How to cut out specific segment from findpeaks function?

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Hi All,
I am trying to figure out how to cut out the specific segment of the signal where I draw the border using findpeaks function.
I am using example of findpeaks link below:
x = linspace(0,1,1000);
Pos = [1 2 3 5 7 8]/10;
Hgt = [4 4 2 2 2 3];
Wdt = [3 8 4 3 4 6]/100;
for n = 1:length(Pos)
Gauss(n,:) = Hgt(n)*exp(-((x - Pos(n))/Wdt(n)).^2);
PeakSig = sum(Gauss);
Then, using findpeaks function,
title('Signal Peak Widths')
I want to cut the segment of the border into 3 segment.
The method I am thinking is find the X value corresponding the Y value, which is not an effective way to do it.

Accepted Answer

Star Strider
Star Strider on 8 Feb 2019
You can get the ‘Borders’ x-coordinates using:
(Previous Code)
title('Signal Peak Widths')
Ax = gca;
Kids = Ax.Children;
Borders = Kids(1:2);
Line = Borders.XData;
You have to search the ‘Axes.Children’ result manually to find the ‘(Border)’ elements. There appears to be no way to get them automatically, or even to specifically search for a ‘Line (Border)’ or ‘Border’ entry in the ‘Kids’ variable. (There may be a way, but it would take more patience than I have tonight to find it. I’ve tried every cell and structure addressing approach I can think of or can find in the documentation, including using strcmp and related functions, and couldn’t get a good result.) It would be very nice if this was more straightforward.
Star Strider
Star Strider on 14 Feb 2019
I am not sure can I plot only specific range and then find the pks in that specific range?
You can. Starting with:
[pks,locs] = findpeaks(-avSpots,year)
(or something similar) then calculate the indices as as:
idx = find((year >= yr1) & (year <= yr2));
where ‘yr1’ is the beginning year and ‘yr2’ is the end year, then analyse your data as:
spots_sub = avSpots(idx);
year_sub = year(idx);
That should work (I did not test it).

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More Answers (3)

João on 7 Feb 2019
Try this,
[pks,locs] = findpeaks(PeakSig,x,'Annotate','extents','WidthReference','halfheight')
The variable locs have the indexes of the peaks found. Use them like this to remove a specific peak (your case the third):
PeakSig(locs(3)) = [];
Matlaber on 9 Feb 2019
Thanks for your reply.
I managed to find the peak point, which is below:
pks =
4.8799 4.0040 3.0328 1.9994 2.2135 3.0039
locs =
0.1031 0.2002 0.2833 0.4995 0.7057 0.7998
peakToFind = 1; % the peak you want to delete (in this case the first one)
% where you get the postion in x corresponding to locs
idx_x = find(x==locs(peakToFind));
Give the answer:
ans =
which is the first peak answer.
However, how can I plot of first segment of the peak?

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Image Analyst
Image Analyst on 9 Feb 2019
The borders appear to be available in findpeaks() at line 558 (in R2018b version):
% get the x-coordinates of the half-height width borders of each peak
[wxPk,iLBh,iRBh] = getPeakWidth(yFinite,x,iPk,bPk,iLB,iRB,refW);
See it with edit, then search for border
>> edit findpeaks.m
Make a copy of findpeaks.m and call it findpeaksandborders() in some utilities folder on your path with all your other m-files. BE SURE NOT TO ALTER THE ORIGINAL ONE!!!
Then have findpeaksandborders() return the borders (iLB and iRB) in the list of output arguments.
Matlaber on 14 Feb 2019
Edited: Matlaber on 14 Feb 2019
I used the above option. I still unable to figure out the error:
Error using findpeaksandborders
Too many output arguments.
Error in Find_All_Peak (line 315)
[pks,locs,Wpk,Ppk, iLB, iRB] = findpeaksandborders(PeakSig,x)
However, it cannot find the first border.
I think it is due to invert of peak.
load sunspot.dat
year = sunspot(:,1);
avSpots = sunspot(:,2);
title('Signal Peak Widths')
hold on
[pks,locs] = findpeaks(-avSpots,year)
plot([locs; locs], [zeros(size(pks)); -pks], '+r', 'MarkerSize',10)
hold off

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Giacomo Echevers
Giacomo Echevers on 18 Jan 2022
Well, I kind of needed exactly this feature so I ventured and fought with the findpeaks code, luckily I found what I needed. I attached the modified version of the findpeaks function (now called Findpeaks.m).
Note: It seems that because MATLAB recognized that this is a custom function, the plotting features are not working, but that's no issue, because you can simply use the orginial function and that'll do, this mod is just to extract the x position for the borders.
I'm going to include here a small example using the previous presented case.
x = linspace(0,1,1000);
Pos = [1 2 3 5 7 8]/10;
Hgt = [4 4 2 2 2 3];
Wdt = [3 8 4 3 4 6]/100;
for n = 1:length(Pos)
Gauss(n,:) = Hgt(n)*exp(-((x - Pos(n))/Wdt(n)).^2);
PeakSig = sum(Gauss);
Now, let's use the modified version of the findpeaks function to get the borders x values. Please note the use of the pair parameters 'BorderExtraction' and logical 1. Basically, logical 1 to get the x values for borders in the loc output variable position and logical 0 to tell it to behave exactly the same as the original findpeaks function.
[~,X] = Findpeaks(PeakSig,x,'Annotate','extents','WidthReference','halfheight','BorderExtraction',1)
X = 6×2
0 0.1451 0.1451 0.2663 0.2663 0.4184 0.4184 0.5866 0.5866 0.7397 0.7397 1.0000
And now let's use this to get the second peak just for fun.
plot(PeakSig(x>=X(2) & x<=X(3)))
Hope this helps
A94 out.
Giacomo Echevers
Giacomo Echevers on 18 Jan 2022
Glad to hear that! Just noticed those borders, literally yesterday and got quite triggered that it didn't have a native way to extract those x coordinates.

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