Re: [Lib2geom-devel] Subpath from path by length?

[mathog <ma...@ca...>]

On 04-Sep-2012 12:28, Johan Engelen wrote:
> On 4-9-2012 21:14, mathog wrote:

>> subpath = make_sub_path(P,from,start,end,&trunc);


>    Have a look at Path::portion. The parameters are "time-values" 
> along
> the path I believe, not distance. To match time values with distance
> along the path, you can do an arc_length_parametrization, but I think
> that is only available for Piecewise<D2<SBasis> > paths. But it may 
> not
> be necessary to do that (expensive) operation, if you can derive your
> start/end points as time-values in a different way.

By "time-values" you mean the "t" paramter 0->1 for a Bezier curve?

Does portion() work for a path that consists of more than one Bezier 
curve? Here is a
specific example using line segments (which could be represented by 
Beziers):

P = {{0,0},{0,1},{1,1},{1,2}};

subpath = make_sub_path(P,{0,0},0.5,1.7,&trunc);

Results in subpath = {{0,0.5},{0,1},{1,1}{1,1.2}}

I don't see what a 0->1 "t" could be in this situation other than by 
first calculating the length of P and then
making it a fraction of that length.

Thanks,

David Mathog
ma...@ca...
Manager, Sequence Analysis Facility, Biology Division, Caltech