Matlab area calculation

Question

If I have an arbitrary shape (attached is a really simple mock up), how would I estimate the area of the enclosed surface in Matlab. To get some random points along the curve, I used the ginput command to get a rough estimate of the curve, with unequal spacing between the points. I want to get an estimate of the area, but I believe the trapz command would overestimate the area due to the overlap (Please correct me if I am wrong here). Is there a more accurate way to obtain the area?

Thanks!

Answer 1

Well, you didn't really give enough info to solve the problem entirely, but here's one approach you can take to find the boundary automatically in order to calculate the area:

% Get image and convert to logical mask
img = ~im2bw(imread('polyarea.jpg'));

% Fill the hole
img = imfill(img,'holes');

% Get boundary
b = bwboundaries(img);

% Approximate area of boundary
area = polyarea(b{1}(:,1), b{1}(:,2));

% Print area
disp(['Area: ' num2str(area)]); 

imshow(img);
hold on;
plot(b{1}(:,2),b{1}(:,1),'go');

The idea is you have an input, you form a logical mask, get the boundary of the mask, and then you can approximate the area enclosed by the boundary by using polyarea.

The output is:

Area: 228003

Additionally, you could also use regionprops(img,'Area') which outputs:

ans = 

    Area: 229154