Question

I’m looking at a simple, programmatic way of detecting whether or not the user has drawn a circular shape. I’m working in C, but am happy to work from pseudo-code. A bit of Googling brings up a number of (hopefully) overly-complex methods.

I’m tracking the mouse coordinates as floats, and have created an array of vectors to track the mouse movement over time. Essentially I’m looking to detect when a circle has been drawn and then disgard all movement data not associated with that circle.

I have a basic idea of how this might be accomplished:

Track all movements using a polling function. Each time the function is polled the current mouse position is stored. Here, we loop through the historic position data and do a rough ‘snap to position’ to compare the two locations. If the new location is within a close enough distance to an old position, we remove all historic data before the old location.

While this works in theory, it’s a mess in practice. Does anyone have any suggestions? Bonus points if the method suggested can detect whether it’s been drawn clockwise or counter-clockwise.

Answer 1

Based on your tracking/polling function, which pushes float pairs on a stack. This must be done on a regular timing interval.

1. Do a threshold-based search for two equal entries in the list. Now you have two indexes in your stack; the first and the second equal entries. Consider this as a line.
2. Get the absolute difference in indices. Then divide by two and get the coordinates of this point. (Center of the line.)
3. You’ve got two points: thus you can get the radius of the circle, by getting the distance between the two points divided by two.

4. Divide the number of step 2 by 2, now you’ve got the quarters.

   If the line at step 1 is vertical and the first point of the line is at the top: If the first quarter is left of the center-point, the circle was drawn counter-clockwise. If the first quarter is right of the center-point, the circle was drawn clockwise. If the first point of the line is at the bottom, reverse (i.e. ccw => cw and cw => ccw)

   If the line at step 1 is horizontal and the first point of the list is at the left: If the first quarter is above the center-point, the circle was drawn counter-clockwise. If the first quarter is below of the center-point, the circle was drawn clockwise. If the first point of the line is at the right, reverse.

5. Check if it was a circle: iterate over all pairs of coordinates and calculate the distance to the center-point. Tweak the threshold of allowed distances from the calculated distance and the actual distance to the center-point.

In step 2 and 4 you can tweak this algorithm further by taking the average of several indices if the timing interval is very low (fast polling). For instance: there are 30 pairs in the array, then you average pairs at 0, 1 and 28, 29 to get the upper point. Do the same for all other points.

I hope this is easy enough.