# Changing Bezier curve control points by dragging the curve

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Hi everyone,
I have created a roading system where roads are drawn by a five control point bezier curve through the center of them and using parallel curves defining the outer edge.

As we all know the way to change a bezier curve is to move the control points but what I want to know is, is there a way to move the control points by dragging the curve?

In other words move the control points based on where the curve is being moved to.

I won't need to worry about the start and end control points because the are being handled differently but how would I figure out the other three control points?

I read online that bezier curves are not the best way to go with this but rather that I should use b-splines. What does everyone think?

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Have you looked into using clothoid splines? There is a library, Spiro, that can transform from clothoid splines (which use points on the curve) to Bezier curves.

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Have you looked into using clothoid splines? There is a library, Spiro, that can transform from clothoid splines (which use points on the curve) to Bezier curves.

Looked into Spiro and it looks too complicated. Anything other suggestions using pure maths?

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What are the constraints on each arc? It's natural to assume you specify (or match with other segments) position, tangent direction and curvature at the extremes, i.e. 8 constraints: too few for 5 control points.
In a sufficiently smart spline representation the position constraint obviously nails the first and last (on-curve) control points, the tangent and curvature constraints determine the second and second last control point (along the required tangent, at a distance that gives the correct curvature) and the middle control point doesn't affect the tangent and curvature at the extremes and it can be used to tweak the shape of the bend near the middle.

So why don't you use some kind of cubic spline instead of raw Bézier curves?

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I have looked into the cubic cplines and they look like what I need. However I can't find anything useful about how to code them in c++. Can anyone help?

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