How do I write a program using MATLAB to find the circumference of a area (white) using the unit as a pixcel (i ever Used to command g = regionprops (label, 'Area');)

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sombat supha
sombat supha 2018-11-17
编辑： Bruno Luong 2018-11-17
1234.jpg

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KSSV
KSSV 2018-11-17
编辑：KSSV 2018-11-17
I = imread(image) ;
I = rgb2gray(I) ; % If gray alredy not needed
[y,x] = find(I) ;
idx = boundary(x,y) ;
imshow(I)
hold on
plot(x(idx),y(idx))
dx = diff(idx) ; dy = diff(y(idx)) ;
d = sqrt(dx.^2+dy.^2) ;
sum(d)
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Bruno Luong
Bruno Luong 2018-11-17
编辑：Bruno Luong 2018-11-17
Sorry but this method proposed by KSSV won't give the correct result for non-convex shape, since boundary will not follow closely the concave part (unpredictable sideeffect of alphashape).
As showed in the test script where the shape is 2 circles that are touching.
true circ = 2092.3
boundary method = 1790.75
Code:
x = linspace(-3,3,1000);
y = x(:);
Img = ((x-1).^2 + y.^2 < 1) | ((x+1).^2 + y.^2 < 1);
close all
imagesc(Img)
colormap gray
axis equal
truec = (length(x)-1)*(2/6*2*pi);
[y,x] = find(Img) ;
idx = boundary(x,y) ;
dx = diff(x(idx)) ; dy = diff(y(idx)) ;
d = sqrt(dx.^2+dy.^2) ;
c2 = sum(d);
hold on
plot(x(idx),y(idx),'r','linewidth',3)
fprintf('true circ = %g\n', truec)
fprintf('boundary method = %g\n', c2)

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Bruno Luong
Bruno Luong 2018-11-17
编辑：Bruno Luong 2018-11-17
Assuming your binary is Img with one connexed component
c = contourc(double(Img),[1 1]);
c = c(:,1+(1:c(2,1)));
d = diff(c(:,[1:end 1]),1,2);
crcumference = sum(sqrt(sum(d.^2,1)))
You must be aware the boundary might have a fractal property, therefore the circumference is something difficult to defined and computed accurately.
circle.png

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