# Hermite Curve Tangents

Hello, and I am working on generating a path which will wind through a number of points. I have selected the Hermite Curve3 type, as it gives me the most correct rendition of the curve I am trying to create, but the programmatically setting the tangent lengths continues to be a problem. The points are tangent to circles which have been placed on a plane, and I would like some advice on how to correctly recreate the curvature of the circle they have been extracted from in the curve.

Please have a look at the following pg:

In the _createSplineSegment function (line 50) I have the center, radius, and tangent points available to interpolate the hermite tangents, but I have yet to come up with a formula which works across all cases. The distances between centers is non-uniform as are the angles between points.

Robert

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I found an article describing how to calculate tangents using the deflection angle of the two points (here), but the retrieved value requires a scale factor which causes other issues.

The formula is essentially: radius * tan(deflection / 2)

Here is an updated playground, which includes the calculation of the intersection point and the formula above.

@carolhmj might be able to help here as she worked on some paths examples previously

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Hey there! I don’t think the Simple Curves calculation is going to work exactly all the time, because the Hermite spline is cubic, not quadratic. I tried creating a simple interface to explore the effect of the tangent length on the curve between two points on a circle, and it can’t fit exactly:

Tangent to circle | Babylon.js Playground (babylonjs.com)

I think in your case, it might be better to calculate the points along the circles using trigonometric functions, and join them with the other sections.

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Hey Carol,

Thanks for looking at this, and for a viable solution to my issue. As I have the center/tan/radius already it should be easy enough to derive a set of points which represent the chord between the two points. The curve segments are used to create points for a Path3d object, and I don’t think I will lose anything to the method.

r