Find a local max and local min of a surface with noise

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Rafael Freire
Rafael Freire 2011-4-21
I got the matrix above How can I find de MAXIMUM & MINIMUM locals and put the X,Y coodinate into a vector X and a vector Y and when i plot X vs Y o got the dots representating the maximum and minumum values with different color for max and min ps. remember to consider the noise
A=[0.5000 -0.2668 -0.4606 -0.0866 -0.1433 -0.2063 0.0020 -0.0684 0.0824 0.0618; -0.0700 -0.1001 -0.6211 -2.0000 -0.6901 -0.5352 -0.5370 0.6527 -0.4742 -0.2398; -0.0159 0.1090 0.0902 -0.3513 -0.5579 -0.2559 -0.1088 0.2090 -0.1592 0.0290; 0.3378 0.3437 0.4101 0.4995 0.3286 0.1963 0.0434 0.1998 0.2853 0.3430; 0.3673 0.3320 0.2836 0.3189 0.3596 0.1729 0.0493 0.2758 0.2314 0.3171; -0.7621 -0.5151 -0.3759 -0.1816 0.1758 0.0306 -0.4316 0.1782 -3.0051 -1.9493; -0.0107 -0.1784 -0.0642 -0.1268 -0.0137 0.0840 -0.1945 0.0944 -0.1449 -0.0296; 0.2212 0.1060 0.0321 3.0020 0.1965 -0.0060 0.0472 0.1958 0.1850 0.3266; 0.1410 0.2704 0.1225 0.2163 0.0337 0.1676 0.1012 0.1511 0.1105 0.3585; 0.1301 0.2741 0.0495 0.0251 0.1299 0.2433 0.0284 0.0727 0.1462 0.1443 ;]
  2 个评论
Rafael Freire
Rafael Freire 2011-4-21
I'm not doing the right way.
i made a isocut in the max an min values using this function that 1 create
function [modecut1,modecut_1]=modecut(X,i);
tic;
modecut1 = X;
modecut_1 = X;
modeX = mode ( mode(X) );
modecut_1 (modecut_1 >= modeX * i & modecut_1 <= -modeX * i ) = 0;
modecut_1 (modecut_1 > 0 ) = 256;
modecut_1 (modecut_1 < 0 ) = -256;
Than i use bwlabel an calculate the centroids of the points.
but one problem of that is when i have 2 max nearby for this way i will consider one max only, so i'm loosing important information.

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Rafael Freire
Rafael Freire 2011-4-29
I use a function extrema2 to do that and works here is the link http://www.mathworks.com/matlabcentral/fileexchange/12275-extrema-m-extrema2-m/content/extrema/extrema2.m and the code I use
z=crop;
[zmax,imax,zmin,imin] = extrema2(z,1);
[m,n]=size(crop);
y=1:m;
x=1:n;
[x,y]=meshgrid(x,y);
crop(crop<=10)=nan
figure
hold on
plot3(x(imax),y(imax),zmax,'bo',x(imin),y(imin),zmin,'ro')
for i = 1:length(zmax)
text(x(imax(i)),y(imax(i)),zmax(i),[' ' num2str(zmax(i))])
end
for i = 1:length(zmin)
text(x(imin(i)),y(imin(i)),zmin(i),[' ' num2str(zmin(i))])
end
hold off
hold on
surf(crop)

更多回答（2 个）

bym
bym 2011-4-21
I am not sure I understand your question but is
imagesc(A)
what you are looking for?
  1 个评论
Rafael Freire
Rafael Freire 2011-4-24
I'm looking for a values (linear indexing) of the points where is the max of the curves, but I have a lot of maximuns and minimuns in my surface...
This function that you pass helps me in another part of my work.
Thanks anyway

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Walter Roberson
Walter Roberson 2011-4-22
On a discrete surface, every point is both a local max and local min -- at least until you define your neighborhood. If you want noise to be taken in to account, you also have to define either a noise detection function (if you want to ignore points with noise) or a smoothing function.
You also need to define the behaviour you want for saddle-points, that are local min in one direction but local max in another.
You might want to use gradient() perhaps.
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Walter Roberson
Walter Roberson 2011-4-25
Post the images on a service such as tinypic or imageshack and post the URL here.

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