draw oriented rectangle around a blob

function y=cadre(x)
info = regionprops(x,'Boundingbox') ;
imshow(x)
hold on
for k = 1 : length(info)
     BB = info(k).BoundingBox;
     rectangle('Position', [BB(1),BB(2),BB(3),BB(4)],'EdgeColor','g','LineWidth',1) ;
end

I am using this code with a segmented image in input, and I have this result :

input output

I would like the rectangles to be the smallest and with the same orientation as the objects, to get the angles for exemple.

How can I modify my code ?

Answer 1

[Scott MacKenzie]

Something like this seems to work:

img = imread('testimage.jpg');
bw = imbinarize(im2gray(img),'adaptive','Sensitivity',0.45);

info = regionprops(bw, 'all');
imshow(img);
hold on;
minSize = 50; % ignore small/noise regions (adjust as necessary)
angles = [];
for k = 1:length(info)
    
    % get region properties    
    w = info(k).MajorAxisLength;
    h = info(k).MinorAxisLength;
    x = info(k).Centroid(1) - w/2;
    y = info(k).Centroid(2) - h/2;
    theta = info(k).Orientation;
    
    if h < minSize % skip the small/noise regions
        continue;
    end
    
    angles = [angles theta];
    ps = polyshape([x, x, x+w, x+w], [y, y+h, y+h, y]);
    ps2 = rotate(ps, -theta, [x+w/2 y+h/2]);
    plot(ps2, 'EdgeColor','r', 'FaceColor','none', 'LineWidth',2);
end

angles

angles = 1×2

67.7787 -44.8710

[Image Analyst]

OK but note that the boxes don't completely contain the blobs.

Answer 2

[Image Analyst]

See John D'Errico's suite of bounding shapes:

https://www.mathworks.com/matlabcentral/fileexchange/34767-a-suite-of-minimal-bounding-objects?s_tid=srchtitle

Answer 3

[Walter Roberson]

See https://www.mathworks.com/help/images/ref/bwferet.html

Note that if you are looking for the angles then regionprops() 'Orientation' will give you the angle of the major axes.