I don't know of any simple means to convert an image into a minimal line drawing. Features like the intersection of the facial lines with the nose or the intersection of the tail lines with the tuft are structurally the same -- they're a transition between a constant-width line and a solid region. Yet whatever operation we use must somehow recognize that they are to be handled in completely different ways and interpret where the stroke should be. I don't know that basic filtering or morphological operations can do that neatly and consistently.
Something like this might work, but it's going to collapse some solid features.
% read image, convert to a logical image
A = rgb2gray(imread('lion.png'))<127;
% reduce line width
B = bwmorph(A,'thin',Inf);
% display the image
imshow(~B); hold on
This is a little bit better at not collapsing cusps, but it leaves them heavy. That's not much better.
A = rgb2gray(imread('lion.png'))<127;
D = bwdist(bwperim(A,4));
C = (D>4 & D<7) & A;
C = bwmorph(C,'thin',Inf);
C = ~bwareaopen(~C,500,4);
Other methods still have similar issues with thick cusps. This destroys open curves and still leaves small islands.
A = rgb2gray(imread('lion.png'))>127;
[L nb] = bwlabel(A);
st = strel('disk',11);
B = false(size(A));
Bm = false(size(A));
for b = 1:nb
% dilate each blob
thisblob = L == b;
thisblob = imdilate(thisblob,st);
% and trim it against the others
maskedblob = thisblob & ~Bm;
Bm = Bm | thisblob;
maskedblob = bwperim(maskedblob);
B = B | maskedblob;
end
B = bwmorph(B,'thin',Inf);
B = imclearborder(B);
Like I said, I don't know of a good way to do this. Maybe someone else does.
The above examples should give you a line image without any need for finding coordinates. If you really truly want to use plot(), just use find(), since B is already a logical image.
% or if you really want to use xy coordinates
[y x] = find(B);
plot(x,y,'.')
