Bézier curves are very common in computer graphics. They also interesting mathematical properties. This post will give a quick introduction to Bézier curves, describing them first in visual terms and then in mathematical terms.

There are different degrees of Bézier curves: linear, quadratic, cubic, etc. Linear Bézier curves are just straight lines. The most common kind of Bézier curve in drawing programs is the cubic and that’s the one I’ll describe below.

A cubic Bézier curve is determined by four points: two points determine where the curve begins and ends, and two more points determine the shape. Say the points are labeled *P*_{0}, *P*_{1}, *P*_{2}, and *P*_{3}. The curve begins at *P*_{0} and initially goes in the direction of *P*_{1}. It ends at *P*_{3} going in the direction of a line connecting *P*_{2} and *P*_{3}. If you move *P*_{1} further away from *P*_{0}, the curve flattens, going further in the direction of *P*_{1} before turning. Similar remarks hold for moving *P*_{2} away from *P*_{3}.

Now for equations. The cubic Bézier curve is given by

*B*(*t*) = (1-*t*)^{3} *P*_{0} + 3(1-*t*)^{2}*t* *P*_{1} + 3(1-*t*)*t*^{2} *P*_{2} + *t*^{3} *P*_{3}

for *t* running between 0 and 1. It’s clear from the equation that *B*(0) = *P*_{0} and *B*(1) = *P*_{3}. A little calculation shows that the derivatives satisfy

*B*‘(0) = 3(*P*_{0} – *P*_{1})

and

*B*‘(1) = 3(*P*_{3} – *P*_{2}).

Moving the points *P*_{1} and *P*_{2} further out increases the derivatives and thus makes the curve go further in the direction of these points before bending.

**Related post**: The smoothest line through a set of points

I think you might have switched the sign of the derivatives. Unless I’m mistaken, it should be B'(0)= 3(P1-P0) and B'(1) = 3(P3-P2).

Can you double check that?

Thanks. You were right. I’ve corrected the post.

You might also be interested in my Animated Bézier Curves demo.