How to get pixel location of object in image [closed] - matlab

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Suppose I have a binary image shown in below. I want to get the location(pixel value in X,Y coordinates) of all rectangular shapes and also circular shapes. How to detect how many rectangular and circular shapes are present there. I want to get the solution in Matlab. The rectangle and circle may be in different size. The small circles are noise here. Thanks in advance.

Answer 1

You need to:
find connected components (bwconncomp)
find some statistic for each connected component (regionprops)
discard connected components that are too small, defining a threshold on the area
find if the connected component is a rectangle or a circle / ellipse. You can use the circularity, defined as 4*pi*Area / (Perimeter^2). Perfect circles will have value 1, squares 0.785, and so on. So you can define a threshold on circularity to determine if a shape is a circle/ellipse or not.
Here the result, with smaller blobs removed, and circles/ellipses and rectangles in different colors:
Code:
% Read image
img = imread('path_to_image');
% Convert to grayscale
img = rgb2gray(img);
% Remove JPEG artifacts
bw = img > 20;
% Prepare output image
output = zeros(size(bw));
% Compute connected components, and their statistics
cc = bwconncomp(bw);
props = regionprops(cc, 'Area', 'Perimeter');
% Will contain x,y coordinates for rectanles and circles
coord_rect = {};
coord_circ = {};
% Define thresholds on size and circularity
th_size = 200;
th_circ = 0.8;
% For each connected component
for i = 1 : cc.NumObjects
% Is big enough?
if(props(i).Area < th_size)
continue;
end
% Rectangle or Circle?
circularity = 4 * pi * props(i).Area / (props(i).Perimeter ^ 2);
[y,x] = ind2sub(cc.ImageSize, cc.PixelIdxList{i});
if(circularity > th_circ)
% Circle
output(cc.PixelIdxList{i}) = 1;
coord_circ{end+1} = [x, y];
else
% Rectangle
output(cc.PixelIdxList{i}) = 2;
coord_rect{end+1} = [x, y];
end
end
% Show results
imshow(bw);
figure();
imagesc(output);