Question about convhull code

Hello everyone,

I'm using the function convhull to create a better looking hill chart. For that purpose I need to know how Matlab determines at which point the hull-curve should start. The documentation of convhull doesnt tell me anything and I'm too unexperienced to dig into the code of the function itself (that and I dont know when something is considered "reverse engineering", which might be illegal anyway).

[Image Analyst]

Please include some screen shots. You can edit any program and see what it does, at least to some extent, for example

>> edit convhull.m

Perhaps you'd be interested in the boundary() or envelope() function instead - not sure until we see your data.

[Jan]

@Ingo: You have mentioned in your other thread, that you need the hull start with the smallest X and Y position. By the way: Whenever you post multiple thread about one problem, add a link to the thread in the other forum also. Otherwise such "cross-posting" can waste the time of the ones, who write an already given answer. Thanks.

Answer 1

[Jan]

Reading the M- and C-files is considered to be legal.

You can resort the output easily to be sure that it has the wanted order:

K = convhull(X,Y);
C = [X(K), Y(K)];
[~, Index] = sortrows(C);  % Only the first entry matters
if Index(1) ~= 1           % Reordering required
  K = circshift(K, -Index(1) + 1);
end

Now the first index in K is the position with the smallest X and Y values.

In the current Matlab version, K is ordered as you want it. But you cannot be sure if this is the general case, such that the above check is safer.