Features

Softage H.263 codec corresponds to the Profile 0 of ITU-T Recommendation H.263 �Video coding for low bit rate communication�. Coding mode and stream format of the Profile 0 are defined in ITU-T H.263 Annex X �Profiles and Levels Definition�. Profile 0, also known as basic profile, is supposed that there is no optional modes are implemented in Soft Age release of H.263 codec.

The following distinctive features of the implemented codec are to be mentioned:

Coding with fixed bitrate. Softage H.263 encoder dynamically changes quality of several parts of coded frames in order to retain approximately the same amount of bits in each encoded frame.

Fast search algorithm of motion vectors for effective interframe compression.

There are no key frames in the compressed video stream (except the only first one). Instead of using key frames Softage H.263 encoder make several parts of image to the key part in different periods of time. Consequently, bitrate dependence of time is smoother as compared to distinctive key frames with perfect quality of key parts.

Compression is made per frame in contrast to reference implementation. It allows more effective management of compression parameters.

Time critical algorithms are implemented using Intel � MMX technology.

Since Intel � MMX technology doesn�t support integer division, Softage H.263 encoder uses specially designed algorithm of Fourier coefficients quantization which bypasses this limitation. Overall performance of quantization is in six times faster then ordinary implementation in pure C.

Performance

All performance tests were made on foreman 176x144 original video sequence. The figure below shows dependence of encoding time on frame number in the sequence. One can see that the encoding time lies between 4 and 5 milliseconds (on PC with Intel � Pentium 4 CPU with 2 GHz frequency) except some sharp peaks. Average encoding time per frame is about 5 ms and slightly varies with output video quality.

Foreman 176x144 original video sequence.

Performance of H.263 encoder.

Also quality of encoding was tested. As distortion measure of whole sequence the averaged peak signal-to-noise ratio (PSNR) was used, i.e. first PSNR for each frame n was calculated according to

,

where M is the number of samples in a frame, and o_i and c_i are the amplitudes of the original and coded frame consequently. Then the PSNR values of each frame are averaged for N frames in the sequence according to

.

All data presented on the figures below were obtained for first 200 frames of foreman original video sequence in 176x144 resolution with using fixed quantizes 31, 24, 20, 16, 12, 10, 8 and 6. Comparison with reference software was made by using data from paper Bernd Girod et al. �Comparison of H.263 and H.261 video compression standards� published in SPIE proceedings vol. CR60, Standards and Common Interfaces for Video Information Systems, October 25-26, 1995, Philadelphia, USA.

Quality dependence of bitrate for H.263 codec.

Per frame PSNR dependence for Softage H.263 implementation for foreman sequence.