Fast conversion of H.264/AVC integer transform coefficients into DCT coefficients

In this paper we propose a fast method to convert H.264/AVC 4x4 Integer Transform (IT) to standard Discrete Cosine Transform (DCT for video transcoding applications. We derive the transcoding matrix for converting, simultaneously, in the transform domain, four IT 4x4 blocks into one 8 x 8 DCT block of coefficients. By exploiting the symmetry properties of the matrix, we show that the proposed conversion method requires fewer operations than its equivalent in the pixel domain. An integer matrix approximation is also proposed. The experimental results show that a negligible error is introduced, while the computational complexity can be significantly reduced.