Photobuilder: 3D Model Acquisition from Photographs
Roberto Cipolla | Duncan Robertson

1 Camera Calibration
We must first determine internal camera parameters (such as the focal length of the lens). This information might be obtained from a database of different types of camera, or from the image itself using vanishing points.
  A vanishing point is the point in the image plane where a set of parallel lines in the world appear to meet. Three vanishing points corresponding to three orthogonal sets of parallel lines provide enough information for camera calibration.

Trinità by Masaccio (1427) illustrates the use of vanishing points	Parallel lines in the world appear to meet at a vanishing point v in the image

 Camera Position and Orientation
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Photobuilder