(cm) 
» 
a 
  
  
  
  
0 
(cm) 
Figure 4. Effect caused with a hammer head. 
4.2 Motion Capture of a Hand 
The system proposed in this paper allows a person to ma- 
nipulate a virtual tool according to his intention, and the 
operationality of the tool to be promoted by giving both his 
virtual hand and the virtual tool the movement reproduced 
using the data obtained from the operation of the real tool 
with his own hand in an ideal real environment. The motion 
of a hand is captured 30 times in every second at both the 
cases when he hits a nail with a hammer and when he uses 
a nail extractor. Figure 5 shows the data acquisition pro- 
cess. The time taken to hit a nail is about 8 seconds and the 
one to pull it out is about 3 seconds. 
  
Figure 5. Environment for experiment. 
4.3 Analysis of Hand Movement 
Figure 6 shows the motion of a hand observed during a 
hammer is hitting a nail. You can see from the figure that 
the motion of hand draws a small ellipse while the nail is not 
yet enough driven into the board, but that the size of ellipse 
becomes big as the nail is firmly fixed to the board. The 
positional relation between a hammer and a nail is not 
shown here, it has become clear that the head of a hammer 
hits a nail not from right above but from a little aslant direc- 
tion. It seems that this is due to the characteristic of our arm 
joints. Consequently an simple up and down motion of a 
hammer is not enough to give an operator feeling that he is 
really hitting a nail. 
(cm) 
  
Figure 6. A locus of a hammer head used as hammer. 
Figure 7 shows how the hand moves observed while a nail 
is pulled out with a claw hammer. You can see that it moves 
along an arc that has the nail as a center and the handle of 
the hammer as a radius. Comparison with the hitting opera- 
tion, the motion is a two dimensional one. 
  
30 
?8 F 
24 | 
(cm) 
  
  
  
  
0 
(em) 
Figure 7. A locus of a hammer head used as puller. 
5. EXPERIMENTAL SYSTEM 
5.1 Models of Tools and Objects 
We provide 3 kind of models for each object or tool accord- 
ing to their objectives to decrease computational costs 
needed to realize a virtual environment. Figure 8 shows the 
3 models corresponding to a given hammer. The first one 
provides a model just like to the real hammer, which is used 
to visualize how the virtual one is operated. The second 
one provides a model for collision detection, which consists 
of several bounding boxes each of which is used to exam- 
ine which portion of the model collides with another object, 
and the last one is defined as a set of directed lines for 
examining what kind of interaction will occur between the 
tool and an object. 
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