A method for drawing a freehand curve (450, 931) on a 2D interactive screen (215, 415) in a first coordinate system (205, 405, 905), wherein a pointer is used to interact with the screen (215, 415), the curve (450, 931) being represented in 3D space in a second, 3D, coordinate system (210, 410, 910), with a third dimension, which requires to be determined, wherein the curve (450, 931) is drawn on a 2D drawing plane (408, 908) positioned within the second, 3D, coordinate system (210, 410, 910), and wherein the drawing plane (408, 908) lies parallel to the plane of the interactive screen (215, 415), wherein the interactive screen (215, 415) is parallel to the x-y plane of the first coordinate system (205, 405, 905), the method comprising: selecting, for display on the interactive screen (215, 415), a perspective view of a 3D drawing object (220, 420, 920), the drawing object comprising features (220, 420, 430, 436, 920), wherein the orientation of the drawing plane (408, 908) is determined by selecting the perspective view; creating a first point (445, 925) in the first coordinate system (205, 405, 905) on the interactive screen (215, 415); capturing (310, 325, 330) the first point (445, 925) in the second, 3D, coordinate system (210, 410, 910) as a start point (446, 926) for a curve (450, 931) being drawn in the drawing plane (408, 908), wherein the capturing of the first point (445, 925) as the start point (446, 926) comprises fixing the first point (445, 925) to a position that coincides with a feature (436, 920) of the drawing object which is nearest to the first point (445, 925) in the selected perspective view, wherein the feature is an edge, a surface, or a line of the drawing object, wherein the feature (436, 920) of the drawing object which is nearest to the first point being of known position in the second, 3D, coordinate system (210, 410, 910), whereby fixing causes the coordinates of the start point (446, 926) to be determined in the second, 3D, coordinate system, and wherein the drawing plane (408, 908) is adjusted to be at the depth of the feature (436, 920) of the drawing object (220, 420, 920) which is nearest to the first point (445, 925) in the selected perspective view; creating a second point (455, 935) in the first coordinate system (205, 405, 905) on the interactive screen (215, 415); capturing (320, 335, 340) the second point (455, 935) in the second, 3D, coordinate system (210, 410, 910) as an end point (456, 936) for the curve (450, 931), wherein the capturing of the second point (455, 935) as the end point (456, 936) comprises fixing the second point (455, 935) to a position that coincides with a feature (436, 920) of the drawing object (220, 420, 920) which is nearest to the second point (455, 935) in the selected perspective view, wherein the feature is an edge, a surface, or a line of the drawing object, wherein the feature (436, 920) of the drawing object which is nearest to the second point being of known position in the second, 3D, coordinate system (210, 410, 910), whereby fixing causes the coordinates of the end point (456, 936) to be determined in the second, 3D, coordinate system. selecting, for display on the interactive screen (215, 415), a perspective view of a 3D drawing object (220, 420, 920), the drawing object comprising features (220, 420, 430, 436, 920), wherein the orientation of the drawing plane (408, 908) is determined by selecting the perspective view; creating a first point (445, 925) in the first coordinate system (205, 405, 905) on the interactive screen (215, 415); capturing (310, 325, 330) the first point (445, 925) in the second, 3D, coordinate system (210, 410, 910) as a start point (446, 926) for a curve (450, 931) being drawn in the drawing plane (408, 908), wherein the capturing of the first point (445, 925) as the start point (446, 926) comprises fixing the first point (445, 925) to a position that coincides with a feature (436, 920) of the drawing object which is nearest to the first point (445, 925) in the selected perspective view, wherein the feature is an edge, a surface, or a line of the drawing object, wherein the feature (436, 920) of the drawing object which is nearest to the first point being of known position in the second, 3D, coordinate system (210, 410, 910), whereby fixing causes the coordinates of the start point (446, 926) to be determined in the second, 3D, coordinate system, and wherein the drawing plane (408, 908) is adjusted to be at the depth of the feature (436, 920) of the drawing object (220, 420, 920) which is nearest to the first point (445, 925) in the selected perspective view; creating a second point (455, 935) in the first coordinate system (205, 405, 905) on the interactive screen (215, 415); capturing (320, 335, 340) the second point (455, 935) in the second, 3D, coordinate system (210, 410, 910) as an end point (456, 936) for the curve (450, 931), wherein the capturing of the second point (455, 935) as the end point (456, 936) comprises fixing the second point (455, 935) to a position that coincides with a feature (436, 920) of the drawing object (220, 420, 920) which is nearest to the second point (455, 935) in the selected perspective view, wherein the feature is an edge, a surface, or a line of the drawing object, wherein the feature (436, 920) of the drawing object which is nearest to the second point being of known position in the second, 3D, coordinate system (210, 410, 910), whereby fixing causes the coordinates of the end point (456, 936) to be determined in the second, 3D, coordinate system.