Phase 2
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Phase 2 - Design and Analysis of Interaction Metaphors for the Co-Located User 

Contents

Overview

Summary

Colocated Interaction 

Experiment Design

Overview

This was the in the second experiment phase we wanted to study techniques that were customised for the user group.  Two main tasks were undertaken:

 

  1. We introduced a new interaction technique, shadow cone selection, that was designed to better support complex selection tasks where it might be difficult to point directly at the object. 

  2. Secondly, we wished to show experimentally that the performance of shadow cone was no worse than common alternatives. Since shadow cone enables some otherwise impossible selections, we would be able to claim that it would be a universal replacement for these techniques.

Summary

IPTs allow small group interaction within the space of the IPT. The most common demonstration scenario, as shown in Figure 2b, is one “pilot” head-tracked with another demonstrator in control of the situation. In this situation the demonstrator may need to indicate items of the display or control the environment, but they are seeing a distorted view, as shown in Figure 3.

An experiment was undertaken in May 2003 to compare head-tracked verses non-head tracked tasks performance [22 ]. It had been suggested by others that interaction on IPTs for non-head tracked users would be either difficult or impossible. We showed that actually the non head-tracked user (typically the demonstrator) could interact very rapidly with a suitable choice of interaction metaphor.

We introduced a new interaction metaphor called shadow cone selection. All the objects with a cone are selected when the user presses a button, and as the user moves their hand only objects that are always within the cone are selected when the button is released. To implement this, when the user presses the button, the potential selection set is initialised with all objects that are within a target angle of the ray from the hand. On each frame we remove any object is no longer within the target angle and we don’t add new objects. When the user releases the button the objects remaining in the potential selection set are then selected. This can be visualized by imaging turning on a spotlight in a dim scene and selecting only the objects that are always in the spotlight before it is turned off.


The experiment compared shadow cone, standard cone and ray casting on selection tasks. The results were complicated because no technique stood out as the best on all tasks. The results can perhaps best be conveyed by the guidelines we proposed in the associated paper:

·          Virtual hand should only be used for SIDs when the user will be head-tracked. We suggest that implementers avoid it for SIDs because of the variety of viewing scenarios.

·          Ray, Cone and Shadow Cone selection are all suitable for non-head tracked SID users. Go-Go hand, image plane selection or aperture selection are not.

·          Close-by manipulation should only be attempted with head-tracked egocentric views. For the non-head tracked view, only manipulation at a distance should be attempted.

We can also propose some tentative guidelines:

·          Consider using Cone Selection or Shadow Cone Selection in preference to Ray Selection if there is any significant jitter in the tracking system.

·          Shadow Cone Selection and Ray Selection are more suitable than Cone selection for complex tasks involving clustered objects.

 

Colocated Interaction 

Co-located interaction concerns interaction of a group of people in an IPT with the virtual environment. It is most common for one user to control both the head-tracking and wand device and we will concentrate on this case. We note for possible future investigation that it is not un-common for the head tracker and wand device to operated by different users. This might occur in a demonstration scenario where the "client" is head-tracked and seeing the correct view, and the "operator" is standing close by effecting selection, manipulation and locomotion with the wand device.  

  
Two views from the IPT of a user using Ray Selection to select the yellow object from a small cluster of distracting blue objects.  (a) Shows a head tracked user’s view of the scene. The ray appears straight. (b) Shows a non-head tracked user’s view of the same situation where the view is generated for a head-tracked user who is standing 0.75 m to their right. Thus the projection is incorrect, and the ray appears bent across the corner between two IPT walls. The non-head tracked user can still select the object.

Little work has been done in the area of co-located interaction, but it is certainly an important area for study since IPT systems are very frequently used by groups of people. With co-located users the key problem is that the non-tracked users see a distorted view of the world. In our context this most seriously manifests itself when the tracked user points at an object in the scene. Because the other users see a distorted view of the world, they are unlikely to be able to detect the object being indicated without there being explicit visual feedback. 

A second problem for co-located users is that they are not in-control of locomotion of the IPT and thus might not be prepared for the sudden visual flow associated with locomotion. Unexpected visual flow is well-known to be a contributing factor to simulator sickness. 

A third problem related to the previous two is that the non-head tracked users might mis-understand the direction and/or target of motion.  

Experiment design

With co-located users the key problem is that the non-tracked users see a distorted view of the world. In our context this most seriously manifests itself when the tracked user points at an object in the scene. Because the other users see a distorted view of the world, they are unlikely to be able to detect the object being indicated without there being explicit visual feedback. 

This 2nd set of experiments seeks to investigate the effect of interacting with an environment from a non head tracked perspective (i.e. with view distortion). We tested the performance of 3 interaction metaphors over several simple scenarios to be performed in the IPT. Half the subjects performed the task with the viewpoint being set by the position of the users tracked head, and so these users were interacting  with no view distortion. The other half of the subjects performed the tasks with a fixed viewpoint (at the centre of the IPT at head height). For these subjects the viewpoint is only correct if there head happens to be at centre of the cave, as they move further from the centre the view distortion will increase.

See Task description for results of this experiment.