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
Projects : http://plib.sourceforge.net
http://tuxaqfh.sourceforge.net
http://tuxkart.sourceforge.net
http://prettypoly.sourceforge.net
http://freeglut.sourceforge.net
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