How to find first index before main peak in a signal ?

Hi all,

I have a signal ( see figure an attached).

I want to identify loactions and values of indices circled bold black (first through before main through indicated by blue star)

I have values and locations of all the remaining indices (attached a b c and d).

I am able to identify indices circled red using the code below. I would assume I can modify it to get the indices in bold black but I fail to do this. Can you help please?

load 'signal'
load 'a' % load all through value
load 'b' % load all through location
load 'c' % load main through value
load 'd' % load main through location

counter = 0;
for i = 1:length(main_trough_location)
    %finds end of movement cycle
    index = all_trough_location > main_trough_location(i);
    if sum(index)>0   
        counter = counter + 1; 
        end_location(counter) = all_trough_location(find(index,1));
        end_value(counter) = all_trough_value(find(index, 1));  
    end   
end

plot(thigh_orient_y,'k')
hold on
plot(all_trough_location,all_trough_value,'ko')
plot(main_trough_location,main_trough_value,'b*')
plot(end_location,end_value,'r*')
h = legend('signal',' all troughs','main troughs','end');

Answer 1

[Simon Chan]

Suppose it is the minimum point before rising, you may try to find the first minumum point counting backwards from each peak.

load('signal.mat');
peak_location = find(islocalmax(thigh_orient_y,'MinProminence',40));
backward = arrayfun(@(x) diff(thigh_orient_y(x:-1:1)),peak_location,'UniformOutput',false);
num_back = cellfun(@(x) find(x>0,1,'first'),backward);
min_location = peak_location-num_back+1;
plot(thigh_orient_y);
hold on;
grid on;
plot(min_location,thigh_orient_y(min_location),'g*')

[Tomaszzz]

thanks ! any idea how to find value of min_location ?

[Simon Chan]

thigh_orient_y(min_location)

[Tomaszzz]

Hi @Simon Chan

Many thanks and sorry to bother again

I have a signal like this.

I get the error:

Error using cellfun
Non-scalar in Uniform output, at index 1, output 1.
Set 'UniformOutput' to false.
Error in findpeaks_XsensDot (line 74)
num_back = cellfun(@(x) find(x>0,1,'first'),backward);

I suppose this is because there is no minimum just before the first main peak. Not sure how to modyfy the code to make this work. Ignoring the first minimum if not exists would work for me as well.

Can you help please?

[Tomaszzz]

@Simon Chan

I have posted thsi question in another post here: https://uk.mathworks.com/matlabcentral/answers/1638060-how-to-discard-beginning-and-end-of-the-signal-when-extracting-elements-of-the-signal-identified-via