Interaction density

Professional apps are often “pro”, because they offer quick access to many abilities. A wide and tall interface, with common actions immediately available. Visually daunting for newcomers, but fast for those who rely on them.

And, if that definition is correct, I think multitouch devices are less suitable for the task than their mouse and keyboard counterparts. Multitouch is significantly impaired when it comes to interaction density.

Imagine a device’s display, broken into a grid of squares. Each square represents the smallest desirable unique interaction zone. Or, in simpler terms, the closest you’d want to put two buttons next to each other.

The entire display typically wouldn’t and shouldn’t be filled with buttons, but the minimum interaction zone still matters. In a vector drawing app, it dictates how far you’d have to zoom in to be able to select individual points.

With a lower interaction density and the same vector app example, you would have to zoom in closer to be able to comfortably tap or click a specific point.

44pt vs 20pt #

For iOS, Apple suggest 44×44pt as a minimum hit target size. This is echoed across the entire system, Apple’s apps and third party apps. The nav bar is 44pt, table rows are 44pt, and many other buttons are 44pt high. There are some smaller targets, but they’re rarer.

For macOS, there’s no direct guidance from Apple, but there are plenty of examples to use as reference — the traffic light buttons at the top left of almost every macOS window are 12×12pt, the standard Open and Save buttons in file windows are 20pt high, the macOS menubar is 22pt high, dropdown menu items are 19pt. Using 20×20pt as a comfortable minimum interaction size for macOS seems sensible, possibly even a little generous.

That means macOS has roughly double iOS’ interaction density on both X and Y axis, resulting in four times the number of zones for a display of the same size. That is significant.

The stark size difference can be seen in many places. iOS Mail’s table rows are 44pt, macOS’s are 22pt. Tweetbot’s share icon is 27pt tall on iOS, but only 13pt tall on macOS. Everywhere you can find a meaningful comparison, the iOS equivalent is roughly double the width and double the height.

Display density #

You may note that I’ve been using iOS points and macOS points for the above comparisons, and haven’t mentioned the difference in display pixel densities. This is intentional — display densities vary a little on iOS devices and a lot on Mac displays, but the UI itself remains the same point size.

Stacking #

Different input methods allow for different ways to stack many functions in a single interaction zone. Each region on iOS can be tapped, double tapped, long pressed or even 3D Touched.

That’s not enough to compete though — macOS has corresponding abilities with click, double click, long click and right click. In fact, mouse input is very frequently augmented with keyboard modifiers (⇧, ⌥, ⌃, and ⌘). Most design tools allow for objects to be duplicated via ⌥-dragging, or duplicated and constrained via ⇧⌥-dragging. Those aren’t bizarre edge cases. They’re actions many professionals use 10s or 100s of times a day. It would be possible to present those abilities in another way, but maybe not in a way that wouldn’t take up additional screen space.

In terms of interaction stacking, multitouch is worse than mouse and keyboard. Yes, I realise keyboards can be paired with multitouch devices, but it is very much an afterthought, not as mature or prevalent as macOS, and if we’re being honest, not a first class input method.

Total interactions per device #

Ignoring stacking, an iPad Pro can house roughly 723 unique interaction zones. A 12-inch MacBook? 2074. The smallest Mac display fits more than double the UI of the biggest iOS display.

Does this matter? For many tasks and apps, no. For professional video, music, and design tasks? Yes, it matters more than any other factor I can think of.

It is possible there are solutions, but none obvious to me. Multitouch will always be constrained by the size of our human fingers. If multitouch is the future of computing, we’re going to need bigger screens.

Published 12 March 2016.