A tester reported that Pawns for iPhone was not working on his 4th generation iPod Touch (with Retina display.) Specifically, many of the menu buttons were very flaky, sometimes requiring multiple taps before they’d register.

After some investigation I isolated the cause [EDIT: or maybe not? See my updates at the end of the post.] It appears that UnityGUI buttons are extremely sensitive to movement of the finger, when run on retina displays. The slightest movement of the fingertip on a retina display causes the tap to be ignored. (Version info: Unity 3.0 for iPhone; non-HD build of the game.)

I believe the problem is with Unity’s translation of touches into “clicks”. There is a movement threshold value (probably 5 to 10 pixels) beyond which a touch is no longer treated as a single tap on a Button. I believe Unity is using the same threshold value on retina displays, and that threshold is too low for the higher-resolution.

If I’m right, anyone shipping an iPhone game that relies on normal UnityGUI click logic is taking a serious risk at the moment. If you are getting complaints about unresponsive controls on retina displays, this is quite possibly the culprit.

I have filed a bug of course, and in due time I expect Unity Tech will change the threshold, or possibly something else I haven’t thought of, and all will be well.

In the meantime, I’ve got a game to ship! So, what to do?

My plan was to add my own touch-processing logic, using the same techniques I recently used to make the ScrollView control respond to finger drags.
However, I found to my dismay that most of my remaining UnityGUI menus happen to be using GUILayout, not GUI. This means that I don’t actually know where the buttons are being displayed on the screen. And without that Rect, I can’t do touch detection.

So I am faced with the choice of waiting for a possible fix, or of converting my GUILayout menus to GUI. Doing the layout myself is not rocket science, but it is tedious to rewrite the menus and retest every iPhone and iPad resolution and orientation I plan to support.

UPDATE: See the comments below. It turns out there IS a way to get the rectangle of a GUILayout control. I will post some sample code once I’ve implemented a workaround for the responsiveness issue.

UPDATE October 28, 2010: I have coded the workaround. But: I am no longer able to reproduce the original problem! Either something has changed about my test app, or I’m missing an important detail.

I wasn’t seeing widespread reports of this problem so the fact that it doesn’t always occur makes sense. In a way, a relief. But I would dearly like to know how it happened originally, and not just because it would have saved me some effort rewriting the tap logic. The effect was very specific to button presses on a retina display, and all my testers noticed it. Unfortunately without owning a retina display myself I am not going to get to the bottom of this any time soon.