What I meant was that some input devices are "more relative" than others, i.e. second derivative, pressure controlling velocity, instead of movement controlling position. With pressure controlling velocity, you're one more level removed from what you actually want to control, which is position. But that level of abstraction does have its benefits, like avoiding the "nulling problem" that makes context switching more difficult.

And in particular, the huge advantages the trackpad benefits from, which overcome the costs of being "more relative", is that it's in a small fixed location, and that location is right where your hands already are, minimizing the (enormous and frequently repeated) cost of switching between pointing and typing tasks (0.75 - 1.75 seconds).

Since the trackpad controls velocity, you can press it for as long as you want to move the cursor as far as you want in one direction. But with a mouse or trackpad that controls position, you eventually have to lift your mouse or finger and recenter (nulling it) when you hit the edge of the desk or trackpad.

Simply taking your finger off of the trackpoint stops moving (nulling it), while a joystick requires a spring to return it to the center (nulling it) to make it stop moving when you take your hand off of it.

http://en.wikipedia.org/wiki/Pointing_device#Joystick

>Isotonic joysticks are handle sticks where the user can freely change the position of the stick, with more or less constant force. Joystick. Analog stick.

>Isometric joysticks – where the user controls the stick by varying the amount of force they push with, and the position of the stick remains more or less constant. Isometric joysticks are often cited as more difficult to use due to the lack of tactile feedback provided by an actual moving joystick.

>Pointing stick: A pointing stick is a pressure-sensitive small nub used like a joystick. It is usually found on laptops embedded between the 'G', 'H', and 'B' keys. It operates by sensing the force applied by the user. The corresponding "mouse" buttons are commonly placed just below the spacebar. It is also found on mice and some desktop keyboards.

Bill Buxton wrote an insightful and classic paper (one of my favorite of his many great papers) called "Lexical and Pragmatic Considerations of Input Structures" that explains what I'm getting at.

http://www.billbuxton.com/lexical.html

It's well worth reading the whole paper (it's short and accessible), but here's the relevant stuff:

Check out "Figure 1: Taxonomy of Input Devices", with the vertical axis labeled "Property Sensed: Position, Motion, Pressure (Sensing Mechanism, Touch Sensitive)", and the horizontal axis labeled "Number of Dimensions (1, 2, 3)".

http://www.billbuxton.com/lexical1.gif

The column labeled "Number of Dimensions / 2 / Small Fixed Location" has the following cells:

"Isotonic Joystick" @ "Position / Sensing Mechanism"

"Spring Joystick, Trackball" @ "Motion / Sensing Mechanism"

"X/Y Pad" @ "Motion / Touch Sensitive"

"Isometric Joystick" (i.e. trackpoint) @ "Pressure / Touch Sensitive".

>Caption: Continuous manual input devices are categorized. The first order categorization is property sensed (rows) and number of dimensions (columns). Subrows distinguish between devices that have a mechanical intermediary (such as a stylus) between the hand and the sensing mechanism (indicated by "M"), and those which are touch sensitive (indicated by "T"). Subcolumns distinguish devices that use comparable motor control for their operation.

>[Discussion...] Before leaving the topic of the tableau, it is worth commenting on why a primary criterion for grouping devices was whether they were sensitive to position, motion or pressure. The reason is that what is sensed has a very strong effect on the nature of the dialogues that the system can support with any degree of fluency. As an example, let us compare how the user interface of an instrumentation console can be affected by the choice of whether motion or position sensitive transducers are used. For such consoles, one design philosophy follows the traditional model that for every function there should be a device. One of the rationales behind this approach is to avoid the use of "modes" which result when a single device must serve for more than one function. Another philosophy takes the point of view that the number of devices required in a console need only be in the order of the control bandwidth of the human operator. Here, the rationale is that careful design can minimize the "mode" problem, and that the resulting simple consoles are more cost-effective and less prone to breakdown (since they have fewer devices).

>One consequence of the second philosophy is that the same transducer must be made to control different functions, or parameters, at different times. This context switching introduces something known as the nulling problem. The point which we are going to make is that this problem can be completely avoided if the transducer in question is motion rather than position sensitive. Let us see why.

>Imagine that you have a sliding potentiometer which controls parameter A. Both the potentiometer and the parameter are at their minimum values. You then raise A to its maximum value by pushing up the position of the potentiometer's handle. You now want to change the value of parameter B. Before you can do so using the same potentiometer, the handle of the potentiometer must be repositioned to a position corresponding to the current value of parameter B. The necessity of having to perform this normalizing function is the nulling problem.

>Contrast the difficulty of performing the above interaction using a position-sensitive device with the ease of doing so using one which senses motion. If a thumb-wheel or a treadmill-like device was used, the moment that the transducer is connected to the parameter it can be used to "push" the value up or "pull" it down. Furthermore, the same transducer can be used to simultaneously change the value of a group of parameters, all of whose instantaneous values are different.