* Corresponding author. Tel.: #31 15 278 3731; fax: #31 15 278 1843. E-mail address: R.L.Lagendijk@its.tudelft.nl (R.L. Lagendijk) Signal Processing: Image Communication 15 (2000) 531}544 Low-complexity rate-distortion optimal transcoding of MPEG I-frames R.L. Lagendijk*, E.D. Frimout, J. Biemond Information and Communication Theory Group, Faculty of Information Technology and Systems, Mekelweg 4, 2628 CD Delft, Delft University of Technology, Delft, The Netherlands Philips Consumer Electronics, Advanced Systems and Applications Lab, Eindhoven, The Netherlands Received 20 October 1997 Abstract The transcoding of MPEG video streams is important in a variety of situations such as in rate reduction for small bandwidth downstream channels and in extracting a fast visual playback stream for digital video cassette recorders. This paper addresses transcoding of MPEG intra-coded frames by selectively copying DCT AC coe$cients from the primary MPEG stream into the secondary MPEG stream. A rate-distortion optimal transcoding approach, as well as several computationally more attractive suboptimal approaches, is presented. The proposed solutions outperform the more traditional codeword extraction methods from literature.  2000 Elsevier Science B.V. All rights reserved. Keywords: Video compression; Transcoding; DCT; MPEG; Rate control; Rate-distortion theory 1. Introduction A typical MPEG encoder that compresses digital video for broadcasting purposes has a target bit- rate approximately between 4 and 10 Mbit/s. The choice of this rate is primarily determined by the available or allocated capacity of the terrestrial or satellite transmission link. In some cases these primary wide-band channels feed into secondary downstream channels of lower capacity. For in- stance, in a cable head-end a network operator may decide to allocate a smaller bandwidth to a speci"c video service than the bandwidth the service occu- pies on reception. Clearly this is at the advantage of providing more di!erent services on the cable net- work however at the cost of a lower quality per channel. Another example is the provision of fast visual playback facilities in digital video cassette recorders. Fast visual playback, i.e., a fast forward or reverse playback mode which displays video pictures, is particularly complicated for MPEG compressed digital video stored on a helical scan video cassette recorder [2,4,5]. One solution to fast visual playback is to store (and display on fast forward or reverse) a low bit-rate stream on the digital cassette in addition to the original MPEG bit stream. This requires the real-time extraction of a low bit-rate MPEG stream when recording the primary MPEG video bit stream. The problem of converting a compressed video stream such as MPEG into another (MPEG) 0923-5965/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 3 - 5 9 6 5 ( 9 9 ) 0 0 0 1 1 - 9