[solved] Addition of Bezier-curves to the Spline Tool

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wombat
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[solved] Addition of Bezier-curves to the Spline Tool

Post by wombat » Wed Jun 08, 2016 10:05 am

Hello,

I wonder if you might consider adding a new Spline Tool to create Bezier-curves. Bezier-curves are very useful for 'lofting' curves on model boat hulls and airplane fuselages, etc. in instances where, except for the start and end point, the position of points lying on a smooth curve is not known. (For reference, I currently create these curves as 'Paths' in the GIMP 2 image manipulation program, then use the image coordinates (X,Y) to reproduce the position of selected points in vector form within QCAD.)

The following quote from the GIMP 2 glossary describes the concept very nicely:

"A Bezier spline is a cubic spline which has four control points, where the first and last control points (knots or anchors) are the endpoints of the curve and the inner two control points (handles) determine the direction of the curve at the endpoints.

In the non-mathematical sense, a spline is a flexible strip of wood or metal used for drawing curves. Using this type of spline for drawing curves dates back to shipbuilding, where weights were hung on splines to bend them. The outer control points of a Bezier spline are similar to the places where the splines are fastened down and the inner control points are where weights are attached to modify the curve."

The GIMP 2 implementation of Bezier splines in the 'Paths' tool is very convenient; it would be very nice indeed to have a similar (vector) feature in QCAD.

Thank you for considering this suggestion.

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andrew
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Re: Addition of Bezier-curves to the Spline Tool

Post by andrew » Wed Jun 08, 2016 10:13 am

In QCAD, you can create a Bezier curve as a special case of a spline curve with exactly four nodes. A spline with more nodes is essentially a concatenation of bezier curves.

Are you aware of the two spline tools QCAD offers?

Spline from control points:
http://qcad.org/doc/qcad/latest/referen ... trolPoints

Spline from fit points:
http://qcad.org/doc/qcad/latest/referen ... eFitPoints

It seems to me that the Draw > Spline > Spline (Fit Points) tool might just be what you are looking for.

If this does not answer your question, please ellaborate as to what specific features you are missing, thanks.

wombat
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Joined: Sun Jun 07, 2015 12:41 pm

Re: Addition of Bezier-curves to the Spline Tool

Post by wombat » Wed Jun 08, 2016 12:03 pm

Thank you very much indeed for your quick reply. And, as you suggested, using only four control points produces the desired result.

Previously, I had tried to make my curves by moving around many anchor points (e.g. 15 to 20) on a theoretical curve, both using the Spline (Control Points) and Spline (Fit Points) tools. Doing this was like chasing my tail; moving one control point immediately affected the shape of other parts of the curve, so I had to keep adjusting many other control points to maintain the desired curve shape. I never thought of using only four points! (Lateral thinking is obviously not one of my strengths.)

Your suggestion provided a very simple solution.

Thank you!

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andrew
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Re: Addition of Bezier-curves to the Spline Tool

Post by andrew » Wed Jun 08, 2016 12:23 pm

wombat wrote:Your suggestion provided a very simple solution.
OK, good to hear.

BTW: Another noteworthy tool might be Misc > Modify > Convert Splines to Bezier Segments. This allows you to break up a longer spline into four-node Bezier segments, maintaining the tangency at connecting points.

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