Joystick sampling rate in games.

Thu 14 June 2018
By Bram

I investigated an interesting conundrum this morning: why was my game running so much differently on my iPad PRO? The tank was snappy, and turning aggressively on the iPad PRO, but not on Linux and Android.

The main difference between iPad PRO and other platforms is its higher display refresh. But I was certain I had this covered, as I step my simulation exactly the same on all platforms: with 1/120s steps. The only difference being, that I render after each step on iPad PRO and only render once after two sim steps on other platforms that have 1/60s display refresh.

First thing to do, is to rule out differences in the iOS port. When I force my iPad to render at 1/60s instead, the iPad behaviour reverts to the same as the Linux/Android ports. Confirmed: it is the display refresh rate that makes the difference, not the platform's architecture.

So why would these two scenarios have different outcome?

[ sim 1/120s, render ] [ sim 1/120s, render ]

[ sim 1/120s,       sim 1/120s,      render ]
|                     |                     |
0ms                 8.3ms                 16.6ms

A logical explanation would be that I somehow influence the simulation somewhere, as I render. But after examining the code, nothing showed up.

It dawned on me that in the high display refresh case, the faster rendering is not the only difference. In 120Hz mode, you not only get more rendering activity, you also get more frequent input events. Touches come in faster when you render 120Hz, as they do when you render at 60Hz. Joystick changes, and touch events are batched with display refresh.

To confirm this, I put in an artificial joystick value, that would simply rotate the joystick at a set pace. Then I adjusted how those joystick changes were relayed for a 60Hz display frame. The result is the video below.

On the right, I adjust the joystick angle with 0.10 radians before each sim step. On the left, I adjust the joystick angle only once for two steps, but at double the the radians.

At 120Hz stick sampling, I get a smoother joystick signal. Even though the joystick rotation speed is the same, the 60Hz sampling shows more jarring deltas. I hadn't expect the effect on the simulation outcome this big.

The reason for the dramatic difference is that the small difference is amplified by the PID controllers I use in my game. In the case of low stick sampling rate, the PID controller will always see a zero change during the second step, and a large change in the first step. The PID controller can react a lot more effectively if it gets a higher frequency signal.

Lesson learned: these two scenarios give different simulation outcomes:

[ read stick,     sim 1/120s,     sim 1/120s,     render ]

[ read stick, sim 1/120s, read stick, sim 1/120s, render ]
|                                                        |
0ms                                                    16.6ms

Although forcing the 120Hz stick signal down to 60Hz is simple to achieve, it will be hard to provide a 120Hz stick signal if you only get your events at 60Hz. So the sweet, reactive control on iPad PRO is hard to achieve on 60Hz devices, unless you interpolate or extrapolate the stick values.