x-coordinate of peaks in 1D plot

4 次查看（过去 30 天）
zozo
zozo 2012-6-21
Hello,
I have the following 1D plot:
http://i46.tinypic.com/35clydu.jpg
I want to find all the x-coordinate values of the peaks in a above plot. I have used the following lines of code and end up having only the y-coordinates(maginutude) values sorted in descending order:
out; %row vector containing the magnitude values(dependent variable)
omega %row vector containing the frequency axis of FFT (independent variable)
peaks=imregionalmax(out);
[label n]=bwlabel(peaks);
peak = imdilate(peaks,strel('disk',1));
G=regionprops(label,'PixelIdxList');
maxForThisBlob=zeros(1,n);
for blob = 1 : n
% Get pixel intensities of this particular blob.
thisBlob = out(G(blob).PixelIdxList);
% Find the max intensity out of those pixel intensities.
maxForThisBlob(blob) = max(thisBlob(:));
end
% Sort them in descending order.
sortedMaxes = sort(maxForThisBlob, 'descend');
%----plotting----%
N=size(omega,2);
figure;
plot(omega(1:N/2),out,'r') %plot one-sided spectrum
The above variable sortedMaxes contains the Magnitude (y-coordinate) values of the peaks(that is 0.02542,0.0191,......so on). How can I extract the corresponding x-coordinates at each of the peaks(that is 3.6,7.2,......so on) ?
please help

请先登录，再回答此问题。

回答（2 个）

Walter Roberson
Walter Roberson 2012-6-21
[sortedMaxes, maxidx] = sort(maxForThisBlob, 'descend');
maxx = omega(maxidx);
  3 个评论
显示 1更早的评论
Walter Roberson
Walter Roberson 2012-6-21
Hmmmm....
Looking again I see that maxidx would be a blob number. I do not have any feel for how blobs correspond to peak positions in your graph.
Change your G=regionprops(label,'PixelIdxList'); to
G=regionprops(label,'PixelIdxList', 'Centroid');
Then
for blob = 1 : n
thiscentroid = G(maxidx(blob)).Centroid;
blobx(blob) = thiscentroid(1);
end
Then blobx will be a vector of x coordinates of the centroids (for lack of anything better to use). The x coordinates would be pixel coordinates.
Then you have the problem of converting pixel coordinates to graph x coordinates. Doing that automatically (without user input) is a challenge. Programs can help, such as http://www.mathworks.co.uk/matlabcentral/fileexchange/7173-grabit
or http://www.mathworks.com/matlabcentral/fileexchange/4316-reverseplot
Though, you could probably automatically detect the pixel coordinates corresponding to the beginning and ending of the x axis without so much difficulty. I guess you could use interp1() to interpolate those to omega values.
XLen = Xaxisend - Xaxisstart + 1; %might need to tweaked as right end of plot does not touch axis
blobomega = interp1( Xaxisstart : Xaxisend, linspace( omega(1), omega(end), XLen ), blobx );
Or more simply,
XLen = Xaxisend - Xaxisstart + 1; %might need to tweaked as right end of plot does not touch axis
blobomega = omega(1) + (blobx - Xstart) / XLen * (omega(end) - omega(1));
zozo
zozo 2012-6-22
@walter: that looks very challenging..!! Is there anyway I can combine the results from sortedMaxes (from my piece of code) and findpeaks() as illustrated below by other experts to achieve my goal?

请先登录，再进行评论。

Honglei Chen
Honglei Chen 2012-6-21
  10 个评论
显示 8更早的评论
Honglei Chen
Honglei Chen 2012-6-22
It seems that you can just use 'SORTSTR' to 'descend' and then pick the first 3 peaks.
However, this only works if you are reasonably confident for your data set because as long as you have noise, there will be a lot of small peaks. Hence, if there are no big peaks, the small peaks will be chosen. To solve this issue, you will need some sort of adaptive minimum peak height. You will need to estimate the noise floor of your data and then use that to come up with a reasonable minimum height
zozo
zozo 2012-6-23
okay..thank you @honglei and @walter

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Descriptive Statistics 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by