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Date:	Sat, 15 Jun 2013 19:33:38 -0400
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Subject:	Re: [geda-user] 3D modelling and gEDA
From:	Evan Foss <evanfoss AT gmail DOT com>
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What do you think of opennurbs?

On Sat, Jun 15, 2013 at 4:56 PM, Peter Clifton <pcjc2 AT cam DOT ac DOT uk> wrote:
> 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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Work
http://forge.abcd.harvard.edu/gf/project/epl_engineering/wiki/

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