Intra mode selection in downsizing video transcoder based on H.264

Abstract

An intra mode selection scheme is proposed in this work, which supports both downsizing transcoding and re-quantization transcoding simultaneously. In the proposal, a total number of nonzero coefficients in precoded frame is used as criterion and a thresholding method is applied to select intra macroblock mode in reencoder. To calculate this threshold, which is related to re-quantization parameter (denoted as Q r ), we propose a Th_I-Q r model which includes direct method and percentage I16MB method. In the former, an exponent model is proposed to describe relationship between the threshold and the Q r ; while in the latter, the threshold Th_I is converted into percentage of macroblocks with I16MB mode in the downsized frame (denoted as per_16), and relationship between the per_16 and the Q r is also modeled as an exponent function. Then the two exponent models are all converted into linear regression model, and least square estimation is used to estimate the parameters of the models. Furthermore, if I4MB mode is selected for one macroblock, the intra prediction modes in precoded frame are utilized to select prediction mode for every 4 3 4 block of the macroblock in downsized frame to reduce computational complexity. We compared rate distortion performance and computational complexity of the proposed method with rate-distortion optimization method. Simulation results demonstrate that on the precondition of compression performance of the proposal being close to the results of the rate-distortion optimization method, the proposed method can save up to 30 and 80% in total encoding time and mode decision time, respectively. V