Only a limited number of methods have been proposed to realize heterogeneous transcoding, for example from MPEG-2 to H.263, or from H.264 to H.263. The major difficulties of transcoding a B-picture to a P-picture are that the incoming discrete cosine transform (DCT) coefficients of the B-frame are prediction errors arising from both forward and backward predictions, whilst the prediction errors in the DCT domain arising from the prediction using the previous frame alone are not available. The required new prediction errors need to be re-estimated in the pixel domain. This process involves highly complex computation and introduces re-encoding errors. We propose a new approach to convert a B-picture into a P-picture by making use of some properties of motion compensation in the DCT domain and the direct addition of DCT coefficients. We derive a set of equations and formulate the problem of how to obtain the DCT coefficients. One difficulty is that the last P-frame inside a GOP with an IBBP structure, for example, needs to be transcoded to become the last P-frame in the IPPP structure, and it has to be linked to the previous reconstructed P-frame instead of to the I-frame. We increased the speed of the transcoding process by making use of the motion activity which is expressed in terms of the correlation between pictures. The whole transcoding process is done in the transform domain, hence re-encoding errors are completely avoided. Results from our experimental work show that the proposed video transcoder not only achieves a speed-up of two to six times that of the conventional video transcoder, but it also substantially improves the quality of the video.

For a conventional downscaling video transcoder, a video server has firstly to decompress the video, perform downscaling operations in the pixel domain, and then recompress it. This is computationally intensive. However, it is difficult to perform video downscaling in the discrete cosine transform (DCT)- domain since the prediction errors of each frame are computed from its immediate past higher resolution frames. Recently, a fast algorithm for DCT domain image downsampling has been proposed to obtain the downsampled version of DCT coefficients with low computational complexity. However, there is a mismatch between the downsampled version of DCT coefficients and the resampled motion vectors. In other words, significant quality degradation is introduced when the derivation of the original motion vectors and the resampled motion vector is large. In this paper, we propose a new architecture to obtain resampled DCT coefficients in the DCT domain by using the split and merge technique. Using our proposed video transcoder architecture, a macroblock is splitted into two regions: dominant region and the boundary region. The dominant region of the macroblock can be transcoded in the DCT domain with low computational complexity and re-encoding error can be avoided. By transcoding the boundary region adaptively, low computational complexity can also be achieved. More importantly, the re-encoding error introduced in the boundary region can be controlled more dynamically. Experimental results show that our proposed video downscaling transcoder can lead to significant computational savings as well as videos with high quality as compared with the conventional approach. The proposed video transcoder is useful for video servers that provide quality service in real-time for heterogeneous clients.

In video downscaling, simply reusing the motion vectors extracted from an incoming video bitstream may not result in good quality pictures. In this paper, we propose an adaptive motion vector re-composition algorithm using two new measures: the diversity and importance measures of motion vectors. Using the importance measure, our proposed scheme manages to differentiate the most representative motion vector as a consideration to recompose a new motion vector. In addition, the diversity measure provides information for the video transcoder controlling the size of the refinement window to achieve a significant reduction of computational complexity. Experimental results show that our proposed adaptive motion vector re-composition scheme provides a high coding efficiency in terms of both quality and complexity.

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.

This paper presents a new frame-skipping transcoding approach for video combiners in multipoint video conferencing. Transcoding is regarded as a process of converting a previously compressed video bitstream into a lower bitrate bitstream. A high transcoding ratio may result in an unacceptable picture quality when the incoming video bitstream is transcoded with the full frame rate. Frame skipping is often used as an efficient scheme to allocate more bits to representative frames, so that an acceptable quality for each frame can be maintained. However, the skipped frame must be decompressed completely, and should act as the reference frame to the nonskipped frame for reconstruction. The newly quantized DCT coefficients of prediction error need to be recomputed for the nonskipped frame with reference to the previous nonskipped frame; this can create an undesirable complexity in the real time application as well as introduce re-encoding error. A new frame-skipping transcoding architecture for improved picture quality and reduced complexity is proposed. The proposed architecture is mainly performed on the discrete cosine transform (DCT) domain to achieve a low complexity transcoder. It is observed that the re-encoding error is avoided at the frame-skipping transcoder when the strategy of direct summation of DCT coefficients is employed. By using the proposed frame-skipping transcoder and dynamically allocating more frames to the active participants in video combining, we are able to make more uniform peak signal-to-noise ratio (PSNR) performance of the subsequences and the video qualities of the active subsequences can be improved significantly.

