MATLAB Answers

finding multiple maxima in a Gaussian distribution

12 views (last 30 days)
PChoppala
PChoppala on 9 Mar 2012
Hi
I have a query and will appreciate your help.
If we try the code below,
>> c1=1;
>> for p=-50:50
c2=1;
for q=-50:50
pp1(c1,c2)=exp(- ( (p-10).^2 + (q-9).^2) /2) + exp(- ( (p+20).^2 + (q-30).^2) /2);
c2=c2+1;
end
c1=c1+1;
end
>> mesh(pp1)
you can see that the figure(matrix Po) has two peaks(maxima), stating that there are two targets, one at (10,9) and another at (-20,30).
If there is only one target(peak), I used [a,b]=find(Po==max(max(Po))), to find the locations (i,j) at the peak.
But in the above code, we have two peaks. Can you help me to find the
1. No of peaks(estimate no of targets)
2. Find the location of the peaks(i,j)
Cheers
[Information merged from duplicate Question]
Hello
I have a simple query, which was unanswered a few days ago. I am re-posting because I need a hint urgently. Please do not direct me to my previous post. It is not quite complete.
You can copy this code and see the result.
s1 = [5 4 0.5 -0.64]';
s2 = [0 -3.9 -0.5 -0.64]';
Sa = [-40:0.2:40];
Sb = [-40:0.2:40];
L = length(Sa);
m = zeros(L,L);
Pr = 2*ones(L,L);
Pr = Pr / sum(sum(Pr));
x1 = [s1(1);s1(2)] + sqrt(R)*randn(2,1);
x2 = [s2(1);s2(2)] + sqrt(R)*randn(2,1);
for i = 1: length(Pr)
for j = 1: length(Pr)
m(i,j) = (1/sqrt(2*pi)) * (exp(-((Sa(i)-x1(1)).^2 + (Sb(j)-x1(2)).^2)/2) + exp(-((Sa(i)-x2(1)).^2 + (Sb(j)-x2(2)).^2)/2));
m(i,j) = m(i,j) * Pr(i,j);
end
end
Po = m/sum(sum(m));
[a,b] = find(Po==max(max(Po)));
Sest = [Sa(a);Sb(b)];
meash(Sa,Sb,Po)
The matrix(surfing of the matrix) 'Po', will result in two peaks at the (s1(1),s1(2)) and (s2(1),s2(2)), show that there are two targets. If I had one peak, a,b would collect it and Sest would collect the location on the grid.
But I am unable to figure out how to find two peaks instead. So the last two statements do not hold now.
I look forward for suggestions on
1. finding the number of peaks(2 in this case)
2. finding the location on grid(Sest in this case) (Sa,Sb)
Will appreciate your help!
Cheers

  1 Comment

Walter Roberson
Walter Roberson on 10 Mar 2012
http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

Sign in to comment.

Answers (2)

Image Analyst
Image Analyst on 9 Mar 2012
See the code I added to yours:
c1=1;
for p=-50:50 c2=1;
for q=-50:50
pp1(c1,c2)=exp(- ( (p-10).^2 + (q-9).^2) /2) + exp(- ( (p+20).^2 + (q-30).^2) /2);
c2=c2+1;
end
c1=c1+1;
end
mesh(pp1)
Here is code I added:
[labeledImage numberOfPeaks] = bwlabel(pp1>0.01);
measurements = regionprops(labeledImage, 'Centroid');
message = sprintf('The number of peaks is %d\n', numberOfPeaks);
uiwait(msgbox(message));
for r = 1 : numberOfPeaks
message = sprintf('The centroid of peak %d is at (%.2f, %.2f)\n', ...
r, measurements(r).Centroid(1), measurements(r).Centroid(2));
uiwait(msgbox(message));
end
Yes I know the numbers aren't the same. I'm hoping you'll be able to figure out how to shift the coordinate system by subtracting 51 from x, and y.

  7 Comments

Show 4 older comments
PChoppala
PChoppala on 12 Mar 2012
Yep, worked. Thanks a lot.
Just to wrap up, I am not able to figure out what number I should be subtracting from x and y when the mesh is [-40:0.2:40, -40:0.2:40], can you please help me out.
I tried a few but I am still wrong.
Image Analyst
Image Analyst on 12 Mar 2012
Well just like you subtracted -51 from x when it was -50 to send it to +1, you would want to subtract -41 (i.e. add 41) from your x and y values so that the indices go from 1 to 81 instead of -40 to +40
PChoppala
PChoppala on 12 Mar 2012
Sorry, but that is not working out. I tried it. The values should be near s1(1,2) and s2(1,2)
I am not able to find the indices on Sa and Sb that correspond!!

Sign in to comment.


Walter Roberson
Walter Roberson on 10 Mar 2012
Your previous question is still here and still active. I have merged your duplicate question into it. Please do not post duplicate questions: doing that just confuses and frustrates people.

  3 Comments

PChoppala
PChoppala on 10 Mar 2012
Could not agree more. But what do you expect me to do, given the fact that the continued question was unanswered for 23 hours?
I had to assume the volunteer is either not interested or busy. But you cannot expect me to keep waiting and not attempt anymore!
Anyway thanks!
Image Analyst
Image Analyst on 10 Mar 2012
Learning how to use the debugger to step through your code is always a good course of action.
Walter Roberson
Walter Roberson on 10 Mar 2012
Re-posting doesn't make the volunteers more interested or less busy.
If you wish to remind people about your question, you can do that by adding a comment to your question. In cases where you realized your question could be improved or did not have enough information, you can edit your question.

Sign in to comment.