Draw a Perfect Circle Game

Perfect Circle started with a conversation in the kitchen. Josie asked, "could you make a game where you have to draw a circle from memory and it scores you on how close it actually is?" and that's pretty much the whole game. There's no second mechanic. You see a guide circle for two seconds, then it disappears and you have ten seconds to draw it freehand. The game lays your stroke over the true circle and grades you from F up to S, based on the average distance from each of your points to the nearest point on the real circle.

Rendering and input were easy, just basic canvas work. The scoring function took real effort, because the obvious approach (measure how far off every mouse-event point is) gives wildly wrong results. On a high-refresh-rate monitor, someone who draws slowly and carefully ends up with thousands of points packed close together. Someone who swooshes across the screen gets fewer points, spread further apart. If you just average the deviation, you end up rewarding the fast drawer for being fast instead of accurate. We fixed it by resampling the stroke at fixed distance intervals before scoring, so the comparison is fair no matter how fast you draw.

The second problem was working out where the player's circle actually was. Say you draw a small loop and then a stray line heading away from it. The algorithm has to recognise the loop as the circle you meant and ignore the stray. Bounding-box detection couldn't handle that, so we switched to median distance from the centroid, which doesn't get thrown off by outliers. Now if you draw a great circle with one wobble, the wobble costs you almost nothing.

How to do well. Don't rush. The game rewards smooth, consistent motion. Start at the top of where you remember the guide circle being and move steadily clockwise (counter-clockwise works too, the algorithm doesn't care about direction), trying to keep a constant rotational speed. Most people give up on the last quarter because they can see it isn't going to close where they expected. Keep going anyway. The whole stroke gets graded, not just where it ends.

We calibrated the grading tiers against real human attempts. Dad's average is a C. Josie holds the household record at A-tier, and the kids' friends who have played land somewhere between D and B. S-tier is genuinely hard and we've only seen it a handful of times. If you hit it, you're a better circle-drawer than 99% of the people who have tried.