thesmallest.com lessonettes: short essays on whatever

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Using a cordless pen and tablet is easy; plug it in and scribble around on the tablet's active area, and the pointer (brush, pen, whatever tool you're using at the moment) will move with you. It simulates the behaviour of a mouse (although with a couple of key differences), while approximating the feel of a regular pen or pencil. Its behaviour seems fairly simple on the surface, but what actually goes on to make this happen involves some fairly interesting physics.

Wacom's technology involves no batteries in the pen, so there's no worry about your pen losing power and refusing to work at an awkward moment. Instead, all the hard work, as such, goes on in the tablet itself.

A fine wire grid is embedded in the tablet's active area, the section where the pen actually makes something happen. This switches between transmit and receive modes at very high speed, around every 20 microseconds. When transmitting, the signal this grid produces is picked up by the resonant circuit inside the cordless pen. The resulting oscillation in the pen's coil and capacitor arrangement is then detected by the tablet's grid circuit when it flips into receive mode. This data is used to determine where the pen is, and as tablets are generally used in 'absolute' mapping mode - where the edges of the tablet relate directly to the edges of the computer's display - this is used to position the on-screen tool. Wacom explains the resonance circuit technology in terms of tuning forks. When a piano string is tuned to a particular frequency, placing the right tuning fork near the vibrating string will cause it to pick up the vibration energy, generating a tone. The tablet causes a sympathetic resonance in the pen then detects that resonance, finding out all the necessary information in the process, flipping from one mode to the other roughly 50,000 times a second.

As well as the 'X/Y' location information that pinpoints the pen's position on the tablet, the data that the tablet picks up includes feedback on which pen buttons are in use and how hard the pen's tip is being pressed down - in effect adding a third dimension, the 'Z' axis, to the equation. This information is ignored by applications such as the Finder, where pressure makes no difference beyond registering as a button click or not. However, in applications such as Photoshop, FreeHand, and other pressure-aware tools, this data can be used to control various tool effects; the opacity of brushes, the width of vector strokes, and more. These days pressure-sensitivity is registered as 8-bit values, allowing 256 different pressure levels to be sensed.

The big behavioural difference between a mouse and a pen lies in the tablet. The pen does nothing until used in the active area of the tablet, whereas a mouse can register movement across almost any surface. As we mentioned a moment ago, tablets are generally used in absolute positioning mode, so a specific point on it will correspond to a specific point on the screen. This behaviour can be switched to a more mouse-like mode where repeated strokes will move the on-screen tool progressively further across the screen, but that's generally not well suited to natural media tool simulation.

Pressure-sensitive trackpads?

There has been recent online discussion about whether or not PowerBook and iBook trackpads provide pressure-sensitive feedback. In fact, there is a form of Z-axis data recorded by the trackpad, but this isn't specifically pressure. The trackpads register the size of the capacitive area of the pad which is covered by your finger; push harder and the area increases slightly, use two fingers and the area becomes much larger (and mouse control becomes more erratic). This data isn't used directly by most applications, but there is a Mac OS X tool available at the FingaMIDI MIDI controller trackpad driver site which uses this to control MIDI music production.