1. A method for estimating a camera pose from which an image of a real environment is viewed, comprising: (a) identifying a line feature within the image; (b) identifying values for each of four variable parameters that define the line feature in a local coordinate system; (c) estimating values for each of a plurality of parameters for the camera pose; and (d) solving a system of equations that involves the four variable parameters and the camera pose parameters by inserting the identified values for each of the four variable parameters and the estimated values for the plurality of parameters for the camera pose,