Curves > Create / Modify Curves allows free-hand drawing and manipulation of curve lines on a point or polygon object.
There may be gaps between the resulting curve line and the surface of the object, so the typical next step is Curves > Project Curves.
Compare this function to Curves > Extract Curves which is semi-automatic and which does not require subseqent use of Curves > Project Curves.)
The controls are:
Points group -
Display Points checkbox - displays all points in the underlying point cloud. Typically, check this checkbox.
Display Fold Vectors checkbox (available after Initialize) - For every point in the object, a Fold Vector illustrates the general direction of a curve line that might be generated by the Line editing mode.
Initialize button - Immediately after the dialog opens, press this button.
Editing Mode group - defines the mouse action. To activate a grayed-out Editing Mode, press Execute.
Define - each click on the object represents a new point on a curve. To Ctrl-Z to un-do previously defined points. While a curve line is being Defined, it can be Optimized.
Edit - Use the mouse to click and drag existing points on a curve. After curve lines are Defined and Edited, you can Optimize (relax) the lines so they adhere more closely to the surface.
Split/Merge - Use the mouse to insert a degree-2 vertex, or point, into a curve line. Degree-2 vertices are break points between contiguous curve lines. To remove existing degree-2 vertices, Ctrl-select them.
Detection Mode - (Curves > Extract Curves is a superior method of detecting the ideal location of curve lines.) The Line and Blocking Points radio buttons are related to automatic placement of curve lines on a polygon object.
Line - In this mode, underlying points that lie in areas of high curvature are highlighted in white. To detect curve lines automatically, click any white point. The application draws a curve line where it finds a trail of white points, and inserts control points that create the best possible fit for that curve. There are two factors that affect the line you create: Increasing the Focus instructs the application to use a larger diameter when seeking a curve. Increasing the Length instructs it to seek longer trails of white points.
Blocking Points - Blocking points are purple points that interfere with the detection of white trails, causing automatically detected curve lines to be shorter. In this mode, use the basic selection tools to select and cause certain points to become blocking points, or use the Ctrl key with basic selection tools to change blocking points to regular points. If Line mode finds a trail of white points that is longer than necessary, press Ctrl-Z to un-do the curve line, go to Blocking Points mode, select some white points to make them purple, go back to Line mode, click a white point, and see that the new curve line stops at the purple blocking points.
Curve Definition group -
Control Points integer field (available before a curve line is Executed) - allows the number of control points on a curve line to be reduced, not increased.
Tension decimal field (0.0 to 1.0; available before a curve line is Executed) - represents a trade-off between accuracy and smoothness. The higher the value, the smoother the curve line and the lower the adherence to original control points.
Focus decimal field (when Detection Mode is Line) - affects the diameter of the curve of a trail of white points that is sought.
Length decimal field (when Detection Mode is Line) - specifies the length of the trail of white points that is sought.
Optimize button (when Editing Mode is Define) - adjusts the positions of defined points to areas of higher curvature. If Define mode is being used to roughly place the control points of a curve line, press Optimize to fine-tune their positions.
Execute button - converts a curve line preview (green) to a real curve line (orange). Next, create or detect another curve line
OK button - Executes any un-executed curve line, stores all curve lines to the object, and closes the dialog.
Cancel button - terminates the dialog without saving curve lines to the object.