About Draw a Circle

What Is It?

Click and drag (or drag a finger) to draw the best circle you can. It can go anywhere in the box, any size above a small minimum, and in either direction. There is no center to draw around: whatever circle fits your drawing best is the one you are scored against.

Scoring

Your drawing is closed off with a straight line from where you finished back to where you started, shown dashed. If you overshoot the start, the extra bit past one full turn is ignored and shown greyed out. The closed drawing is then resampled into 512 points spaced evenly along it, so a stretch you drew slowly doesn't count any more than one you drew quickly.

Next comes the circle that fits those points best. A quick algebraic fit gives a first guess, and then a hill climb nudges the center and radius until no nudge makes the fit any better. Each point misses that circle by some distance, and the error is the root mean square of those misses as a fraction of the radius. Squaring the misses means a few big ones, like a lopsided side, cost more than lots of small ones. Every 1% of error costs 5 points, so a 2% error scores 90 and a 20% error scores 0.

The fitted circle is drawn over yours, and your line is colored by how far each part of it strays: green on the circle, through yellow, to red.

Not Counted

A drawing isn't scored if it leaves the box, is too small, doesn't go all the way around, goes around more than about one and a half times, or leaves a gap between its ends of more than half the radius.

Best Score

Your best score is shown above the box and is saved in your browser, so it sticks around between visits. The button below erases it. It asks first, because this can't be undone.

clear best score