1. Lossless watermarking of compressed media using reversibly decodable packets
Authors：Bijan G. Mobasseri、Domenick Cinalli
Source：Signal Processing 86
Reporter：楊蹕齊

2. Outline Objective Two-way decodable packet Watermark detection
Establishing synchronization in watermarked bitstream
Experimental result
Conclusion

3. Objective
Entropy coded multimedia signals such as JPEG and MPEG-1 and 2 have little or no error resiliency.
To counter this situation, researchers have proposed using reversible VLCs (RVLC) to identify and limit error propagation.
The objective in this paper is to embed the watermark directly in the compressed bitstream of a popular compression standard, specifically MPEG-2, and restore the cover signal with no loss.

4. Two-way decodable packet
Bidirectional packet
Watermark embedding
Instant error detection and reverse decoding

5. Bidirectional packet
Define a packet P consisting of N consecutive VLCs as follows: If

6. Watermark embedding

7. Instant error detection and reverse decoding
To ensure forward detection failure right at the edge of a watermarked VLC, the decoded Cw must begin with a sequence of flag bits of length lf. The flag bits have a single job: to guarantee detection failure.

8. Watermark detection Packet length known to the decoder
Packet length unknown to the decoder-reverse flag

9. Packet length known to the decoder

10. Packet length unknown to the decoder-reverse flag

11. Establishing synchronization in watermarked bitstream
Variable length packet watermarking can only be implemented if the end-of-packet can be reliably identified.
Authors propose the following resynchronization algorithm: once the forward flag is encountered, the decoder will parse the bitstream forward and pause at every end-of-packet emulation it finds. They call these emulations potential end-of-packets (PEOPs).

13. Experimental result

15. Conclusion
Authors proposed a lossless watermarking algorithm for compressed media.
They used a two-way decodable packet, originally designed to counter channels errors, to recover watermark bits.
Decoding does not require access to the original title; thus, the algorithm is both lossless and blind.