Conversion between DCT coefficients and it coefficients in the compressed domain for H.263 to H.264 video transcoding

For transcoding a video sequence from the H.263 format to the H.264 format, it is beneficial to reuse as much information as possible in the original sequence. However, given the significant differences between the H.263 and the H.264 algorithms, transcoding is much more complex. Motivated by this, a set of operators is derived for converting the DCT coefficients to integer transform (IT) coefficients in the compressed domain. Using these operators, the proposed architecture is able to transcode the DCT coefficients to the integer transform coefficients directly. In other words, no complete decoding and reencoding processes are required. To further speed up the transcoding process, an approximation form of the DCT coefficients is proposed. Experimental results show that the proposed video transcoder can reduce substantially the amount of computation and provide transcoded videos with better quality as compared with those obtained from the conventional cascaded video transcoder. The proposed video transcoder can combine any existing fast searching algorithm for multiple reference frames and variable block sizes estimation to speed up the transcoding process.