On Wed, Apr 4, 2012 at 6:32 PM, Edward <55549...@qq.com> wrote:
> Hi!
> when I input draw sph.brep in command line,It only draws a half
> circle.It is different from draw sph(draw a frame model). I‘m confused
> about it. Is it related to the structure of brep?
>
Short answer - yes, it has to do with the details of how a BREP sphere is
defined using a NURBS surface.
Long answer - when BRL-CAD draws a wireframe view of a NURBS-based
primitive, what you're seeing by default is a 3D representation of just the
parts of the surface corresponding to the "edges" of the NURBS surface.
For a sphere, which is pretty much worst case, all of those edge point
projections into 3-space fall along that one curve you're seeing, with the
net result of being a very unhelpful illustration of the 3D shape being
defined.
If you think of the NURBS "surface" defining the sphere as a square that is
mapped into three-space using an equation, and only the lines at the edge
of the square get to be visible, what ends up happening in this case (if I
recall correctly) is two edges map entirely to points at the top and bottom
of the sphere, and the other two sides both end up mapping to the arc you
see. So we would need to produce more lines by sampling the interior of
the NURBS surfaces to show a better sphere.
In many real world situations a sparse wireframe isn't a bad thing - it
lets you get manipulable views of large models - but the sphere case is an
illustration of how this approach can fall apart in certain cases.
CY
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