Plain rotation, scaling, and/or translation of an image can lead to
loss of watermark synchronization and thus authentication failure.
The block based approaches in particular are sensitive to
geometric distortions due to the need for repositioning the block’s
rectangular grid. In this paper, we propose a novel block based
approach for watermarking image objects in a way that is invariant
to RST distortions. With the term “image object” we refer to
semantically contiguous parts of images that have a specific
contour boundary. The proposed approach is based on shape
information since the watermark is embedded in image blocks, the
location and orientation of which is defined by Eulerian tours that
are appropriately arranged in layers, around the object’s skeleton.
The skeleton of the object (MAT transform) is defined by the
shape of its boundary and not only is invariant to RST
transformations but is also robust to cropping, clipping and other
common deformation attacks. Results on the Caltech database are
reported, showing the advantages of the proposed scheme.