1 in = 25.399999999972 mm not exactly 25.4 but close enough for layout work if you use high enough precision.
25.4 to 1 might not be close enough if you are trying to put a satellite in orbit around mars. Steve M. On Wed, 2008-11-19 at 18:58 -0700, John Doty wrote: > 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!) > > > > > > > > _______________________________________________ > > geda-user mailing list > > [email protected] > > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user > > > > John Doty Noqsi Aerospace, Ltd. > http://www.noqsi.com/ > [EMAIL PROTECTED] > > > > > _______________________________________________ > geda-user mailing list > [email protected] > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
