On 2012-09-17, Steven D'Aprano <st...@pearwood.info> wrote: > On 17/09/12 11:15, Pete O'Connell wrote: >> Hi, I have a bezier line with 20 points on it and I am trying to reduce >> this to a line with 4 points, keeping the first and last points and 2 >> evenly spaced points in between like so: > > In general in Python, it is faster and much easier to create a new list > rather than to mess about deleting items from a list. > > So instead of calling del 16 times, which has to move list items around, > it will be faster and simpler to create a new list: > > new = [old[0], old[6], old[13], old[19]] > > then just use the new list. If you really need to modify the old list in > place (perhaps because other parts of the code need to see the same > change) then you can do a slice-assignment: > > old[:] = new # note the [:] slice >
I certainly agree with Steven's comments here that there are much less convulated ways to replace a list with a new list containing a subset of its elements. I would also like to add, though, that I'm not sure if what you're doing makes sense from a mathematical/geometrical viewpoint. It's not totally clear to me from your original post what you're trying to do. I can think of two interpretations: 1) You have a Bezier curver consisting of 20 control points and would like to approximate it with a simpler cubic Bezier curve consisting of only 4 points. I would call this "approximating a Bezier with a lower order Bezier". 2) You have a line that goes through 20 points and you would like to represent it as a cubic Bezier with 4 control points. I would call this "fitting a cubic Bezier to a sequence of points along a line". In either case I think that choosing 4 of the points and using them as a cubic Bezier is incorrect. For case 1) I don't think it's possible to approximate a Bezier using a subset of its control points. I'm not sure how to prove it but I think that typically a significantly higher order Bezier will have control points that are closer to the curve than an equivalent lower order Bezier. For case 2) it's not appropriate to think of a Bezier as interpolating its control points because it doesn't go through it's control points (except for the two at the ends). In either case I would expect a good method to use information from all of the original sequence of points rather than ignoring all but a small subset. Oscar _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
