Re: [Visualpython-users] VPython 5.2

[steve m. <mwi...@gm...>]

Bum! it is for Windows and Mac only!
why there is no Linux?

On Wed, Jan 27, 2010 at 4:05 AM, steve mtangoo <mwi...@gm...> wrote:
> Hope this will have no that make bug in Linux Box when compiling from
> source, downloading hoping to have VP up and running in My Mandriva
> Box!
>
> On Wed, Jan 27, 2010 at 1:36 AM, Bruce Sherwood <Bru...@nc...> wrote:
>> Available at vpython.org: VPython 5.2 for Windows and Mac which adds a
>> new feature for smoothing objects made of faces.
>>
>> After creating a faces object named (say) myfaces,
>>
>>    myfaces.smooth()
>>
>> will average similar normals at a vertex and make the object look
>> smoother. You must make sure that all vertices have nonzero normals
>> before using the smooth function; a simple scheme is to make all the
>> normals perpendicular to their respective faces and then let the smooth
>> function adjust them for improved appearance.
>>
>> The issue that this addresses is that if you try to make a
>> smooth-looking curved surface but adjacent faces have different normals,
>> the surface has creases in it. Smoother appearance is obtained if
>> similar normals at a vertex shared by two or more triangles are averaged
>> and the average normal applied to all these triangle vertices. However,
>> at  a vertex shared between two surfaces with very different
>> orientations, such as perpendicular surfaces, no such averaging should
>> be applied. The new smoothing routine only averages shared normals if
>> they don't differ too much in direction (about 18 degrees).
>>
>> The example program "faces_heightfield.py" has been revised to use the
>> new smooth() function.
>>
>> Bruce Sherwood
>>
>> P.S. Maybe someone would find the algorithm interesting, and I'll
>> describe it in terms of Python code, though it is implemented in C++ for
>> speed:
>>
>> First pass: Construct a dictionary whose keys are vector positions of
>> vertices and whose values are lists (or rather Python "sets") of indices
>> of vectors in the faces pos (or normal) array.
>>
>> Second pass: For each key (spatial location of a vertex) in the
>> dictionary, get the value (the set of indices of pos or normal vectors
>> each of which corresponds to the same position in space). Pick an index
>> in this set, evaluate its (normalized) normal, and one at a time take
>> the dot product of this normal with the (normalized) normals of all the
>> other members of the set, creating a list of indices for which the dot
>> product of the normals is > 0.95 (so the normals differ in direction by
>> no more than 18 degrees). For this subset, average their normals and
>> assign them to this subset, then remove the subset from the larger set
>> and repeat until the set is empty.
>>
>> This procedure assures that neighboring faces that don't differ much in
>> orientation will not have a sharp crease between them, but neighboring
>> faces that have very different orientations, including for example
>> perpendicular faces, will have very different normals.
>>
>> There is an inappropriate bias in this scheme in that the starting
>> normal for searching through the set might be an extreme one and
>> therefore one might fail to identify the right bundle of normals to
>> average. Maybe someone will think of an unbiased way to do the bundling.
>> I can say that having tried the smooth() function on various objects
>> made of many faces, the scheme seems to work pretty well.
>>
>> Another possibility would be to look at edges shared by triangles rather
>> than at vertices shared by triangles. At the moment I don't see how to
>> do that.
>>
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>