A security document comprising artwork including a security pattern,
characterized in that the security pattern has the form of a line
structure in which lines width or line spacing is adjusted to carry
predefined data. The artwork is generated by the modifying at least one
color of at least a part of the artwork to embed the security pattern in
the artwork. Methods for detecting the security pattern include use of a
frequency domain structure to detect the security pattern despite
rotation and scaling of the document, use of signal tiling to improve
signal detection, and use of statistical analyses to verify detection of
the security pattern.