Curve Extraction from Binary Image

Question

Zachariah Harris 2020-11-24

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https://ww2.mathworks.cn/matlabcentral/answers/660833-curve-extraction-from-binary-image

回答： Amrtanshu Raj 2021-2-4

Hi Mathworks community. I have a black and white image and I am trying to extract the polynomial equation of the third line from the left in the image shown below. I am having trouble extracting the locations of the non-zero pixels and then determining the polynomial coefficients (fitting the line). My code is below.

% Label image with 1,2,3,4 for each connected component

L = bwlabeln(BW3);

% return the row and column locations of label #3 within the image

[row,col]=find(L(:,:) == 3);

% Extract the polynomial coeffecients of each line in the image

n=2;

p = polyfit(col,row,n)

% Show the original black and white image and hold on axis

imshow(BW3)

hold on

% Plot the determined polynomial line as a red line on the same figure

x=0:.1:500;

plot(polyval(p,x),x,'r')

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Answer 1

Amrtanshu Raj 2021-2-4

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https://ww2.mathworks.cn/matlabcentral/answers/660833-curve-extraction-from-binary-image#answer_614232

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Hi,

In the image there are only 2 labels generated by bwlabeln,

Assuming that you wish to generate a polynomial and plot it for the curve in middle (which is label 2 in L) of the image, I have made edits to your code so that it generates the required plot.

The major error was in the line

p = polyfit(col,row,n)

Code -

% Label image with 1,2,3,4 for each connected component
L = bwlabeln(BW3);
% return the row and column locations of label #3 within the image
[row,col]=find(L == 2)
% Extract the polynomial coeffecients of each line in the image
n=2; 
p = polyfit(row,col,n);
 
% Show the original black and white image and hold on axis
imshow(BW3)
hold on
% Plot the determined polynomial line as a red line on the same figure
x=0:.1:500;
plot(polyval(p,x),x,'r')