Reaching for objects in VR displays: lag and frame rate

ACM Transactions on Computer-Human Interaction (1994), pp. 331-356, doi: 10.1145/198425.198426
Experiments
Tasks
Findings

(1) Test extended Fitts’ Law: If the lag model described in the introduction is correct then it should account for most of the variance in a variable lag target acquisition experiment. (2) Test to see if motion into the screen obeys Fitts’ Law: It is reasonable to presume that there is no significant difference between vertical and horizontal motion in the plane of the screen, and the available evidence supports this. But motion in and out of the screen has to rely on stereopsis and on the lower resolution in depth that is available in a stereo display. It is plausible that when the critical dimension of motion is in and out of the screen target acquisition will be significantly harder. The present study compares horizontal motion (X direction) and motion in and out of the screen (Z direction) to find out if they can be accounted for by the same model. These are not constants, but clarify the conditions. There was a single base condition, with 2 different conditions that varied. Below are all 5 conditions: Base condition Head lag (msec): 114 Hand lag (msec): 87 Head Lag conditions Head lag (msec): 214 364 Hand lag (msec): 87 87 Hand Lag condition Head lag (msec): 114 114 Hand lag (msec): 187 337

Variables and Constants

    Visual Fidelity
  • latency - head lag varied between 114, 213 and 364 preciesely. hand lag varied between 87, 187 and 337.
Independent Variables
Constants

Display form factor, resolution, FOV, frame rate and the enabled stereoscopy, among others, remained constant.

System Info

Displays
  • 3d monitor - the silicon graphics iris crimson contains a 3d monitor with stereographcs cystaleyes lcd active shutter glasses, and inside the glasses are logitech ultrasonic head tracking sensors.
Input Hardware
Software
None

The computer is s Silicon Graphics Crimson VGX. I am assuming that this computer came with its default monitor, PowerVision Graphics, that supports stereoscopic viewing, hardware enabled texture mapping, 1280 x 1024 px, 24-bit color, (source: http://www.sgistuff.net/hardware/graphics/powervision.html) The Silicon Graphics Crimson VGX has specs approximately the following, in addition to the graphics specifications above: Processor: 100-150 MHz Cache: 8-16 KB, with a secondary cache of 1 MB Memory: 16-256 MB RAM Hard drive space: 1-2 GB Network: 4 Ethernet

Participant Info

Twelve computer-literate subjects from the authors’ university served as paid volunteers. Three of the subjects had prior experience with the apparatus used in the experiment.

Total # Age Range Gender Balance
12 -

The user had to select a target in 3D space.

Interaction and Environment

Interface

The only interaction was moving hte hand to the target, and pressing a button that was attached to the hand sensor.

The screen background was set to a dark grey color, and two light grey wire mesh grids were drawn in the horizontal plane at the top and bottom of the screen. The target consisted of two purplish-red, 5 cm square tiles with solid borders (l-pixel wide antialiased lines) and translucent faces

Dimensionality Scale Density Visual Realism
3D Small Low Low
Metrics

  • time - correctly select the object
  1. There was a significant interaction between latency and target width on time for a object selection task.

    The smaller target and increased lag lead to longer task completion times for target selection tasks in 3D,

    Specificity: Neither
    The claim is that lag * 1.4 * difficulty, measured by size of target = delay in task completion time. It is mentioned to be significant in it of itself, derived from a series of models, with a decent r^2 of .86.

The second experiment had the following two goals: (1) Test extended Fitts’ model for 3D cube targets: Whereas Experiment 1 was designed to be a task for which only one dimension of movement was critical (either X or Z), Experiment 2 was designed to investigate the problem of the capture of three-dimensional targets which are small in all three dimensions. We undertook to investigate the matter in a formal experiment in which the targets were cubes of different sizes. (2) Measure performance under conditions of diplopia: The first experiment was designed to minimize the occurrence of double images (diplopia). However, in many situations diplopia will occur because the binocular disparity is too great, and it is important to determine if this is a significant factor in target acquisition times.

Variables

    Visual Fidelity
  • latency - head lag varied between 114, 213 and 364 preciesely. hand lag varied between 87, 187 and 337.
  • stereoscopy - increase the occurrence of diplopia, or the occurrence of double images.
Independent Variables
System Info

Displays
  • 3d monitor - the silicon graphics iris crimson contains a 3d monitor with stereographcs cystaleyes lcd active shutter glasses, and inside the glasses are logitech ultrasonic head tracking sensors.
Input Hardware
Software
None

The computer is s Silicon Graphics Crimson VGX. I am assuming that this computer came with its default monitor, PowerVision Graphics, that supports stereoscopic viewing, hardware enabled texture mapping, 1280 x 1024 px, 24-bit color, (source: http://www.sgistuff.net/hardware/graphics/powervision.html) The Silicon Graphics Crimson VGX has specs approximately the following, in addition to the graphics specifications above: Processor: 100-150 MHz Cache: 8-16 KB, with a secondary cache of 1 MB Memory: 16-256 MB RAM Hard drive space: 1-2 GB Network: 4 Ethernet

Participant Info

Twelve computer-literate subjects from the authors’ university served as paid volunteers. Seven of the subjects had prior experience with the apparatus used in the experiment.

Total # Age Range Gender Balance
12 -

The task is the same as the last, except that the position also varied in both the X, Y & Z direction simultaneously instead of independently.

Interaction and Environment

Interface

Same last the last experiment

Same last the last experiment

Dimensionality Scale Density Visual Realism
3D Small Low Low
Metrics

  • time - time to select the object
  1. There was a significant interaction between latency and target width on time for a object selection task.

    The smaller target and increased lag lead to longer task completion times for target selection tasks in 3D,

    Specificity: Somewhat general
    This was derived from a model that was refined from the first experiment. What makes this model more compelling than the last was that they evaluated independently and in combination with targets varying in X, Y & Z. The claim is that lag * 1.5 * difficulty, measured by size of target = delay in task completion time. It is mentioned to be significant in it of itself, derived from a series of models, with a good r^2 of .95. Z has the biggest significance in movement.