On 02-May-06 Mike Saunders wrote: > I am modeling a trend surface using trmat and want to trim the > resulting matrix to the area enclosed by my real data (i.e., > remove all the extrapolated areas). I was using chull and > in.chull to calculate the convex hull and change all the other > values created by trmat to NA. However, my real data has portions > that are slightly concave so chull would give me slivers that are > extrapolations from my data. Is there some other type of "chull" > function that will allow the resulting polygon to be slightly > concave? (I can send a picture to show what I am trying to do, > if needed)
If I understand you correctly, what you are trying to do is not well-determined, though one can think of ways of achieving the general aim. One approach might be to replace each segment of the convex hull by a circular arc, pulled perhaps towards the centroid until it first meets an interior point. Or you might, for each bounding segemtn fo the convex hull: 1: Select the nearest interior point to a boundary segment, and make it the vertex of a triangle whose other two are the end-points of the segment; then remove the segemnt. Or: 2: Select the two nearest points to the segment and, along with the two end-points, make the convex hull of these four points; then remove the segment. Or: ... k: Select the k nearest points to the segment and make the convex hull of these k+2 points; then remove the segment. ... Come to think of it, perhaps your picture might help! Best wishes, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 02-May-06 Time: 23:16:52 ------------------------------ XFMail ------------------------------ ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
