kicad-developers team mailing list archive

[Bevan Weiss <bevan.weiss@xxxxxxxxx>]

Normally culling of polygons with non-camera facing normals is an optimisation and can be turned off.
Is this not possible in the 3d render libraries we're using?


Regards,
Bevan


On 1 Oct. 2020, 06:23, at 06:23, Cirilo Bernardo <cirilo.bernardo@xxxxxxxxx> wrote:
>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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