%% Image Segmentation with Distance Transform and Watershed Algorithm
% Sample code showing how to segment overlapping objects using Laplacian
% filtering, in addition to Watershed and Distance Transformation.
%
% In this demo you will learn how to:
%
% * Use the OpenCV function |cv.filter2D| in order to perform some laplacian
%   filtering for image sharpening
% * Use the OpenCV function |cv.distanceTransform| in order to obtain the
%   derived representation of a binary image, where the value of each pixel is
%   replaced by its distance to the nearest background pixel
% * Use the OpenCV function |cv.watershed| in order to isolate objects in the
%   image from the background
%
% Sources:
%
% * <https://docs.opencv.org/3.2.0/d2/dbd/tutorial_distance_transform.html>
% * <https://github.com/opencv/opencv/blob/3.2.0/samples/cpp/tutorial_code/ImgTrans/imageSegmentation.cpp>
% * <https://docs.opencv.org/3.2.0/d3/db4/tutorial_py_watershed.html>
%

%%
% load image
fname = fullfile(mexopencv.root(), 'test', 'cards.png');
if exist(fname, 'file') ~= 2
    disp('Downloading image...')
    url = 'https://cdn.rawgit.com/opencv/opencv/3.2.0/samples/data/cards.png';
    urlwrite(url, fname);
end
src = cv.imread(fname, 'Color',true);
imshow(src), title('source image')

%%
% change the background from white to black, since that will help later to
% extract better results during the use of Distance Transform
mask = all(src == 255, 3);
src(repmat(mask,[1 1 3])) = 0;
imshow(src), title('black background image')

%%
% create a kernel that we will use for accuting/sharpening our image
% (an approximation of second derivative, a quite strong kernel)
if true
    kernel = [1 1 1; 1 -8 1; 1 1 1];
else
    kernel = [0 1 0; 1 -4 1; 0 1 0];
end

%%
% do the laplacian filtering, with intermediate calculations performed in
% floating-point precision (given the kernel used, Laplacian image will likely
% have negative values which would otherwise get truncated too soon)
if true
    imgLaplacian = cv.filter2D(src, kernel, 'DDepth','single');
else
    imgLaplacian = cv.Laplacian(src, 'DDepth','single');
end
src = uint8(single(src) - imgLaplacian);
imshow(src), title('sharpened image')

%%
% create binary image from source image
bw = cv.cvtColor(src, 'RGB2GRAY');
bw = cv.threshold(bw, 'Otsu');
imshow(bw), title('binary')

%%
% perform the distance transform algorithm, and normalize it to [0,1] range
% so we can visualize and threshold it
D = cv.distanceTransform(bw);
D = cv.normalize(D, 'Alpha',0, 'Beta',1, 'NormType','MinMax');
imshow(D), title('distance transform image')

%%
% threshold distance image to obtain the peaks and dilate it a bit
% (these will be the markers for the foreground objects)
D = cv.threshold(D, 0.4, 'MaxValue',1);
D = cv.dilate(D);
imshow(D), title('peaks')
if ~mexopencv.isOctave()
    %HACK: transparency not yet implemented in Octave
    image('CData',src, 'AlphaData',0.3)
end

%%
% find total markers
D = uint8(D * 255);
contours = cv.findContours(D, 'Mode','External', 'Method','Simple');
contours = cellfun(@(C) cat(1,C{:}), contours, 'UniformOutput',false);
N = numel(contours);

%%
% create the marker image for the watershed algorithm and draw the markers
% (seed regions are marked with positive labels 1,2,3,etc. everything else 0)
markers = zeros(size(D), 'int32');

% foreground markers
for i=1:N
    markers = cv.drawContours(markers, contours, 'ContourIdx',i-1, ...
        'Color',i, 'Thickness','Filled');
end

% background marker
markers = cv.circle(markers, [5 5], 3, 'Color',N+1, 'Thickness','Filled');

%%
% perform the watershed algorithm
markers = cv.watershed(src, markers);

%%
% in output, pixels are assigned to regions (1,2,etc.), boundaries set to -1
L = uint8(markers);  % -1,1,2,..,N,N+1 -> 0,1,2,..,N,N+1
if ~mexopencv.isOctave() && mexopencv.require('stats')
    %HACK: tabulate in Octave behaves differently
    tabulate(L(:));  % 0=boundaries, 1:N=regions, N+1=background
end

%%
% show segmented objects color-coded according to labels
clr = [1 1 1; hsv(N); 0 0 0];
rgb = uint8(255 * ind2rgb(L, clr));
imshow(rgb), title('segmented image')

kyamagu
github-repos/MATLAB/kyamagu-mexopencv/mexopencv-d29007b2a484d0fd92e6e941dc5fd4750014fa6a/samples/watershed_segmentation_demo.m
mexopencv
github-repos/MATLAB/kyamagu-mexopencv
d29007b2a484d0fd92e6e941dc5fd4750014fa6a