Re: [Plib-users] CAM using plib was: Wanted--bitmap

[Steve B. <sjb...@ai...>]

Marlin Mixon wrote:

> This is for a CAM application using foam and hotwire, so textures don't
> actually come into play. Here's what I'm doing and I think I'm on the right
> track:
> 
> I'm taking an aircraft fuselage and presenting it in a true sideview with an
> ortho projection.

OK.

> (ortho-projection = oxymoron?)

No - there are other kinds of projection - Perspective for example.

> (I'm not actually
> interested in the textures, they are just a bonus in that they are pleasing
> to the eye.)

I see.
 
> Then, by clicking my modified-viewer's right mouse button, it starts the
> recording process:  Using Steve Bakers snippet of code, I'll take a snapshot
> image of the sideview, rotate the fuselage like it was on a barbecue spit 10
> degrees, take a snapshot, 10 more degrees, snapshot, etc. until I've got 18
> images for 180 degrees.

Sounds painful.  I would have taken the 3D model, saved in some simple ASCII
format like 'TRI' (PPE can convert models to that) - and write a program to
plot out the profiles in (say) Postscript with a grid behind them...but that's
quite a bit of work.
 
> Now I've got 18 profiles.  I'll process them in such a way that I've got a
> silhouette, that is I'm only interested in two colors: model vs background.
> Finally, I'll need to figure out a way to compute the edge

(A Sobel filter for example - GIMP has one of those)

> i.e. find all
> the white pixels sitting next to black pixels and figure out the set of line
> segments that delineates the edge.  This I will call the "linear profile."

That's tremendously hard work...and a lot less accurate than going directly
from the triangle data.

I would:

 * Read the model into PPE.
 * Delete the wings and tail fins. (Select primitives and Edit/Cut)
 * Write the fuselage data out in ".TRI" format.
 * Read the data into a C++ program from a 'TRI' format file (it's
   *REALLY* easy to write a parser for that format).
 * Rotate the model whatever number of degrees is needed using routines
   from libplibsg
 * Compute the surface normal for each triangle (there is a PLIB/SG routine
   for that) - and discard those that are facing away from us.
 * Treat each triangle as three edges (each with two vertices).
 * Make a table of the UNIQUE vertices in the model...discard duplicate
   vertices.
 * Make a table of edges in the model - according to which vertices
   they share.
 * For each edge, check to see if there is a duplicate edge somewhere
   on another triangle.  Delete all edges that have duplicates - along
   with those duplicates.
 * Since the only edges remaining came from triangles whose neighbours
   were back-face culled, we have a pretty good profile.

> With the linear profiles, I can scale them up to real-world units and apply
> them to my neighbor's computerized foam cutter:  Situate the foam and cut
> the first linear profile.  Rotate 10 degrees and cut the second linear
> profile, rotate, cut, etc.

That should work.
 
> After the 18 profiles have been cut, you will have a real-world replica of
> the virtual fuselage.  I know there will be a few limitations, like if you
> left the tail feathers on, there would be some peaks and valleys that would
> not be cut enough--excess material.  Also any compound concavity won't be
> rendered correctly such as scoop inlets.

Yes...that's a problem...but it's inevitable with a hot-wire core cutter.

So, if this works, you owe me a 2 foot tall foam model of Tux - OK?  :-)

-- 
Steve Baker   HomeEmail: <sjb...@ai...>
              WorkEmail: <sj...@li...>
              HomePage : http://web2.airmail.net/sjbaker1
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