Fiducial marker

A fiducial marker or fiducial is an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument.{{{annotations}}} A fiducial marker or fiducial is an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument. In high-resolution optical microscopy, fiducials can be used to actively stabilize the field of view. Stabilization to better than 0.1 nm is achievable. In physics, 3D computer graphics, and photography, fiducials are reference points: fixed points or lines within a scene to which other objects can be related or against which objects can be measured. Cameras outfitted with Réseau plates produce these reference marks (also called Réseau crosses) and are commonly used by NASA. Such marks are closely related to the timing marks used in optical mark recognition. Airborne geophysical surveys also use the term 'fiducial' as a sequential reference number in the measurement of various geophysical instruments during a survey flight. This application of the term evolved from air photo frame numbers that were originally used to locate geophysical survey lines in the early days of airborne geophysical surveying. This method of positioning has since been replaced by GPS, but the term 'fiducial' continues to be used as the time reference for data measured during flights. In applications of augmented reality, fiducials help resolve several problems of integration between the real world view and the synthetic images that augment it. Fiducials of known pattern and size can serve as real world anchors of location, orientation and scale. They can establish the identity of the scene or objects within the scene. For example, a fiducial printed on one page of an augmented reality popup book would identify the page to allow the system to select the augmentation content. It would also serve to moor the coordinates of the augmented content to the three dimensional location, orientation and scale of the open book, helping to create a stable and accurate fusion of real and synthetic imagery. A slightly more complex example would be multiple fiducials, each attached to an individual piece in an augmented reality board game. The appearance of markers in images may act as a reference for image scaling, or may allow the image and physical object, or multiple independent images, to be correlated. By placing fiducial markers at known locations in a subject, the relative scale in the produced image may be determined by comparison of the locations of the markers in the image and subject. In applications such as photogrammetry, the fiducial marks of a surveying camera may be set so that they define the principal point, in a process called 'collimation'. This would be a creative use of how the term collimation is conventionally understood. Some barcode readers can estimate the translation, orientation, and vertical depth of a known-size barcode relative to the barcode reader. Some sets of fiducial markers are specifically designed to allow rapid, low-latency detection of the 2D location, 2D orientation, and identity of hundreds of unique fiducial markers.For example, the WhyCon marker, WhyCode markers, 'amoeba' reacTIVision fiducials, the d-touch fiducials, or the TRIP circular barcode tags (ringcodes).