Area between a curve and a baseline

I want to create a base line (red line) from the plot and calculate area between the base line and curve (blue line).Curve is a vector. Baseline is drawn manually using code below. [x y]=ginput(2); line(x,y);

Any help/ suggestions is highly appreciated.

Thanks

Answer 1

[Andrei Bobrov]

data - your array < 2 x N >, data(1,:) - abscissa; data(2,:) - ordinate.

plot(data(1,:),data(2,:));
[x y]=ginput(2);
t = x(1) <= data(1,:) & x(2) >= data(1,:);
X = data(1,t);
out = trapz(X,diff([interp1(x,y,X);data(2,t);]))

[Teja Muppirala]

Ah, right. With linear interpolation and TRAPZ, I see you only really need to interpolate at the abscicca, not at a whole bunch of points like I did below.

[Andrei Bobrov]

corrected

[Joe Abraham]

Sorry for delayed reply. I got the following error when computed ur code.

??? Attempted to access xx(:,1); index out of bounds because size(xx)=[3,0].

Error in ==> specgraph.areaseries.refresh at 82 v = [xx(:,k) yy(:,k)];

Error in ==> graph2d.series.schema>LdoDirtyAction at 108 refresh(h);

Warning: Error occurred while evaluating listener callback. > In area at 100 In DSC at 27

Answer 2

[Teja Muppirala]

You could try something along the lines of this:

% Just making some data  
x0 = 0:0.001:5;
y0 = sin(x0);
plot(x0,y0);
grid on; set(gca,'layer','top');

% Get your two points
[x y]=ginput(2); line(x,y);

% Break up the line segment into a sufficiently fine grid
N = 10001; 
x_fine = linspace(x(1),x(2),N);
dx = x_fine(2)-x_fine(1);

% Interpolate the values at the grid points 
top = interp1(x0,y0,x_fine); % Top of the area
bottom = interp1(x,y,x_fine); % Bottom of the area

% Get the area using TRAPZ and show it
area = dx*trapz(top-bottom)
patch([x_fine fliplr(x_fine)], [top fliplr(bottom)],'r');

Just replace the sine with whatever data you are using (the example above uses row vectors, so if your data is in a column instead, you'll probably have to make some minor modifications)

[Joe Abraham]

Hi. Thanks for the help. I get the following error when tried executing your code. Do you have any idea what went wrong?

??? Error using ==> interp1 at 259 The values of X should be distinct.

Error in ==> DSC at 28 top = interp1(T1,HbyC,x_fine); % Top of the area