Calculate area under graph between two points

Hello,

We want to calculate the area between two points where the graph is zero like in the graph below. So we want the area under every single peak separately. We think we have to use trapz for it, but how do we use this one right? Or do we need to use something else?

Also we want to know which area is the biggest. Is it possible to get this value out of matlab?

Answer 1

[dpb]

Start with

doc findpeaks

the optional output peak widths function with proper WidthReference level should give you all the info you need.

Does require Signal Processing Toolbox...

Answer 2

[Star Strider]

Try this:

x = linspace(0, 5*pi);                                                  % Create Data
y = sin(x);                                                             % Create Data
y = y .* (y>0);                                                         % Create Data

zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);                    % Returns Approximate Zero-Crossing Indices Of Argument Vector
idx1 = zci(y);                                                          % Indices: y=0
idx2 = find(diff([0; idx1])>1);                                         % Find Discontinuities
yint = cumtrapz(x, y);                                                  % Integrate
yints = diff([0 yint(idx1(idx2))]);                                     % Peak Areas
[pkv,pki] = maxk(y, numel(yints));                                      % Used In ‘text’ Call

figure
plot(x, y)
text(x(pki), pkv/2, sprintfc('Area = %.2f', yints), 'HorizontalAlignment','center','VerticalAlignment','middle', 'Rotation',90)

EDIT —

Added plot figure —

Answer 3

[darova]

Simple solution:

k = 1;
thresh = 0;
s = zeros(1,100);           % how many peaks?
for i = 1:length(y)-1
    % if a new peaks starts:
    if (y(i) <= thresh) && (y(i+1) > thresh)
        k = k + 1;
    end
    h = x(i+1) - x(i);        % width of a current rectangle
    y1 = (y(i) + y(i+1))/2;   % average height
    s(k) = s(k) + h*y1;
end
s = s(1:k);