Watermark generation, which is also called watermark preprocessing, randomly scrambles watermark information in a video asset to enhance the security of the watermarking algorithm. Watermarking in the compressed domain is often preferred over watermarking in the spatial domain since decompressing and then recompressing the entire video data might not always be feasible due to high storage capacity requirements. Reencoding an MPEG stream also significantly increases the processing time, sometimes even rendering it unsuitable for real-time applications where embedding processes go parallel to compression.
However, some constraints in compressed-domain video watermarking are the increase in computational complexity and restriction in the amount of watermarking information (going beyond the capacity of watermarking information might enhance the bit-rate of the watermarked video). Hence, watermarking schemes are implemented during the compression of video by partially modifying the structure of compression standards, such as MPEG-2, MPEG-4, AVC (Advanced Video Coding)/H.264, HEVC (High Efficiency Video Coding)/H.265, etc.
MPEG is a compression scheme for standard digital television and high-definition TV, and has a coding rate ranging from 3 Mbit/s to 100 Mbit/s. MPEG-2 program streams and MPEG-1 system streams are multiplexed streams containing at least two elementary streams, i.e., an audio and a video elementary stream. An efficient watermarking scheme should be able to deal with multiple streams, which is the primary requirement in an OTT environment which pushes DRM protected content. Typically, the original stream is demultiplexed into the constituent elementary audio and video streams which are then processed to embed the watermark. The watermarked elementary streams are then multiplexed again to produce the final MPEG stream. However, this method has a high computational cost and complexity. Hence, many watermarking algorithms in the compressed domain only perform partial decoding in the embedding process and deal with the multiplexed stream itself.
H.264, which is also the 10th part of MPEG-4, is a highly compressed digital video codec standard that has a higher data compression ratio and video picture quality. Most robust watermarking schemes based on AVC/H.264 embed watermarks into DCT coefficients. H.265 is the successor of H.264 and has a better compression performance. It can transmit higher quality network video with limited bandwidth, and only needs half of the original bandwidth to play a video with the same quality.
