fractional sample interpolation in a first direction on the retrieved reference samples to generate a first set of fractional samples represented with a second bit depth to which the first bit depth is scaled up by a scaling-up factor, wherein the second bit depth is constant and set equal to a number of bits available to represent the fractional sample, and the scaling-up factor is set equal to the second bit depth minus the first bit depth and is variable to keep the second bit depth constant and independent from a change of the first bit depth; performing a second fractional sample interpolation on the first set of fractional samples in a second direction to generate a second set of fractional samples represented with the second bit depth; deriving a prediction sample from the second set of fractional samples, wherein the prediction sample is represented with the second bit depth; scaling down and clipping the prediction sample to generate a prediction picture scaled down from the second bit depth to the first bit depth; and adding the prediction picture and the residual to reconstruct the target picture represented with the first bit depth.