1. A method of decoding video data in uni-directional prediction, 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 a reconstructed pixels using the prediction block and the residual block, wherein prediction pixels of the prediction block is generated using an interpolation filter selected based on the motion vector, the interpolation filter is a 7-tap asymmetry filter if the motion vector indicates quarter pixel position, and the interpolation filter is a 8-tap filter if the motion vector indicates half pixel position, and wherein the quantization parameter is generated using a quantization parameter predictor and a differential quantization parameter, and the quantization parameter predictor is generated by averaging two quantization parameters of a left quantization parameter, an above quantization parameter and a previous quantization parameter if two or more quantization parameters are available.