Sparse signal modulation schemes encode a data channel on a host image in a manner that is robust, flexible to achieve perceptual quality constraints, and provides improved data capacity. The host image is printed by any of a variety of means to apply the image, with sparse signal, to an object. After image capture of the object, a decoder processes the captured image to detect and extract data modulated into the sparse signal. The sparse signal may incorporate implicit or explicit synchronization components, which are either formed from the data signal or are complementary to it.

The invention relates to a security element applied onto printing substrates (bank notes, securities, product packagings, identity cards/labels or other similar documents) by printing, comprising a unique identifier as primary information visible to the naked eye and secondary information protecting against copying non-visible to the naked eye. The unique identifier is typically a point code. Said secondary information is represented by a structure with the largest dimension of 2 to 40 microns, and due to printing distortions arising when the security element is applied onto a printing substrate, said secondary information is non-reconstructable from the print of the security element and a statistically analyzable inherent feature is associated to it. The invention also relates to a printed matter with such an inventive security element and to a method to check authenticity of a printed matter with the inventive security element in visible light (380-750 nm).

A document verification system may verify modification to a physical document through digital image processing. Imaging sensors capture images of a modified physical document. A digital image processing circuit measures pixel parameters for a plurality of regions of the digital images for comparison to a verification pattern to verify the document.

The present disclosure relates generally to data hiding for retail product packaging and other printed objects. One embodiment embeds an information signal in a spot color for product packaging. The spot color is screened, and overprinted with process color tint. The tint is modulated prior to overprinting with optimized signal tweaks. The optimization can include consideration of a detector spectral dependency (e.g., red and/or green illumination). Many other embodiments and combinations are described in the subject patent document.