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The recent desire to integrate knowledge and experience from the field
of cognitive science into the design of user interfaces and new
interaction paradigms has led to an appreciation of the importance of
presenting data within its context [3][4]. Our goal is
to allow interactive access to 3D information spaces while maintaining
context. This is achieved by providing a viewer aligned visual access
distortion which clears a line of sight to the object of interest,
permitting examination from all angles.
Figure 1. 3D Lattice with central focus made visible
This method extends the ideas and techniques that to deal with
occlusion in 3DPS [2]. Our algorithm proceeds as follows. Let L
be a line segment extending from the focal object or region to the
viewpoint (the line of sight), and a vector v be the shortest
vector from any other object O at position P to a point N on the
line L (used to establish the direction of the displacement). A
gaussian distribution is used to calculate the magnitude of the displacement
based on the length of v.
For a given value of |v| we determine the height h of the
gaussian. The characteristic bell-curve of the gaussian means that
for points near the line of sight (when |v| is small) h,
will be greatest. Points further from the line of sight will have
progressively smaller displacement values (Figure
\ref{crossection}). The distribution of the distortion can be
controlled by adjusting the height and standard deviation of the
curve. Since the viewing direction is along the line of sight the
resulting distortions will appear to be radially symmetrical about
the focus.
Figure 2. Cross section of expanding access distortion
Furthermore, the visibility of the focus will be maintained under
rotation of the data or motion of the viewpoint, smoothly deflecting
potentially occluding nodes away from the line of sight as they
approach it and returning them smoothly to their original positions (Figure 3).
Figure 4. The access function may be applied simultaneously to
more than one focus
This method scales well to multiple focal points. Here the
displacement D of any point P relative to a set of n line
segments Li, where each segment Li begins at an object of
interest Oi and terminates at the viewpoint V, may be
summarized as follows:
where Ni is the point on the ith line segment nearest the
point P and g() is the function that returns the height of
a gaussian given the distance from the point P to the line Li.
The resulting displacement is the average of several independant
distortions, each in a radial direction away from the line segment
Li. Thus it is possible to clear lines of sight to several objects
simultaneously. For example in Figure 4 the upper left
object is one layer deep into the 9x9x9 cube and the lower right one
is 8 layers deep, but still visible.
The viewer aligned visual access distortion provides a clear line of
sight to the internal structures of 3D data sets while maintaining
smooth integration within its context. It is view rather than data
aligned and provides control over the degree and extent of the
distortion.
Future research includes the application of this method to both
general 3D graph structures and solid 3D data, and an examination of
the potential use of perceptual cues to reveal the nature of the distortions.
This research was suported by NSERC and the Algorithms Lab, Graphics
and Multimedia Research Lab and the School of Computing Science,
S.F.U.
2. M.S.T. Carpendale, D.J. Cowperthwaite, and F.D. Fracchia. 3-dimensional pliable surfaces: For effective presentation of visual information. In UIST: Proceedings of the ACM Symposium on User Interface
Software and Technology, pages 217--226, 1995.
3. E.G. Noik. A space of presentation emphasis techniques for visualizing graphs. In Graphics Interface'94, pages 225--233, 1994.
4. R.Spence. A taxonomy of graphical presentation. Information Engineering Section report 93/3, Imperial College of Science, Technology and Medicine, 1993.
Abstract
We describe a novel solution to the problem of occlusion in viewing
three-dimensional data. A distortion function is used to clear a
line of sight to previously obscured interior elements.
Keywords:
Distortion viewing, 3D interactionINTRODUCTION
There is accumulating evidence [1] supporting the
idea that three-dimensional representations of data are advantageous.
Unlike 2D techniques, 3D viewing encounters a fundamental
problem in the display of information in that it is possible for an
object of interest to be partially or wholly occluded by other
objects. Current solutions provide access to such internal details
through the use of cutting planes, layer removal, fly-through, and
transparency. However, such techniques result in the loss of
contextual information.
VISUAL ACCESS DISTORTION
The problem of occlusion arises when objects lie on or near the line
of sight, between the viewpoint and the focus. The solution is to
move data objects away from the line of sight where necessary,
clearing a path to the focus. A distortion is applied radially about
the line of sight, displacing data items in gradually decreasing
amounts as their distance from the line of sight increases (Figure 1).
Here the scale and hue of the focal node have been adjusted to
distinguish it from the other nodes.
Figure 3. The distortion remains aligned to the viewer as the data is rotated (Quicktime)
CONCLUSION
Acknowledgments
References
1. K.W. Arthur, K.S. Booth, and C.Ware. Evaluating 3d task performance for fish tank virtual worlds. In ACM Transactions on Information Systems, 11(3):239--265, 1993.