When the first version of Sketchpad was released, you didn't see many constructions involving curves other than circles. With Version 2, people did a lot of curve stitching and curved locus tracing. With Version 3, you have arcs, and you have dynamic loci. That, combined with dynamic plotting, makes for a lot of wonderful curve construction!

Squaring the Circle
This is a quick demo of what it means to square the circle that Dan Bennett at Key Curriculum Press did using a very early alpha of Sketchpad/3. After years of explaining that Sketchpad was restricted to Euclidean constructions, he couldn't resist. The buttons animate a circle deforming into a square and vice versa brought to you by arcs and by dynamic transformations tied to calculated values.
Rolling Reuleaux
This sketch rolls a Reuleaux triangle between two "plates," allowing you to change the size and shape of the triangle and to trace the path of the center of the triangle as it rolls. It makes use of arcs to handle the rounded vertices of the triangle.
Continuous Conics
The search for the ideal conics sketch continues. Scott Steketee, programmer for the Windows version of Sketchpad created this one. It's based on the definition of a conic as the locus of points which are equally distant from a given point F1 (the focus) and a given circle (the directrix).

In this construction, the focus F1 and point P on the circle are kept fixed. The radius of the circle is changed by moving the center F2 along the line joining F1 and P.

You can adjust the size of the circle, and change the eccentricity of the conic, by moving a point labeled "drag me," (not shown) or you can double-click an "Animate" button.

Bezier Curves
This curve demonstrates Bezier's description of a cubic. Four control points define the curve: two "endpoints" on the curve, and two which specify tangent velocities of the curve at the two endpoints. The sketch also demonstrates connecting Bezier curves by joining endpoints, and smoothly connecting Bezier curves by both joining endpoints and matching tangents. Bezier curves frequently appear in computer-graphics, computer-modeling, and computer-aided-design applications.

The sketch was constructed by entering the curve's parametric equation (into the Calculator) based on some variable t (the parameter) and the coordinates of the four control points. Two separate equations were used to calculate the X and Y components of a point on the curve for some value of the parameter. These two equations were then plotted as a single point in the sketch, and the curve was then defined as the locus of that point as t varied from 0 to 1.

Bezier Patch (Warning: 1136K)
This "bi-parametric cubic patch" is the result of multiplying two independent Bezier curves, resulting in a surface. Again, control points (16 of them now) determine the curvature of the surface, both by "on-patch" points and by "tangent velocity" points. Bicubic patches are a standard representation of non-planar surfaces in computer modeling situations. Warning: You will need a very fast computer to manipulate this sketch--and even then it will be slow! (Each time Sketchpad draws the patch, it must evaluate a cubic function almost 64,000 times. In real modeling situations, more efficient algorithms are used for drawing bicubic patches.) On a Macintosh, you'll also want to allocate extra memory to the Sketchpad application: we recommend as large a partition as possible.