Trackpads & touch screens

A trackpad, like a mouse, is a pointing device. Specifically, it’s a way of moving a pointer around on the display. When you click on the trackpad, the computer uses the position (x, y) on the screen to determine which element on the screen you’re trying to interact with.

Often you can use two or three fingers on a trackpad to indicate you want a different effect from just using one.

Why wet fingers won’t work

There’s more than one way the position of your finger (or fingers) is being detected, but a common mechanism uses capacitance. The pad or screen is sandwiched between two conductive layers. When you bring your finger close enough it affects the capacitance in that location enough to be detected.

This is why other materials between your finger and the screen (such as rain drops on your mobile phone screen) mess this up. Similarly, tapping with a pen won’t have the same conductive effect as your finger. These things interfere with the conductive characteristics the surface is trying to detect.

Programming touch devices

Like mice, trackpads and touch screens generate events that you can respond to if you’re programming interface behaviour.

Trackpads provide similar capabilities to mice, with the extra consideration of using multiple fingers.

But there is a significant difference with touch screens. Touch screens generally don’t move a pointer around, so there’s no hover or similar behaviour. Interfaces that change button colour when you mouse over them, or have tool tips on hover, usually won’t work the same way.

The JavaScript web API has a Touch interface. The Pointer interface applies to both mouse and touch events.