Hard and soft controls cooperate
When the mechanical team are working on the hard controls and the software team are programming the GUI controls, it's important that an overall view of the combination is kept in mind. Some devices do a beautiful job of marrying the two types of input. On my digital camera, there is a dial which allows the user to choose the mode. The dial contains small icons. When you turn the dial, the display shows the name and description of the new mode. This vanishes after a few seconds, so the extra information does not clutter the image of the picture to be taken. By showing a rounded outline around the icons being selected, the on-screen image looks like an extension of the physical dial. In other words, rotating the off-screen dial also rotates a disk segment that is displayed on the GUI as shown in Figure 5. While this is tricky to describe, the brief video available at www.panelsoft. com/clip.htm makes it a lot clearer.

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While many other cameras show the icon for the selected mode on the GUI, Sony makes this feature far better in two ways. One is that the extra text means that the user can learn the meaning of the icons without resorting to the user manual. The second aspect is the smooth integration of the on-screen and off-screen controls makes this device feel like a single user interface, rather than two distinct interfaces to two different parts of the device.

If you can't achieve a smooth integration of hard and soft controls, then at least try to ensure that they do not interfere with each other. Buttons above the screen cause the hand to obscure the GUI while they are being used. Usability reviews should check that icons and text used on and off screen are consistent. If different developers are doing the hardware and software, it's not unusual for terminology and symbols to differ unintentionally.

References on the GUI to hard controls should be carefully checked for any causes of confusion. Figure 6 illustrates an on-screen message where the placement of ACCEPT and CLEAR are the reverse of the off-screen positions. While the instructions are not incorrect, they do increase the chance that the user will press the left hand button thinking that it is the ACCEPT button.

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Staying in control
Using a GUI opens up boundless possibilities for the designer, but hopefully some of the advice above will help you to carefully consider the tradeoffs when moving some or all of your functionality from mechanical controls onto a graphical interface.

Niall Murphy has been designing user interfaces for over 14 years. He is the author of Front Panel: Designing Software for Embedded User Interfaces. Murphy teaches and consults on building better user interfaces. He welcomes feedback and can be reached at nmurphy@panelsoft.com. His web site is www.panelsoft.com.