# How to find the midpoint between two curves in an image

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Suraj Sudhakar on 27 Jul 2022
Commented: Suraj Sudhakar on 10 Aug 2022
I have attached the image of the curve, I would like to extract the midpoint between the curve. (Ideally, the thickness of the given curve is constant). I tried extracting the gradient direction and using the line along the gradient direction to extract the midpoint, but the gradient direction is not perfectly tangential due to the discrete nature of the iimage
for (points in the upper line)
gx=(bw(i+1,j)-bw(i-1,j))/2;
gy=(bw(i,j+1)-bw(i,j-1))/2;
theta=atan(gy/gx);
m = sin(theta); %slope of the line
%use the slope and known initial point (and assumption that thickness is constant to get a midpoint)
%issue is gradient can only take a few directions, hence getting wrong results
Wanted to know if there is any better way of doing this

Akira Agata on 7 Aug 2022
How abou the following?
% Binarize the image
I = rgb2gray(I);
BW = imbinarize(I);
% Apply bwdist
J = bwdist(~BW);
% Find maximum position for each row
[~, pt] = max(J, [], 2);
% Create the output image
pt_ind = sub2ind(size(BW), 1:size(BW, 1), pt');
BW2 = false(size(BW));
BW2(pt_ind) = true;
% Let's check!
figure
imshowpair(BW, BW2)
Suraj Sudhakar on 10 Aug 2022
That works, thanks

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