chull function behavior

Dear Carsten,

the function chull finds the hull of a curve, which consists of the upper and
the lower part of the hull. In your case, you might be only interested in the
upper part. You can extract this with the following commands:
     c.hull <- chull(y)
     c.hull <- c.hull[(which(c.hull == 1):length(c.hull))]
     c.hull <- sort(c.hull)

The calculation of the hull can be performed by:
     hull[i,] <- approx(x=x[c.hull],y=y[i,c.hull], xout = x,method = "linear",
ties = "mean")$y

Best regards

Lukas Lehnert

Am Donnerstag, 13. Dezember 2012, 16:38:43 schrieb Carsten Neumann:

Dear List members,

sorry for the non specific geo question, but working with image spectra
led me to the following problem calculating a convex upper hull curve
using the function "chull {grDevices}".

Imaging there is a image spectra that you need to normalize using
continuum removal (deviding spectra by upper hull), then you do not
expect values greater than 1. Unfortunately the "chull()" functions did
not identify all points of intersection on a hull curve. At the
beginning there are also points beneath a hull, see example data (first
3 points).

############################################################################
################################# wavelength<-seq(350,449,1)
dummy<-c(0.204054, 0.204828, 0.204161, 0.202444, 0.199883, 0.200960,
0.203838, 0.205723,
   0.203404, 0.204835, 0.207257, 0.205682, 0.209670, 0.212622, 0.212032,
0.211062,
   0.212727, 0.215129, 0.216259, 0.217435, 0.218270, 0.219039, 0.220333,
0.222520,
   0.223502, 0.223225, 0.225050, 0.225485, 0.226832, 0.230962, 0.231891,
0.231264,
   0.231170, 0.233540, 0.235799, 0.238165, 0.240494, 0.240793, 0.242218,
0.244078,
   0.244795, 0.246343, 0.247142, 0.247406, 0.249348, 0.250772, 0.251937,
0.253166,
   0.253301, 0.253941, 0.255231, 0.256503, 0.257881, 0.258632, 0.258916,
0.260414,
   0.261584, 0.262598, 0.263891, 0.264702, 0.265681, 0.266892, 0.267861,
0.268923,
   0.270050, 0.271112, 0.271767, 0.273130, 0.274604, 0.275065, 0.276157,
0.277331,
   0.278321, 0.279585, 0.280577, 0.281469, 0.282497, 0.283157, 0.284381,
0.285984,
   0.286866, 0.288173, 0.289387, 0.290084, 0.290770, 0.291659, 0.292730,
0.293823,
   0.294841, 0.295763, 0.296607, 0.297420, 0.298529, 0.299691, 0.300580,
0.301300,
   0.302290, 0.303378, 0.303885, 0.304686)

id<-sort(chull(wavelength, dummy))
plot(wavelength,dummy,type="l")
points(wavelength[id],dummy[id],type="p",col="red")
############################################################################
################################# Can anybody explain this behavior or give
some advice on improving functionality?

Thanks in advance!

Best

Carsten