kicad-developers team mailing list archive

[Cirilo Bernardo <cirilo.bernardo@xxxxxxxxx>]

Hi Mario,

 I never thought of a good way to manage the doubling of the vertices /
viewing from both sides. Since this is a problem of IGES as well as
VRML models (especially models not produced via MCAD) and it
makes no sense to me to complicate KiCad or the 3D viewer, maybe
one way to fix this would be to mark a bad model file by putting a
'_' at the end of the name, for example 'broken-model_.igs". Files
with that tag will be assumed to have bad normals and the vertices
will be doubled, otherwise normals are assumed to be good.

Cirilo


On Thu, Oct 1, 2020 at 12:36 AM Mário Luzeiro <mrluzeiro@xxxxx> wrote:
>
> Thanks Cirilo!
> It has been small additions but resulted in good improvements...
>
> > results in a doubling of the triangular surfaces
>
> Could an option be added to this? ( both exporter and 3D-Viewer )
>
> I didn't know / don't remember this condition, but I have an idea of some inverted models cases too...
>
> Mario
>
> ________________________________________
> From: Kicad-developers <kicad-developers-bounces+mrluzeiro=ua.pt@xxxxxxxxxxxxxxxxxxx> on behalf of Cirilo Bernardo <cirilo.bernardo@xxxxxxxxx>
> Sent: 29 September 2020 04:51
> To: KiCad Developers
> Subject: [Kicad-developers] Thanks for all the new work on the 3D rendering.
>
> I see the latest changes from Mario Luzeiro and Oleg Endo.  Wow .. the
> 3D viewer has improved so much since Mario and I rewrote the code 6+
> years ago.  I'm very glad to see people continuing to make
> improvements.  If anyone has time and wants a challenge there are 2
> items I'm not very happy with in the 3D system. One is due to our
> early support for VRML only models as well as IGES models, and the
> second issue is due to mistakes I made in interpreting the VRML
> specification.
>
> 1. Legacy issue: the import of a VRML model (including those exported
> by kicad) or an IGES model results in a doubling of the triangular
> surfaces. So if you import a box described by 12 triangles that is
> double to 24 and if you export the model and import it again you get
> 48 triangles, and so on. The reason is that every facet is rendered
> from *both* sides and there is no algorithm for decimating the
> triangle data.  The reason triangles were rendered from both sides is
> that many (well, *most*) VRML models which we used before the 3D
> overhaul were defective and the viewing normals were not guaranteed to
> be correct (a facet described as viewable from the wrong side).  IGES
> models also suffer this problem because the specification does not
> provide a requirement of a 'right-hand rule' to determine the Inner
> side vs Outer side of a surface and various CAD packages seemed to
> either use a right-hand rule, left-hand rule, or no rule.  The
> triangle bloating problem will obviously have disastrous consequences
> for rendering time.  As an alternative to decimating the triangles,
> any VRML model exported by KiCad could be specially tagged (similar to
> how PDF uses some special comments) so that the importer does not
> double the triangles. The advantage of this would be that the VRML
> models generated from solid models via scripts could also be tagged
> and the doubleg could be avoided altogether where appropriate.
>
> 2. Incorrect interpretation of VRML spec.  This is really a fairly
> minor issue and I don't believe there is any evidence that it has an
> impact on VRML models generated from solid models via
> OpenCascade/FreeCAD.  Unfortunately I can't remember the exact problem
> now, but at least one of the VRML groups are not correctly implemented
> and as a result cannot be correctly referenced by more than 1 other
> group. Fixing the implementation could produce a more efficient
> exported model in some use cases.
>
> Cirilo
>
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