Tangent line to the edge of binary image

Attached please see a binarized image of water droplets spraying out of a sprinkler.

I am trying to define a the angle between a normal line and (1) the right edge of the blob (dotted line) and left edge of the blob (dashed line). A solution that comes mind is to get rid of smaller blobs and is to extract the (x,y) values of the edges and fit a curve to each section. Can anyone suggest a better solution to me?

thanks you all :)

Answer 1

[Matt J]

A solution that comes mind is to get rid of smaller blobs and is to extract the (x,y) values of the edges and fit a curve to each section.

Sounds good to me.

load Image

BW0=BW;

BW=bwperim( bwconvhull(bwareaopen(BW,100)) );

[I,J]=find(BW);

[~,p]=min(I);
[~,l]=min(J);
[~,r]=max(J);

Ip=I(p); Jp=J(p);
Il=I(l); Jl=J(l);
Ir=I(r); Jr=J(r);
Ic=min(Il,Ir);
Im=round((Ip+Ic)/2);

BW(Ic:end,:)=0;
BW(1:Im,:)=0;

reg=regionprops(BW,'PixelList');

for i=1:2
  x = reg(i).PixelList(:,1);
  y = reg(i).PixelList(:,2);
  
  reg(i).p=polyfit(x,y,1);

end

imshow(BW0,[]);
hold on

    x1=1:Jp;
    y1=polyval(reg(1).p,x1);
    
    x2=Jp:size(BW,2);
    y2=polyval(reg(2).p,x2);
    
    plot(x1,y1,'rx')
    plot(x2,y2,'rx')
    
hold off