Re: gEDA-user: pcb, how to remove mm garbage (very short lines)?

[John Doty <jpd@xxxxxxxxx>]

On Nov 19, 2008, at 4:08 PM, Peter Clifton wrote:

> On Wed, 2008-11-19 at 15:44 -0700, John Doty wrote:
>
>> If you insist on no artifacts, it's zero.
>>
>> If you round imperial units to the nearest 0.01 mil, you have no
>> additional roundoff error if your fundamental unit is 1 nm, because
>> 0.01 mil is *exactly* (by definition) 254 nm.
>
> Good point... meaning it would be sensible to move towards some  
> kind of
> metric internal units, or an intrinsic coordinate system with units in
> metric being given the divisor of 254 * 10^n.
>
>> It's much harder to go the other way with a humanly comprehensible
>> rounding because of the factor of 127 in the definition of the inch.
>
> I get that 127 * 2 = 254.. which definition of the inch mentions 127
> explicitly though?

I have multiple reference books that state 2.54 cm = 1 inch exactly.  
Presumably there's some SI or NIST definition.

>
>> 2^32 nm is about 4.3 meters, large enough for any PCB I've ever seen.
>> 2^64 nm is about 0.12 astronomical unit ;-)
>
> Well.. I can't recall if we use signed or unsigned numbers throughout,
> so we might end up limited at half those.

If it's signed, you can go 2.1 m either way, so it's the same.

> Aren't 64 bit integers going
> to be slow on a 32bit CPU?

How slow. What fraction of computation is arithmetic in world  
coordinates?

>
>
> Practicalities of changing:
>
> 1. Change internal units: ought not to be _that_ hard since DJ nicely
> abstracted things away when he refactored to introduce the HID (IIRC).
> Make loading / saving convert to the old units, (rounded to integers).
>
> 2. Add unit specifiers to file-format, so users could, if they want,
> write out coordinates such as:
>
> 5mm, 10mm or 5 mm, 10 mm. from their footprint-generation scripts.
>
> Save the files out with native units, e.g. "123456000 nm"
>
> Perhaps, for optimal flare, we could make it more human readable, by
> writing out something decimalised, like: 123.456 mm (*)
>
> If the coordinates for a point happened to be a convenient decimal
> fraction of an inch (requiring less decimal places / significant  
> figures
> than the metric representation), then we could choose to emit in mils,
> or inches etc..
>
> * (Taking care not to loose precision if we were to read it back via a
> floating point number).
>
>
>
> -- 
> Peter Clifton
>
> Electrical Engineering Division,
> Engineering Department,
> University of Cambridge,
> 9, JJ Thomson Avenue,
> Cambridge
> CB3 0FA
>
> Tel: +44 (0)7729 980173 - (No signal in the lab!)
>
>
>
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>

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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