Experiment 2

The second experiment consisted of one user study designed to test whether adding cast shadows to the principal curvature texture technique had any benefit and whether allowing limited animation (surface rocking) in Experiment 1 masked that benefit. I expected that the rocking animation did, in fact, mask the benefits of cast shadows because the kinetic depth effect is triggered by the object motion and produces a very strong perception of surface shape.

This experiment used only the distance task. Including only one task deviates from the previous decision to accommodate our collaborators and perform two shape perception evaluations. The dis- tance task is the one for which I expected principal curvature texture to realize the most benefit from cast shadows. It was also more expedient to run only the one study in this experiment.

5.4.1 User Study 2.1: Distance task Methods

Subjects: Seven undergraduate and graduate students performed the distance task. Two partici- pants also took part in User Study 1.2. All participants had normal or corrected-to-normal vision and reported normal color sensitivity but were not tested. Participants were compensated for their time.

Stimuli: The bump data sets from Experiment 1 were re-used for this experiment. The interface and view parameters were also the same as Experiment 1.

Design: The principal curvature texture technique was the basis for all techniques in this experi- ment. This study used a two-by-two factorial design for the display technique with the presence of cast shadows and the availability of rocking as the two factors. When rocking was part of the technique, participants were allowed to initiate the torsion-pendulum animation at will after its initial occurrence. When rocking was not part of the technique, it did not ever occur.

For this study, only the distance task was considered. It was presented in exactly the same manner as in User Study 1.1 (with allowances for rocking). The range of differences between region dis- tances was restricted to 0.5 grid units through 3 grid units (approximately 1 mm through 8 mm) in 0.5 grid unit increments. The analysis of User Study 1.1 found that at the short-distance end partici- pants performed near chance and at the long-distance end participants percentage of correct responses plateaued. Because this experiment involved four visualization techniques, reducing the number of trials also helped control the time to complete a session.

Each participant viewed 33 trials for each of the four visualization techniques for a total of 132 trials. Other aspects of each session were as in User Study 1.1. Trials were randomly ordered for each participant. Training was similar to Experiment 1.

Participants were asked to respond to the following question:

In which circled region do the two surfaces appear to be closer together? This experiment group does not include a questionnaire.

Hypothesis

The hypothesis is that the percentage of correct participant responses depends on the presence or absence of shadows but that the presence of rocking can mask the effect.

Independent variables:The design directly manipulates three independent variables – the distance difference between regions, the presence of shadows, and the presence of rocking. Also recorded are the following predictor variables: a random unique identifier for each participant, the participant’s gender, the participant’s response time, and the number of times the rocking animation is triggered.

Dependent variable: The dependent variable in this experiment is the percentage of correct re- sponses.

I expected shadows without rocking to enable better task performance than principal curvature texture with no shadows and no rocking. I expected rocking to provide more benefit than shadows, thus explaining why Experiment 1 showed no benefit for shadows.

Results

Unless otherwise noted, independent variables have no statistical significance (p> .05).

Analysis: Two-way ANOVA analysis finds significant main effects for rocking (p< .001) and response time (p< .01). Rocking can be said to have an overall impact on task performance, but the same can not be said for shadows. A significant interaction effect was also found between shadows and rocking (p< .001). The interaction can not be interpreted without a post hoc test. Figure 5.8 shows the overall percentages of correct responses and 95% confidence intervals by visualization technique; the figure also shows that the performance of participants is better than chance. A Tukey’s HSD test finds the following:

• using any combination of rocking and shadows is better than using none,

• combining shadows and rocking is not separable from using either alone, and

User Study 2.1: Distance Task

Neither Shadows Only Rocking Only Rocking + Shadows

Display Technique

Correct (%)

Figure 5.8: This figure shows the overall percentages of correct responses and their 95% confidence intervals for the distance task while comparing the effects of rocking and shadows applied to the principal curvature texture technique. Tukey’s HSD test finds that introducing shadows or rocking to the curvature texture technique conveys inter-surface shape better the texture alone.

Given thepost hocTukey’s HSD test, it can be said that shadows provide a benefit in the absence of rocking.

The fact that response time is a predictor of correct responses but animation count is not a predictor suggests that response times may not be lengthened solely by triggering the animation, as I suggested in Experiment 1. However, it is still the case that participants provide more correct responses the longer the response time.

Discussion: Experiment 2 appears to support my claim that including a rocking animation in the first experiment masked any benefit shadows would have provided. This strongly suggests removing the animation component from further user studies to better control the experiments. Of more general

interest, it can be said that adding shadows to the principal curvature texture technique is beneficial for comparing inter-surface distances in static images (like those intended for print publication). If prioritizing between animation and shadows, animation should be added to the visualization first.

On the other hand, I claim a potential benefit of including shadows even in interactive visualiza- tions (such as the exploratory visualization of interest to the scientists). Consider the perception of shape from animated point samples and how quickly the perception of shape fades once the motion stops. I claim a similar perceptual loss for principal curvature texture is possible. Cast shadows could help minimize the difference.