Merging overlapping Bounding boxes in a loop

Hello! I'm a beginner MATLAB user.

I am looping through a binarized image where I am drawing bounding boxes around every object. If 2 bounding boxes overlap I'd like to merge them into one big bounding box like in the 2 images below:

Currently I am looping through all the objects in this large image and drawing bounding boxes around each of them. But I can't get the merge working at all. I need help. I understand I could dilate the image so they touch, but I do not want to do this. Thanks.

hold on;
for k = 1 : length(st) % for every object
  thisBB = st(k).BoundingBox;
  cropBB = imcrop(bw, [thisBB(1),thisBB(2),thisBB(3),thisBB(4)]);
  
  % merge bounding boxes if they overlap (HOW???)
  %newBox = zeros(1,4)
  %newBox(1)

  rectangle('Position', [thisBB(1),thisBB(2),thisBB(3),thisBB(4)],'EdgeColor','r','LineWidth', 2); % draw bounding box
end
hold off;

[Walter Roberson]

See also https://www.mathworks.com/matlabcentral/answers/265382-how-to-merge-multiple-bounding-box-into-one#answer_207609

Answer 1

[DGM]

There are probably other ways to do this, but here's an example.

Let's start with a binary image and plot its bounding boxes and labels so that we can get an idea of which ones overlap.

% get a binary image of some blobs
inpict = imread('indexedblobs.png');
mask = inpict>0;

% get bounding boxes
PS = regionprops(mask,'boundingbox');

% plot things 
imshow(mask,'border','tight'); hold on
for k = 1:numel(PS)
    % plot bbox
    thisbox = PS(k).BoundingBox;
    rectangle('position',thisbox,'edgecolor','g')
    
    % indicate each blob label
    % this will help clarify the next example
    x = thisbox(1) + thisbox(3)/2;
    y = thisbox(2) + thisbox(4)/2;
    text(x,y,sprintf('%d',k),'color','r','horizontalalignment','center', ...
        'fontsize',12,'fontweight','bold')
end

Now let's try to group them and plot the group bounding boxes. I assume that for most uses, it will be desired to know which blobs are members of each group. Accordingly, I'll get a list of members for each blob group and plot those as well.

% get a binary image of some blobs
inpict = imread('indexedblobs.png');
mask = inpict>0;

% get bounding boxes and subscripts for bounding box areas
PS = regionprops(mask,'boundingbox','subarrayidx');

% create union of bounding boxes
boxmask = false(size(mask));
for k = 1:numel(PS)
    boxmask(PS(k).SubarrayIdx{:}) = true;
end

% get properties of bbox mask
% this will be needed if it's necessary to know which
% blobs belong to each group
L = bwlabel(mask);
PSbox = regionprops(boxmask,L,'boundingbox','pixelvalues');

% find members of each blob group
groups = cell(numel(PSbox,1));
for k = 1:numel(PSbox)
    thisgroup = unique(PSbox(k).PixelValues);
    groups{k} = thisgroup(thisgroup~=0);
end

% plot things
imshow(mask,'border','tight'); hold on
for k = 1:numel(PSbox)
    % plot group bbox
    thisbox = PSbox(k).BoundingBox;
    rectangle('position',thisbox,'edgecolor','g')
    
    % indicate the blobs which are members of each group
    x = thisbox(1) + thisbox(3)/2;
    y = thisbox(2) + thisbox(4)/2;
    thislabel = sprintf('%d\n',groups{k});
    text(x,y,thislabel(1:end-1),'color','r','horizontalalignment','center', ...
        'fontsize',12,'fontweight','bold')
end