• To mark a curve on a photograph:
•
Pick the Curve mode tool from the toolbar: 
The
application will go into NURBS Curve marking mode and the cursor will change.
• Click the mouse at the start of the curve. For an open curve (where first and last point in curve are different) you should click on a Marked Point (to be referenced an open curve must share a common Marked Point). For a closed curve you can start at any location. A curve point will be placed on the photo or if you clicked on an existing Marked Point, no new point will be placed. A dynamic line will connect the first point to the cursor.
• Move the cursor a short distance along the curve and mark a second point.
• A curve point will be place on the photo, a line will be drawn between the first and second point and a dynamic curved line through the first two points and the cursor will appear.
• Move along the curve in the photograph placing points so that the curve mark (and dynamic curve line) line up with the curve in the photo. The better the line matches the curve data in the photo the better the results will be. In other words, you need to make sure that not only do the points you place lie on the curve in the photo but the curved spline drawn between the points must also lie on the photographic curve.
• The right click menu has an option to ‘Remove Last Corner Point’ which will delete or undo the last curve point so that the curve continues from the previously defined point.
• When the curve is complete, you can mark the last point as a curve point, on top of a Marked Point, or join to the first point in the curve to make a closed curve. A curve can also be closed by using the right-click menu "Close curve" item. See the Curve Properties section for more information on open vs closed curves.
• The curve drawing will stop automatically if you close the curve, otherwise you click the right mouse button to bring up the menu and pick "End Draw" to end the current curve. The ESC key will also end curve drawing at the last marked point. Examples of an Open Curve (top curve) and a Closed Curve (curve on the wheel rim):
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