In this post, I would like to share a story of solving one of the recent bugs I had in qdEngine.
First, let me tell you about the bug. The problem was with the minigame in the game called “Dog-n-cat: In the Footsteps of Unprecedented Beasts”. In the minigame, players swap and move around triangle pieces to assemble the picture of an animal. The problem was that when swapping triangles, the triangles would move to swapped positions, instead of folding/unfolding in place. And the animation itself had artefacts. This is how it looked before the fix:
and how it looks now, after the fix:
I first thought the problem was with the code logic of this minigame. Even though we already had the sources of the minigames, as it turned out, some of the parts of code were not present there. This led me to think that maybe some crucial piece of code responsible for the rotation was absent. I told this to my mentor – Eugene Sandulenko (aka sev), and he provided me with the decompiled code of the dll for the minigame. But, it turned out to be the same. Then, he went with reversing the several games which were developed in later versions of the engine to see if important differences were introduced. Again, differences that could help were not found.
We started to lose hope on this, so I started to think about the problem from beginning and try a different approach. Sev adviced to look again at what exactly was responsible for rotation and tranformation of the triangle. And I looked very closely at qdScreenTransform::change(), the method that changes object’s angle and scale when transformation happens. In our case, it was called when triangle is transformed during swapping. I noticed that the angle was always 0.0 (wasn’t changing), but the scale in y-axis was decreasing. What’s important is that I was testing it by swapping the triangles vertically. And then, it really came to my mind, that no rotation should not even be happening in this case. Turned out, to simulate the effect of folding, the game was only scaling down the y-component of the triangle object. This meant that I only needed to check the drawing methods with scale parameter. And indeed, after comparing with the original source code, the issue was in the wrong conditional operator:
This bug a took quite some time to solve, because of wrong a wrong assumption I had. However, with the help of my mentor and an insight that came to me eventually, the problem was successfully solved.
Thanks for reading.