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copied!<blockquote> <p>when I move the marker across the table the Y and Z axis is updated, although all I want to be updated is the Z axis, because the height of the marker doesnt change only its distance to the camera.</p> </blockquote> <p>Only true when your camera's view direction is aligned with your Y axis (or Z axis). If the camera is not aligned with Y, it means the transform will apply a rotation around the X axis, hence modifying both the Y and Z coordinates of the marker.</p> <blockquote> <p>So my guess is I need a Transformation Matrix with which I multiply each point so that I have a new coordinate System which lies orthogonal on the table surface.</p> </blockquote> <p>Yes it is. After that, you will have 2 transforms:</p> <ol> <li>T_table to express marker's coordinates in the table referential,</li> <li>T_camera to express table coordinates in the camera referential.</li> </ol> <p>Finding T_camera from a single 2d image is hard because there's no depth information.</p> <p>This is known as the Pose problem -- it has been studied by -among others- Daniel DeMenthon. He developed a fast and robust algorithm to find the pose of an object:</p> <ul> <li>articles available on its <a href="http://www.cfar.umd.edu/~daniel/Site_2/Research.html" rel="nofollow noreferrer">research homepage</a>, section 4 "Model Based Object Pose" (and particularly "Model-Based Object Pose in 25 Lines of Code", 1995);</li> <li>code at <a href="http://www.cfar.umd.edu/~daniel/Site_2/Code.html" rel="nofollow noreferrer">the same place</a>, section "POSIT (C and Matlab)".</li> </ul> <p>Note that the OpenCv library offers an implementation of the DeMenthon's algorithm. This library also offers a convenient and easy-to-use interface to grab images from a webcam. It's worth a try: <a href="http://opencv.willowgarage.com/wiki/" rel="nofollow noreferrer">OpenCv homepage</a></p>

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