Things to consider when writing for touch screen?

A few things to consider:

• You need to account for parallax error when touching controls. Basically, the user may touch the screen above or below your actual control and therefore miss the control. This is a combination of the size of the control (eg you can have the active area larger than visual control to allow the user to miss and still activate the control), the viewing angle of the user (which you may or may not be able to predict/control) and the type of touch screen you're using. If you know where the user will be placed relative to the screen when using it, you can usually accommodate this with appropriate calibration.

• Depending on the type of touch screen, you may need to ensure that your users aren't wearing gloves or using an implement other than their fingers (eg the end of a pen) to touch the screen. Some screens (eg those depending on conductance) don't respond well to anything other than flesh and blood.

• Avoid using double clicks because it can be very hard for users to reliably double click a control. This can be partly mitigated if you've got experienced/trained users working in a fairly controlled environment where they're used to the screens.

• Linked to the above, if you are using double clicks, you may find the double click activated when the user only wants to single click. This is because it's very easy for the user's finger to bounce slightly on touching the screen and, depending on how sensitive the double click settings are, trigger a double rather than a single click. For this and the previous reason, we always disable double clicks and only use single clicks (or similar single activation controls).

• However big you think you need to make the controls to allow for touch activation, they almost certainly need to be bigger still. Make sure you test the interface with real users in the real deployment environment (or as close to it as you can get). For example, w

If you decide to put in analog controls (scrollbars, rotation widgets, etc) be sure to put in a digital control also. Some companies think that a touch screen means perfect control over something with your fingers. In real life, this translates to minutes of frustration trying to fix a number that's just a little off.

The larger gui elements is the major thing. But it applies to all elements, scroll bars, tabs and even text fields.

The other major thing that I can think of, it's hard for the user to right click. So things that require a right click should be avoided, context menus are the only thing that comes to mind at the moment.