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Figure 12.18 Flexible pointer
ambiguities in selection. This is the “Aperture'' technique presented by Forsberg et al.
Zhai et al. (1994) extended single ray casting to a ray to the tip of which is attached
a semi-transparent three dimensional cross (the Silk Cursor). This metaphor is based
on Fitt's law that says that, in 2D, selection time reduces when the surface to be selected
increases. Authors have applied this law to a 3D environment and to a selection volume:
greater the selection volume, lesser the selection time. While transparency helps to
avoid occultation, information density helps to better locate the ray in space. Olwal
and Feiner (2003) imagined the flexible pointer (Figure 12.5.2), which is an extension
of classic ray casting that makes it possible to point more easily to objects that are
obscured, fully or partially, by other objects. The ray can bend in space in order to
reach its target. The curve and length of the ray are controlled with the help of two
hands and the curve thus created is a Bezier curve with 3 control points.
Other methods make the selection of distant objects that are beyond the reach
of the user's hand easier. Pierce et al. (1997) developed a group of techniques called
“Image Plane Techniques''. Their principle is to reduce the task of 3D selection to a
2D task: the user selects the object by pointing to its projection. The selected object
is found on the right through the eye-point and the user's outstretched pointing finger
(“Sticky Finger'') (Figure 12.19). The “Head Crusher'' technique is similar, as selection
is done when the user positions his thumb and forefinger around the projection of the
object (Figure 12.20). The selection is easy and intuitive, but the modification of the
distance to the object is difficult, since only the projection of the object is controlled.
Schmalstieg et al. (1999) completed the projection plan with the help of a trans-
parent plexiglass tablet that the user holds in his non-dominating hand. It is possible to
select one or more objects in two ways. On the one hand, with the help of a stylus, the
user wraps one or more objects, which are visible due to transparency, on the tablet.
On the other hand, the user can centre align an object in the middle of the tablet for
selecting it. To make a modification in the distance of the selected object possible,
Bowman et al. (1997) proposed a technique derived from ray casting to which a “fish-
ing reel'' feature was added. Once the object is captured, the user can move the “line''
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