MATLAB: How to Envelope a Dataset

enveloping

I have a series of data points (in some cases they're in the order of over 1000 points), after some data processing and iterations i.e. sorting, duplicate elimination etc. they will look like a numerical array like "B". B(:,1) will represent the data set on the X-Axis and B(:,2) is the corresponding Y-Axis data.
B =
                        20                      0.01
                        34                     0.017
                        40                      0.02
                        56                       0.5
                        71                       0.2
                        71                       0.5
                        80                      0.04
                       100                     0.075
                       200                       0.2
                       260                      0.04
                       400                       0.5
                       800                      0.04
                      2000                      0.01
                      2000                      0.02
*Please note that there are 2 corresponding values for 71.
I'd like to create an array that envelopes these data to represent the characteristics of "B". I came up with the enveloping dataset manually (which is acceptable for this case). I called this enveloped data set, "Enveloped":
Enveloped =
                        20                      0.01
                        30                      0.02
                        45                       0.5
                       400                       0.5
                      2000                      0.02
I have uploaded a figure at the following link; it can visually show you what I'm looking for:
http://imgur.com/7o6wA
The Red plot represents "B" and the blue plot is the "Enveloped". I'm looking to recreate "Enveloped" in Matlab. Any help is greatly appreciated!

Best Answer

Dear Aviator, Sorry I was away so could not reply on time. Actually you can play trick with this.
k = convhull(x,y)
So you can process k after getting convhull. To avoid box around point it is straight forward that delete the last point. However there can be many exceptions. One good trick can be that. You loop over k and check values of K. Keep plotting while K is increasing. Whenever K start decreasing then you stop. You will see that you don't get box and get the exactly same figure which you showed as example. Sorry I don't have installed matlab to test but I am sure it will work. Try and let us know.
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