intruding on usage. It adds to the sense that the devices are a family and helps
users form a conceptual model of how the system works.
Where visual elements also convey meaning it is vital that they are used in the
same way. This is called semantic consistency. To continue with the heating
example, you may use red/orange/blue colors to indicate temperature. Or a
particular icon might indicate that the water tank is heating up. The icon may
be higher resolution on devices with better screens, but it must be recognizably
the same thing (see figure 9. 24 ).
Figure 9. 24 : An icon (tbc) shown on a smartphone screen and Pebble watch
Interaction architecture and functionality
Interaction architecture is the logical hierarchy (or other structure) of the UI as
mapped to the controls. This is likely to be less consistent across devices and
more platform-dependent. Devices may have different functions in the
service. Even where there is an overlap between functions, they may be
optimized for different purposes. A wall thermostat might be optimized for
small adjustments and switching mode (e.g. turning on the hot water). It might
need to support changing the heating schedule, but that’s always going to be a
better experience on the mobile or web app. In optimizing the thermostat for
quick adjustments, the designers might knowingly create a less-good UX for
schedule changes. But they might view this as acceptable if users are likely to
change schedules on a smartphone or website anyway.
As devices are used for different things, it’s not necessarily desirable to group
functions in exactly the same way. For example, a mobile or tablet screen can
provide one touch access to many functions, facilitating a broad, shallow
functional hierarchy. Fitting the same functions into a heating controller with
an LCD screen plus 3 buttons may require a narrower, deeper hierarchy.
You may also need to use modes, in which the same buttons perform different
actions in different states. Modes are typically more difficult to use, but they
may be an essential compromise if you’re stuck with the hardware (see e.g.
figure 9.25). Structure your mobile or tablet app to be a great solution for that
device, and don’t let it be constrained by the limitations of the embedded
device.
Figure 9. 25 : heating controller with modal functions