Search code examples
matlabplotfigure

Data evaluation

I have some data in MATLAB, and want to distinguish the start and stop point when these data cross specified threshold (for example -50), and save them and then calculate the approximate area of that section under -50 and if it was below some defined value neglect those points and check for the next two points. See the following image:

Figure

The two points on the left side of the figure are marked with x in red and the required area is shown in green. I want to do this for the whole figure.

Any idea? Thanks.

Solution

  • Regarding the plotting, there were possible ways mentioned in the comments, whereas I commonly would use patch to plot filled polygonal regions. For the area approximation, you can use the trapz function for a trapezoidal numerical integration.

    That'd be my solution, including detection of the intervals, and also neglecting intervals with insufficient area (it's a bit lengthy, and full of loops for plotting all the intervals; can be certainly optimized):

    % Set up function, and parameter(s)
x = linspace(-0.125*pi, 4.125*pi, 10001);
y = linspace(60, 100, 10001) .* sin(x);
thr = -50;
thr_area = 30;

% Find y values lower than threshold
y_idx = find(y <= thr);

% Get start and end of intervals
idx_int = find(diff(y_idx) > 1);
n_int = numel(idx_int)+1;
s = zeros(n_int, 1);
e = zeros(n_int, 1);
s(1) = y_idx(1);
e(end) = y_idx(end);
for k = 1:n_int-1
  e(k) = y_idx(idx_int(k));
  s(k+1) = y_idx(idx_int(k)+1);
end

% Calculate areas
Q = zeros(n_int, 1);
for k = 1:n_int
  Q(k) = abs(trapz(x(s(k):e(k)), y(s(k):e(k))-thr));
end

% Visualization
figure(1);
hold on;
plot(x, y);
xlim([x(1), x(end)]);
ylim([min(y)-10, max(y)+10]);
plot([x(1), x(end)], [thr thr], 'k');
for k = 1:n_int
  patch(x(s(k):e(k)), y(s(k):e(k)), 'k');
  plot([x(s(k)), x(e(k))], [y(s(k)), y(e(k))], 'r.', 'MarkerSize', 15);
  text(x(s(k)), thr+20, num2str(Q(k)));
  if (Q(k) < thr_area)
    text(x(s(k)), thr+10, 'Area too low');
  else
    text(x(s(k)), thr+10, 'Area OK');
  end
end
hold off;

    The result looks like this:

    Output

    You should have all information by now to do whatever further calculations, analyses, etc. you have in mind.

    Hope that helps!

    Disclaimer: I tested the code with Octave 5.1.0, but I'm quite sure, that it should be fully MATLAB-compatible. If not, please leave a comment, and I'll try to fix possible issues.