Day 7 - Finger Physics is Hard


After struggling with this problem for quite some time, I think the issue is finally licked. The problem relates to transferring you fingers momentum to a virtual object on the screen. The natural way to display finger position is to plot a sprite at the touch sensors coordinates. The problem with this method is the physics engine doesn’t think their is any momentum built up. The physics engine just thinks your finger teleported there. So if I teleport on top of a cow it just explodes. To make things worse if the touch events come in too slowly you’ll teleport over the cow completely missing the free lunch.

The initial solution I had for the issue was horrible. I figured I could plot the sprite at the point of contact like before, then calculate a force vector based on the last coordinate form the previous touch event, divide by time and then tell the physics engine to apply the force to the target. This gave me accurate transfer of momentum, but looked ridiculous. The finger sprite kept jumping forward as I moved my finger across the screen like a horizontal jack hammer. So I thought I could divorce the finger sprite from the physics engine, but then I’d have to keep track of more objects and states of when to use physics (during a release) and when to not use it (during a drag). Anyway, I chose not to go down that path mainly because it felt like too many parts and I have no time for debugging AND I was still teleporting. Doh!

The solution ultimately came to something very unnatural, but completely sufficient. And it cut down my code significantly. Probably in half. The affect I needed was to have the object I’m dragging show up at my finger location and have momentum maintained at release or collision. Did you catch that word? Dragging? That’s how it hit me. The previous solution was to push the object, but if I drag it things get simpler. In drag mode I just need to apply enough force to the object to get it to my finger location. The math for that is simple and the same as I mentioned in the first solution: (current finger sprite position - current touch position) / time. When I apply the force, the physics engine moves the sprite under my finger perfectly, and if it misses the next touch event comes in to get it even closer. It a self correcting system.

The best part of the final solution is no more teleporting. We don’t do it in real life and we shouldn’t do it in our games either. Maybe one day I’ll add the complexity to push the object instead of drag, but for now I’ll leave that as an exercise to the reader.