A method for drawing a freehand curve on a 2D interactive screen 415 in a first coordinate system 405, the curve being represented in 3D space in a second coordinate system 410, the method comprising: selecting, for display on the interactive screen, a perspective view of a 3D object 420, the object comprising features 430-438. A first point 445 is created in the first coordinate system on the interactive screen and the first point is captured in the second coordinate system as a start point for a curve. A second point 455 is created in the first coordinate system and captured in the second coordinate system as an end point for the curve. Capturing at least one of the start point and the end point comprises fixing the point to a position that represents a feature of known position in the second coordinate system. Preferably, fixing the point comprises snapping the point to a pre-existing object line.
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