Section: New Results
The impact of control-display gain on user performance in pointing tasks
Participant : Géry Casiez.
We theoretically and empirically examine the impact of control-display gain (CD) on mouse pointing performance. Two techniques for modifying CD gain are considered: constant-gain (CG) where CD gain is uniformly adjusted by a constant multiplier, and pointer acceleration (PA) where CD gain is adjusted using a non-uniform function depending on movement characteristics. Both CG and PA are evaluated at various levels of relationship between mouse and cursor movement: from low levels which have a near one-to-one mapping, through to high levels that aggressively amplify mouse movement. We further derive a model predicting the modification in motor-space caused by pointer acceleration. Experiments are then conducted on a standard desktop display and on a very large high-resolution display, allowing us to measure performance in high index of difficulty tasks where the effect of clutching may be pronounced. The evaluation apparatus was designed to minimize device quantization effects, and also used accurate 3D motion tracking equipment to analyze users' limb movements.
On both displays, and in both gain techniques, we found that low levels of CD gain had a marked negative effect on performance, largely due to increased clutching and maximum limb speeds. High gain levels had relatively little impact on performance, with only a slight increase in time when selecting very small targets at high levels of constant gain. On the standard desktop display, pointer acceleration resulted in 3.3% faster pointing than constant gain, and up to 5.2% faster with small targets. This supported the theoretical prediction of motor-space modification, but fell short of the theoretical potential, possibly because PA caused an increase in target overshooting. Both techniques were accurately modeled by Fitts' law in all gain settings. From our results, we derive a usable range of CD gain settings between thresholds of speed and accuracy given the capabilities of a pointing device, display, and the expected range of target widths and distances.