Robust Watermarking in Slides of Presentations by Blank Space Coloring: A New Approach

A new robust method to embed imperceptibly a watermark image into the slides of a presentation is proposed. The watermark is partitioned into blocks and embedded into the space characters existing in the slides in a repeating pseudo-random sequence. The embedding is achieved by changing the colors of the space characters into new ones which are results of encoding the contents and indices of the blocks. The embedded watermark is resilient against many common modifications on slides, including copying and pasting of slides; insertion, deletion and reordering of slides; slide design changes; and file format conversions. A security key is used during embedding and extraction of a watermark, such that if an offending presentation contains slides taken from presentations watermarked with different security keys, each watermark can be extracted reliably in turn with the respective key using a weighted voting scheme also proposed in this study. Experiments conducted in Microsoft PowerPoint confirm the feasibility of the proposed method. On average, a recognizable watermark of size 64 × 64 can be extracted from a presentation containing five watermarked slides. The proposed method is useful for various applications of information hiding involving slides, including slide copyright protection, slide authentication, covert communication through slides, etc.