Frank Mitchell

Hit boxes matter on the mobile web

Mobile development doesn’t happen on mobile devices. I wrote Hard Vacuum: Recon on a Raspberry Pi, not the iPhone 5 it was built to run on. Design, development, and testing took place on the Pi, and it wasn’t until late in the cycle that I actually fired it up on a phone to see how it felt.

Turns out it felt all wrong.

Hard Vacuum: Recon is a memory game. As you fly over rocks, you scan them by touching them, which reveals the ice underneath. Match two icicles of the same size, and your copter gets a little more fuel so you can keep flying. Icicles come in three sizes, big, medium, and small, but rocks are just one size, small.

Icicles from Hard Vacuum: Recon

My first attempt bound an onTouchBegin event handler to the <img> tag that held a rock. The base tile size is 20x20 pixels, so the hit box for the rock was the same size. On the Pi, this felt just fine. I easily clicked on rocks and kept my copter flying. But the iPhone was a different story.

Every time I went to touch a rock, it would slip past my fingers. Maybe one in three touches got through, but it didn’t feel fun or fluid. Mash your finger on a phone screen, and you can see why. On a 320x480 pixel screen, your finger tip covers a square about 40x40 pixels. Since my hit boxes where a quarter the size they needed to be, my rock collecting wasn’t cutting it.

Bumping the hit box size to 40x40 pixels helped. My hit rate went up. My copter stayed in the air longer. Yet every once in a while I found myself saying, “Hey, I touched that. Why didn’t it flip?”

Hard Vacuum: Recon is a vertical scroller. Rocks move up the screen as the game progresses. On a comptuer with a mouse, the delay between click and hit test is zero. Since the mouse is already over the pixel you’re clicking, the vertical motion isn’t an impediment to precision picking.

On a phone, when your finger touches the screen, it’s now in the way of you seeing what’s happening. So we tend to keep our fingers back, darting them forward to tap as needed. The delay between “I want to touch hat” and your finger actually hitting the screen is about 0.25 seconds. Hard Vacuum: Recon scrolls at 20 pixels per second, turning that quarter second reaction time into a 5 pixel error. Couple that error with the soft, round, imprecise nature of fingers and you have touch screen selection disaster.

My solution was to make the hit boxes for rocks 40x60 pixels. Those extra 20 pixels of vertical space account for reaction time and vertical motion. Since rocks are 20 pixels square, centered in the hit box, you actually get a full three seconds of time to touch them. Picking the space a rock was in a second ago, or where it will be a second from now, counts.

By iteratively accounting for the imprecise nature of touch and the vertical motion of the game, I arrived at a solution that feels right. Try it yourself. Load up Hard Vacuum: Recon on your iPhone and see how long you can keep your copter flying.

P.S. The graphics for Hard Vacuum: Recon came from Daniel Cook’s “Hard Vacuum” art on If you’re looking for free game graphics, his work is an excellent choice.