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Intra frame prediction is one of the most compression efficient tools of the H.264/AVC codec over its predecessor. In video transcoding for picture size reduction, pattern directionality of the output frames may differ from the input, affecting the H.264/AVC intra prediction modes. This paper proposes two simple algorithms for efficient mode decision by exploiting the input modes. To further improve the conversion performance two refinement methods are also proposed. One is to look at the neighboring of the selected modes and the other to look at horizontal, vertical and DC modes, as the most frequently used modes. Compared with the full rate-distortion mode decision, the proposed algorithms achieve more than 50% complexity reduction while showing a negligible PSNR penalty of only 0.1 dB and an excess bitrate of 1-6%.