# Find sagitta of a cubic Bézier-described arc

I have a situation where I have an arc that was mangled (irrelevant: by c#'s GraphicsPath.AddArc() function). The original arc is guaranteed to be circular, and the new data I have describes the Bézier approximation for the arc instead. I'm not hugely up on Béziers, or complex geometry, so am hoping someone can help me!

Knowns:

• I have four points which describe a circular arc
• Points 1 and 4 are the two end points of the arc
• Points 2 and 3 are the cubic Bézier control points, i.e are tangential to the arc at each end

It looks like this:

I'm trying to find the length of the line labelled as 'x', which I understand is the Sagitta.

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From the equation of a cubic Bézier curve, the midpoint of the curve is $\frac{1}{8}(P_1 + 3 P_2 + 3 P_3 + P_4)$. $x$ is the distance from that midpoint to $\frac{1}{2}(P_1 + P_4)$, i.e. $|\frac{3}{8}(-P_1 + P_2 + P_3 - P_4)|$.

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This actually gets me closest easiest, with an added step at the end to get the length of the resulting line. Thanks! –  Geoff May 9 '11 at 16:06

First off, cubic Béziers can only approximate circles.

Anyway, get the midpoint of points 1 and 4 (the Bézier implementation you have does keep the control points around, yes?), get the slope of the segment joining points 1 and 2 (call it $F$), and the segment joining points 3 and 4 (call it $G$), get the intersection point of the line perpendicular to $F$ at point 1 and the line perpendicular to $G$ at point 4 (you still remember how to get the slope of a perpendicular?), determine the distance from that intersection point to either of points 1 or 4 (the radius of that circle), and subtract from that the distance from the intersection point to the midpoint of points 1 and 4.

For explicitness, here's a less verbal way of putting the solution. Let us set point $i$ to have coordinates $(x_i,y_i)$ in what follows:

1. "get the midpoint of points 1 and 4" $$(x_m,y_m)=\left(\frac{x_1+x_4}{2},\frac{y_1+y_4}{2}\right)$$

2. "get the slope of the segment joining points 1 and 2 (call it $F$), and the segment joining points 3 and 4 (call it $G$)" $$m_F=\frac{y_2-y_1}{x_2-x_1},\quad m_G=\frac{y_4-y_3}{x_4-x_3}$$

3. "get the intersection point of the line perpendicular to $F$ at point 1 and the line perpendicular to $G$ at point 4", i.e. solve \begin{align*} y_{int}-y_1&=-\frac{x_2-x_1}{y_2-y_1}(x_{int}-x_1) \\ y_{int}-y_4&=-\frac{x_4-x_3}{y_4-y_3}(x_{int}-x_4) \end{align*} for $(x_{int},y_{int})$

4. "determine the distance from that intersection point to either of points 1 or 4"; using point 1 for instance $$r=\sqrt{(x_{int}-x_1)^2+(y_{int}-y_1)^2}$$

5. "subtract from that the distance from the intersection point to the midpoint of points 1 and 4."

$$\text{sagitta}=r-\sqrt{(x_{int}-x_m)^2+(y_{int}-y_m)^2}$$

The method is exact for circles. If it helps, think of this as the computer adaptation of the usual compass/straightedge method for finding the center of a circle.

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Yes, from what I read Béziers only approximate circles (note my wording in my question). I do have the control points (points 2 and 3), although my maths is weak enough that I had trouble putting all the pieces together in your solution (although I'm sure it's right) –  Geoff May 9 '11 at 16:09

Can't you just use the equation of the Bezier curve (this is a cubic one) and then find the midpoint? It seems easy to find the coordinates of all four points.

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That's where I was going, but I was kind of stuck - Peter's answer helped make sense of it for me –  Geoff May 9 '11 at 16:07