Re: [rp-ml] STL files

From: Wesley Brooks <wesbrooks_at_gmail.com>
Date: Thu Dec 07 2006 - 10:52:05 EET

PLY? This file format is something that I've come across when using the open
source "Visualisation Toolkit - VTK" supported by Kitware. This was a
project developed for the Human Visualisation Project, which I'm afraid I
know very little about. What I do know is that having a heavy emphasis on
Medical applications, it deals with slices of solids alot, almost doing the
exact reverse of what I want at times.

Anyway, going back to the three rules;

1. All triangles much conform to the right hand rule
2. All exterior surfaces must be devoid of gaps
3. Each triangle must share two of its vertices with each adjacent triangle

Should point three be changed to; 'Each triangle must share at most two of
its vertices with each of the adjacent triangles'? Consider the simplest 3D
shape that can be represented by triangles, a triangular base pyramid. In
this case the rule holds. Go to a square based pyramid and you get triangles
which share only one point, the sharp end of the pyramid.

I guess there are enough high up staff from RP machine manufactures on this
mailing list, why not add the ability to read in a new format such as a
points and IDs style format? Keep everything public domain open source and
review the standard yearly, in a similar way to large open source projects
such as python are managed. If the software for the machines are written
well all it would take is a simple bit of code to read in the new format,
which would be smaller, and more robust. Here's a set of rules which are an
obvious development on the first three;

1. All triangles much conform to the right hand rule.
2. All exterior surfaces must be devoid of gaps.
3. Each triangle must share at most two of its points with each of the
4. Where a triangle shares two points with an adjacent triangle the order of
the points must be a-b on one triangle ID list, and b-a, on the other.
5. Each point must be shared with at least three triangles.
6. Each edge must be, and only be common to two triangles.

These rules said there should also be a addition where by if 'surface' is in
the header then the triangular mesh can be open, relaxing rules 2, 5, and 6.

I agree with keeping to a triangular mesh format file for the time being,
but I do believe it's time we stopped talking about it and started to
propose a new format. Perhaps develop (or limit) an existing format such as
those in VTK (www.kitware.com)?

If the format (let's say *.orm, Open - Rapid - Manufacture file), is decided
on I guess it wouldn't take much to politely ask the main CAD package
manufactures to include outputs for it in there software too?

Cheers,

Wesley Brooks

On 07/11/06, steve <sjbaker1@airmail.net> wrote:
>
> ...snip...
>
> What you are describing is exactly what PLY does - except
> that you can squish the file still further by using quads
> and higher order polygons as well as triangles - and you
> can specify which vertex attributes there are and at what
> precision they are represented. (So you can have colours
> and/or texture coordinates, etc)
>
> PLY can also store 'List of Edges' so you can get a better
> handle on the topology of the model when cracks and gaps
> are a problem.
>
> But at the basics, what you laid out there was almost
> exactly what a minimal PLY file looks like.
>
Received on Thu Dec 07 09:35:30 2006

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