Some implementations of the invention provide hidden static images. Some such images can only be perceived when viewed from an angle to the normal to a surface. Such images may appear as a solid color when viewed along an axis perpendicular to a surface, but may reveal a hidden image when viewed from an angle relative to that axis. The hidden image may be defined according to interfaces between areas that pass substantially the same wavelength when viewed along the axis, but which pass noticeably different wavelengths when viewed from an angle relative to that axis. The hidden images may or may not be discernable to a human observer. The hidden image may comprise a code that is not readable by an optical scanner positioned to read along an axis normal to a surface, but that is readable by an optical scanner positioned to read along another axis.