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Subject:	Re: [geda-user] 3D modelling and gEDA
From:	Peter Clifton <pcjc2 AT cam DOT ac DOT uk>
To:	geda-user AT delorie DOT com
Date:	Sat, 15 Jun 2013 21:56:03 +0100
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On Sat, 2013-06-15 at 02:03 -0400, Evan Foss wrote:
> I would like to point out that many if not most 3D CAD formats are BOT
> (Bags Of Triangles). Yes it stinks and the mechanical people hate it
> just as much as we do. STL is in use basically everywhere and it has
> the same problem.

This mostly applies to 3D modelling programs, targeting things like
games or artwork rendering etc.., where the rendering engine takes
triangular primitives. "Real" CAD is not this way. (Aside from STL
export for manufacture on a 3D printer or whatever).

This said, CAM programs do often use triangle approximations rather than
compute complex intersections of higher level curves.

> IGES/STEP would have been better because they uses cubic splines. This
> same functionality is available in Eagle right now using google
> Sketchup. I know that is based on COLLADA which I think is also not
> based on BOT.

Yes, but the above three mentioned formats are incredibly complex
(especially STEP). I would estimate tens of thousands of lines of code
go into a STEP -> object structure parser, and that's only the beginning
of using the format.

The machine-readable format validation file for STEP AP-214 (EXPRESS
schema) runs to 18,000 lines!

AP-203 is simpler, but still 5347 lines of schema.


STEP tries to cover a HUGE amount of scope, but sadly - it seems to be
the most important CAD format to support for interchange.

|Transfer is rarely perfect too, due to differences in the details of
the various CAD kernels which originate the data.


[snip]

> This is an off
> topic comment but I wish I could have footprint hole sizes based on
> desired open size diameter so that I could have PCB adjust the drill
> size based on the copper thickness I will be using.

The usual convention (in my world at least), is to specify that the
drill sizes in the PCB / gerber files are _finished_ size, and the
manufacturer will adjust as necessary for their process.


-- 
Peter Clifton <peter DOT clifton AT clifton-electronics DOT co DOT uk>

Clifton Electronics

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