| Polygon Problems [message #19611] |
Wed, 29 March 2000 00:00 |
Struan Gray
Messages: 178
Registered: December 1995
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Senior Member |
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This may be related to Rick Towler's problem (see "Q:how to use
MESH_OBJ") but I'm not sure so I've started a new thread.
I'm playing with IDLgrPolygon objects and have hit two blank walls
simultaneously. I can't decide if it's me or IDL which is being dumb.
I am making Platonic solids, and want to have each solid in a
single IDLgrPolygon object, using a vertex array and polygon array to
specify the vertices and faces. The first problem is that IDL only
seems able to associate normal vectors with vertices, and not with
faces.
Note that the default method IDL uses to compute and store the
normal vectors used in the light-shading calculations takes each
vertex and calculates an average of the surface normals of the
polygons which share that vertex. This is an appropriate and useful
way to do things for smoothly-varying mesh structures, but is totally
wrong for sharp corners.
This doesn't matter for polygons like tetrahedra and cubes because
they have more vertices than faces, so provided you make sure that
each face as a unique first vertex in the polygon array you can
calculate your own normals and the light-shading routines happily use
them instead of the defaults.
However, if the polygon has more faces than vertices there is
nowhere to store some of the normal vectors. Something as simple as a
regular octahedron (eight faces, six vertices) becomes impossible to
display correctly as a light-shaded solid 3D model. Oops.
The only way round this that I have found is to make duplicate
vertices and/or split the polygon into two seperate ones. This is not
only ugly, it complicates the setting of parameters, confuses
programmatic ownership of the vertex data and is going to make my life
dirty and slow when I start calculating things like whether arbitrary
3D points lie inside or outside the polygon.
My second problem is that I would like to display my polygons with
an edge colour which is different from the face colour. I would also
like to do this with IDLgrSurface objects, which seems to be a related
problem.
For both types of object you can either colour the edges, or the
faces, but I can find no combination of properties, texture maps or
whatever which lets me do both. I have made an ugly hack where I
share data with a second IDLgrPolygon object or with an IDLgrPolyline,
which draws the lines in the right places, but the plotted lines look
spotty, even when I make them several pixels thick and independently
of the order in which I plot the two objects.
The *really* annoying thing is that several of the object graphics
demos (like the teapot, knot and shell) do this very nicely, but
because they are locked up in .sav and .dat files I can't tell how
they do it. This at least tells me that my problem is not the OpenGL
implementation on my machines.
Below is a function which makes an octahedron. If anyone can come
up with an elegant way to a) plot it correctly as a shaded solid
object, and b) colour the edges differently from the faces, I will be
forever in their service.
Struan
; ************************************************************ ***
function Octahedron
; make a unit octahedron centred on [0,0,0]
rt2 = sqrt(2.0)
vertex_array = [ $
[ 0.0, 0.0, rt2], $
[ rt2, 0.0, 0.0], $
[ 0.0, rt2, 0.0], $
[-rt2, 0.0, 0.0], $
[ 0.0, -rt2, 0.0], $
[ 0.0, 0.0, -rt2] $
]
poly_array = [ $
[3, 0, 1, 2], $
[3, 2, 3, 0], $
[3, 3, 4, 0], $
[3, 4, 1, 0], $
[3, 5, 1, 4], $
[3, 1, 5, 2], $
[3, 5, 3, 2], $
[3, 5, 4, 3] $
]
return, obj_new('IDLgrPolygon', $
data=vertex_array, polygons=poly_array)
end
; ************************************************************ ***
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