try to use this
img=imread('i2.png');
figure(1)
imshow(img)
hold on
[lab,num]=bwlabel(img);
[r,c]=find(lab==1);
[r1,c1]=find(lab==2);
D = pdist2([r c],[r1 c1],'euclidean'); % euclidean distance
[r2,c2]=find(D==min(D(:)));
point_1=[r(r2) c(r2)];
point_2=[r1(c2) c1(c2)];
plot([point_1(2) point_2(2)],[point_1(1) point_2(1)],'r')
hold off
distance=sqrt( (point_1(1)-point_2(1)).^2 + (point_1(2)-point_2(2)).^2)
distance =
42.1070
