1. A method of decoding video data in uni-directional prediction, the method comprising: deriving a reference picture index and a motion vector of a current prediction unit; generating a prediction block of the current prediction unit using the reference picture index and the motion vector; generating a quantized block by inverse-scanning quantized coefficient components; generating a transformed block by inverse-quantizing the quantized block using a quantization parameter; generating a residual block by inverse-transforming the transformed block; and generating reconstructed pixels using the prediction block and the residual block, wherein prediction pixels of the prediction block are generated using an interpolation filter selected based on the motion vector, the interpolation filter being a 7-tap asymmetry filter if the motion vector indicates quarter pixel position, the interpolation filter being an 8-tap filter if the motion vector indicates half pixel position, and wherein the quantization parameter is derived using a differential quantization parameter and a quantization parameter predictor, and if only one of a left quantization parameter and an above quantization parameter is available, the quantization parameter predictor is an average of a previous quantization parameter and the available one of the left quantization parameter and said above quantization parameter.