Checkers: Explosion bug

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A video posted by Maciej Konieczny (@narfdotpl) on


I had a funny bug. My brother noticed that explosions of the light pieces were “weaker” than explosions of the dark pieces. One early morning before the day job, I decided to look into it.

Fixing the problem was easy. Tracking it down, on the other hand, took about two hours, during which I looked in all the wrong places and had no luck with the usual “print a lot of stuff and then step into the debugger” approach. I won’t go into details here, but tweaking the live system was instrumental in diagnosing the exhibited behavior.

Explosions are implemented like this: first I recursively divide a piece into small fragments, then I apply an impulse of momentum to each fragment — from the center of the piece outwards. The cause of the bug was the fact that I applied momentum vectors in the “physics world” coordinate system, but computed them using positions of fragments in the coordinate system of each piece. You see, the dark pieces use the “normal” coordinates and the light pieces are rotated by 180°, so that the dark triangles and pentagons point downwards and the light ones point upwards.

Rotating by 180° around the (0, 0) point is equivalent to multiplying both x and y coordinates by -1. The moment when I realized this was the moment when I understood what was happening on the screen:

After breaking into fragments, the light pieces would be pushed in the wrong direction, inwards instead of outwards. They would immediately collide with each other, loose some kinetic energy due to not perfect elasticity, and bounce outwards, with a smaller velocity. All of this whithin a single frame of animation.

A funny bug. :)

It’s especially visible when I don’t randomize impulse magnitudes, as in the video at the top of this post. Notice how the “implode and bounce away from each other” behavior affects not only the velocity of fragments but also the overall shape of the debris “cloud”: normal explosions create “rings” while the buggy ones are amorphic.


Checkers series

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