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(2) Open forms There are various kinds of projection drawings as
mentioned above. Each has its good points, however, as for its weak points,
it is impossible to achieve the real shape in three-dimensional space,
that is it is not a complete shape. As the plan for drawing a solid, especially
in respect to manufacturing the shape, it is very convenient if there is
an ideal method to relate to the manufacture of the real shape of the main
plane. So there are open forms for this. In the case of solids, the external
shape that covers the shape has the most important meaning. Also in two-dimensional
space, the outline had an important meaning. There was a big difference
in an observer's sense between the filled shape and the hollow shape. On
the contrary, in the case of a solid, once it is covered by the external
surface, the inside can not be seen. Therefore it is visually the same
as a filled shape. Sculptures made of metal or non-transparent plastic
use this principle. In the case of making a large work with expensive materials,
it is unnecessary to make a heavy filled type work taking much time. In
many cases, it is much easier to construct it from drawing materials in
terms of manufacturing.
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