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Date: Tue, 5 Jan 2016 07:56:31 +0100
From: "Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Subject: Re: [geda-user] Merging stuff. How to make it happen (Geometric
 library)
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Regarding geometric libraries http://www.cgal.org has an impressive developers list. Stephan Böttcher brought up clipper http://angusj.com/delphi/clipper.php a short while ago and started to write on a gerber file clearance checker http://www.psjt.org/code/drc/screenshot.png, have the project here http://www.psjt.org/code/

Nicklas Karlsson



> >> > With regards rewriting geometry functions, I'd be interested in a why -
> >if
> >> > they already worked. The old code looking ugly would be a fine reason,
> >just
> >> > be extra sure the replacements will be well behaved ;)
> >>
> >> They were being duplicated all over the place.  We discussed this a
> >> while back and agreed that collecting all the 2D/3D generic math
> >> functions in one spot made sense.
> >
> >I'm drawing a distinction here between "collecting" and "rewriting". The
> >later needs at least some justification.
> >
> 
> I am not sure about the state of the git head mainstream version, I'm 
> talking about pcb-rnd (forked from a 2013 verison of pcb).
> 
> While working on the cycle drag and semi-related negative box selection, I 
> needed a few calls that decided whether objects touch an axis aligned box.
> 
> I though there was some easy way to do this with existing infra, but 
> couldn't find anything trivial. I figured there were two problems:
> 
> 1. There was no much generic geometric "does this line intersect that 
> circle" like code; instead, search.c has some IsPointOnLine() and co., 
> which take complex PCB object type as one end of the comparison. This is 
> totally fine for DRC, but if you want to implement something else and you 
> really need to check aritrary theoretical objects, they can't be reused.
> 
> 2. Other parts of the code seem to struggle with this. The solution is, as 
> DJ said, is that everyone coded something locally. A random example is the 
> "rubber_callback()" that checks if a line touches a selection box.
> 
> Since the coordinate type is an integer (I consider it sort of a fixed 
> point number, as it is in nanometer), and values are large, calculations 
> may overflow. E.g. assume signed 32 bit coords, you get about 2.1 meters 
> as max board size. Now look at rubber_callback(): at some point it does an 
> x*=x. This means x has to be wider than int32_t else the actual limit 
> comes down to sqrt(2.1). In that piece of code the "intermediate 
> calculation type" is double. This could work well with 32 bit coords as 
> double provides 53 bit base. However if the user configures pcb to use 64 
> bit coords, this is not true anymore (although the board needs to be 
> ireally big to reach 53 bits). Theoretically we'd either need some 
> clipping on the int coord side or long doubles for intermediate 
> calculations. When converting between ints and floats, there may be some 
> system specific behaviour around rounding - some code doesn't seem to care 
> about this.
> 
> For all these, I think it's reasonable to create a centralized, generic 
> (non-pcb-complex-type-oriented) geometry lib that could remove code 
> duplication, could centralize the solution for clipping/rounding. Ideally 
> the complex-pcb-type functions would deal with interpreting the types 
> and then call the generic geometry functions too. It could not only help 
> removing potential bugs but would ease adding new code that needs to deal 
> with geometry.