A Data Embedding Scheme for H.263 Compatible Video Coding

1. Internet Security Principal – Paper Presentation A Data Embedding Scheme for H.263 Compatible Video Coding By Cho-Ting Huang chuang12@ecs.syr.edu November 20 2001

2. Introduction • Video compression algorithm such as H.263 can compress video sequences efficiently. • The compressed information is very sensitive to transmission errors and losses caused by channel impairment. • Numerous methods have been proposed for error concealment. • Here is a method aims at obtaining good recovery performance.

3. Data Embedding at Half-Pixel Motion Estimation • How and where can we embed the data in the H.263 stream?? @ In the H.263 encoder, for every INTER mode coded macroblock(MB), an integer-pixel motion vector(MV) is as in Fig.1, half-pixel motion prediction is done as Fig.2. Fig.1 Fig.2

4. Data Embedding at Half-Pixel Motion Estimation @ If the two bits we want to embed are: bnbn+1, we match bn to MVx and bn+1 to MVy: (bnbn+1==00), MV is at A, no half-pixel prediction (bnbn+1==01), MV is selected from half-pixels 2 and 7 (bnbn+1==10), MV is selected from half-pixels 4 and 5 (bnbn+1==11), MV is selected from half-pixels 1, 3, 6 and 8 @ For video of resolution 176144, there are 99 micro- blocks of size 16-by-16 in one frame. The number of bits we can embed in a frame is equal to twice the number of INTER-mode coded macroblocks.

5. Data Embedding at Half-Pixel Motion Estimation @ The number of INTER-mode coded MBs depends on the motion activity of video sequences. • What kind of information to embed? @ Pictures Header : If a picture header information is lost, the whole picture cannot be decoded correctly and the error will propagate to the following frames. @ Motion Vector

6. The Error Recovery at Decoder • If there is no error, the decoder just works as a standard H.263 decoder. • Assuming at most one GOB(Group of Micro-blocks) data is corrupted in one frame and the round-trip delay of the network is D frame intervals, the error recovery scheme works as below(assume no other errors) : 1. The decoder sends a message to encoder with the location of the damaged GOB when errors occur at frame k.

7. The Error Recovery at Decoder 2. The motion vectors of the GOB can be recovered after frame k+1 is decoded using the data embedding scheme above. 3. Replace the residual data of the corrupted GOB by zero. The reconstructed frame is f’(n), n=k, k+1, k+2, …, k+D-1 4. The residual data, arrives with frame k+D , is used to recover frame f(k). The accumulated motion compensations are reproceeded within the affected area to get the losslessly recovered frame f(k+D-1). 5. The losslessly recovered frame f(k+D-1) is used to decode framed f(k+D).

8. Simulations • Using the base-mode H.263 on QCIF sequence Carphone coded at 10frames/s with bitrate 24kb/s and 48kb/s. @ Below shows the number of bits that can be embeded per frame. We can see in most situations the parity bits can be totally be embedded in the next frame. @ The decoder has to wait for one more frame to fully recover the MVs of the corrupted GOB, if the parity bits can’t totally be embedded in the next frame.

9. Conclusions • This method has advantages in terms of channel utilization, error recovery perfor-mance and compatibility with H.263. • Has the trade-off of coding efficiency versus robustness to error propagation when compared with the original H.263 standard.