You can do something similar with something like this (that I have turned into a funciton) —
x = linspace(0, 10*pi, 250);
y = sin(x) + linspace(-2, 2, numel(x));
level = 1.5;
xv = FindLevel(x, y, level);
figure
plot(x, y)
hold on
plot(xv, ones(size(xv))*level, 'rs')
hold off
grid
function xv = FindLevel(x,y,level)
cxi = find(diff(sign(y-level)));
for k = 1:numel(cxi)
idxrng = max(1,cxi(k)-1) : min(numel(x), cxi(k)+1);
xv(k) = interp1(y(idxrng), x(idxrng), level);
end
end
The ‘xv’ output of ‘FindLevel’ are the interpolated x-coordinates of the points where the curve equals ‘level’, so they may not be exact points in the ‘x’ vector. Those approximate points (actually indices) are returned by the ‘cxi’ assignment in my function.
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