A desktop stereoscopic virtual environment, MITS (Manipulation In Three Space), was developed to conduct the experimental research in this thesis. MITS consists of a SGI IRIS 4D/310 GTX graphics workstation, a SpaceballTM, an Ascension BirdTM, CrystalEyesTM stereoscopic glasses, several input controllers and a software system developed in C and GL by the author. MITS allows the user to perform a variety of 3D object manipulation tasks in 6 DOF with various display options (e.g. monoscopic or stereoscopic) and control modes. Unless otherwise specified, the default display update rate was 15 Hz.
The origin of the {x, y, z} co-ordinates of the MITS virtual environment
was located at the centre of the computer screen surface, with
the positive x axis pointing to the right, the y axis pointing
upwards and the z axis pointing towards the viewer. All objects
were drawn using perspective projection. MITS also provided feedback
and instructions to the subjects, regulated the procedures of
the experiments and logged experimental data. This ensured that
minimum interference was needed from the experimenter when collecting
subjects' data with MITS.
2.5.2 Experimental Task - 6 DOF Docking
Figure 2.3 illustrates the 6 DOF docking task used in the experiment. The aim in designing this task was to incorporate all 6 degrees of freedom in 3D object manipulation and yet be simple enough to be generalisable to various applications in 3D graphics, virtual environments and telerobotics. In the experiment, subjects were asked to move a 3D cursor as quickly as possible to align it with a 3D target. The cursor and the target were two tetrahedra of equal size. The tetrahedra edges and vertex markers (bars and spherical stars) were coloured so that there was only one correct match in orientation. The markers superimposed on each corner of the tetrahedra served multiple purposes. The stars on the target tetrahedron indicated the acceptable target tolerance for each vertex. The two types of markers (stars and bars) served also to differentiate the target from the cursor, as seen in Figure 2.3(c).
The target stayed at the centre of the screen throughout the experiment. At the beginning of each trial, the cursor appeared in one of four locations/orientations in the periphery, according to a random order within a block of 4 trials. During the trial, whenever a corner of the cursor entered into the tolerance volume surrounding the corresponding corner of the target, the star on that corner changed its colour as an indication of capture. If all four corresponding corners stayed concurrently matched for 0.8 seconds, the trial was deemed completed. At the end of each trial, the trial completion time was printed on the screen. The beginning of each trial was signalled with a long auditory beep and the end of each trial was signalled with a short beep.
Since the objective of this experiment was to evaluate 6-DOF input
interfaces, the emphasis in designing the display was to provide
the largest possible number of 3D spatial cues. This was to ensure
that the task performance was driven predominantly by differences
in input controller conditions rather than by difficulties in
perceiving depth information (visual feedback). A 120 Hz sequential
switching stereoscopic display was employed, which has been shown
to be a necessary feature for this kind of experiment, because
without stereopsis, much more orientation ambiguity would be perceived
by the subjects. To enhance the 3D effect, perspective projection
and interposition cues were also implemented. The tetrahedra were
drawn in wireframe so that all edges and corners of the objects
could be perceived simultaneously. Subjects were asked to sit
on a chair approximately 60 cm away from the computer screen for
all experimental conditions.
2.5.4 Subjects
Eight paid volunteer subjects completed the experiment. All subjects
were screened with a Bausch and Lomb Orthorater. Two of the original
ten applicants were rejected, one for having poor stereopsis and
one for having poor (corrected) near vision. All of the eight
accepted subjects were (incidentally) male and right handed, as
determined by the Edinburgh inventory (Oldfield, 1971). Four of
them were undergraduate engineering students, three were graduate
engineering students, and one was a software engineer. All of
the subjects had experience with a computer mouse but none had
previous experience with 6 DOF input devices. All subjects were
asked to use their dominant hand to manipulate the input devices.
Subjects' ages ranged from 20 to 40.
2.5.5 Experimental Design and Procedure
Each subject participated in four separate experimental sessions ranging over four consecutive days, one session per day. Each session involved only one of the four control techniques and was divided into four phases (Figure 2.4). Each phase comprised 10 minutes of training, followed by 12 trials of data collection. Each training phase consisted of demonstrations and suggestions by the experimenter, combined with practice trials. The data from the 12 trials were grouped into 3 blocks of 4 trials, each block comprising 4 different randomly shuffled starting locations for the manipulated cursor. After every set of 12 trials, the mean completion time was shown to the subject.
A within-subjects design was used in the experiment. In order to minimise fatigue and skill transfer (either positive or negative) from one technique to another (Poulton, 1974), three preventive measures were taken. First, different manipulation schemes were tested on different days, so that different conditions for any one subject were at least 12 hours apart. Second, the 10 minute practice and training session preceding the first set of data collection trials served as a buffer to reduce transfer from the session of the previous day. Third, the order of the four conditions being tested was counterbalanced over the eight subjects by using two Latin square patterns, which resulted in every control interface being presented an equal number of times as the first, second, third or fourth condition.
A subjective evaluation of each condition was collected after
each subject completed all four conditions.