For the conventional downscaling video transcoder, the video server would be to first decompress the video, perform the downscaling operation in the pixel domain, and then recompress it. This is computational intensive. However, it is difficult to perform video downscaling in the DCT-domain since the prediction errors of each frame are computed from its immediate past higher resolution frames. Besides, the motion vector need to resample due to the size of the video is changed. Due to the mismatch of the resampled motion vector with the incoming DCT coefficients, the video transcoder need to recalculate the new DCT coefficient with lower resolution in pixel domain; this can create undesirable complexity as well as introduce re-encoding error. In this paper, we propose a new architecture to obtain the new DCT coefficients and the new motion vector by reuse the incoming motion vector and DCT coefficients. Since our proposed transcoder is mainly performing in DCT domain, low computational complexity can be achieved as well as the re-encoding can be reduced. Experimental result show that our proposed video downscaling transcoder can lead to significant computational savings as well as provide a high video quality compared to the conventional approach.

A new architecture of video combiner for multipoint video conferencing is proposed. The proposed video combiner is wavelet-based which extracts the motion activities information from the video bitstreams produced by a wavelet-based video coder. Using the progressive properties of wavelet transform, the encoded bitstream become scalable. Hence, the video quality of inactive sub-sequences can be easily adjusted in the video combiner by discarding the fine detail information bitstreams. In other words, more bits can be reallocated to the active sub-sequences to achieve a good visual quality with smooth motion. In addition, the video coder is region-based so that different wavelet kernels can be used for the foreground and the background. This setting can on one hand reduce the computational complexity significantly. On the other hand, by considering the unequal importance of various regions, a high video quality in foreground can always be guaranteed and an acceptable quality in background can be maintained even under low bitrate environments. Since the video combiner only needs to rearrange the video quality level according to their motion activities, no re-encoding process is required. Therefore, a significant computational complexity saving can be achieved as compared to the conventional video combiner using a transcoding approach. The new video combiner is then used to realize a multipoint video conferencing and some results are presented to show the improvement in performance due to our proposed architecture.

Transcoding is a key technique for reducing the bit rate of a previously compressed video signal. A high transcoding ratio may result in an unacceptable picture quality when the full frame rate of the incoming video bitstream is used. Frame skipping is often used as an efficient scheme to allocate more bits to the representative frames, so that an acceptable quality for each frame can be maintained. However, the skipped frame must be decompressed completely, which might act as a reference frame to nonskipped frames for reconstruction. The newly quantized discrete cosine transform (DCT) coefficients of the prediction errors need to be re-computed for the nonskipped frame with reference to the previous nonskipped frame; this can create undesirable complexity as well as introduce re-encoding errors. In this paper, we propose new algorithms and a novel architecture for frame-rate reduction to improve picture quality and to reduce complexity. The proposed architecture is mainly performed on the DCT domain to achieve a transcoder with low complexity. With the direct addition of DCT coefficients and an error compensation feedback loop, re-encoding errors are reduced significantly. Furthermore, we propose a frame-rate control scheme which can dynamically adjust the number of skipped frames according to the incoming motion vectors and re-encoding errors due to transcoding such that the decoded sequence can have a smooth motion as well as better transcoded pictures. Experimental results show that, as compared to the conventional transcoder, the new architecture for frame-skipping transcoder is more robust, produces fewer requantization errors, and has reduced computational complexity.

Transcoding is a process of converting a previously compressed video bitstream into a lower bit-rate bitstream. When some incoming frames are dropped for the frame-rate conversion in transcoding, the newly quantized DCT coefficients of prediction error need to be re-computed, which can create an undesirable complexity as well as introduce re-encoding error. We propose a new architecture for a frame-skipping transcoder to improve picture quality and to reduce complexity. It is observed that re-encoding error is reduced significantly when the strategy of direct summation of DCT coefficients and the error compensation feedback loop are employed. Furthermore, we propose a frame-rate control scheme which can dynamically adjust the number of skipped frames according to the incoming motion vectors and the re-encoding error due to transcoding such that the decoded sequence can have smooth motion as well as better transcoded pictures. Experimental results show that, as compared to the conventional transcoder, the new frame-skipping transcoder is more robust, produces smaller requantization errors, and has simple computational complexity.