How can I detect rectangles and get their coordinates in a binary image

10 views (last 30 days)
I'm looking for a way to extract the rectangles of a binary image and get the coordinates of the corners of the rectangles. The location and size of the rectangles can vary, but it will always be black rectangles on a white background. Also, the black border at the edge of the image will be made up of 4 rectangles, see the image below (the numbers are just there to give you an idea of the numbers of rectangles).
I have tried to use the detectHarrisFeatures function, but the problem is that I only get the coordinates of the rectangles, thus I don't know which coordinates makes up a rectangle.
I have also tried to use the bwconncomp function, as I was suggested in this post. It works, but it is slow for larger images and it is possible to end up with a lot of small row rectangles or a lot of small column rectangles. For example the top border can be seen as 2 connected rows, or 120 connected columns.
So I'm looking for a more efficient way to detect the rectangles and retrive the corner coordinates for each.
I have attached two example image in the question.
Any help is highly appriciated
Niklas Persson
Niklas Persson on 15 Sep 2022
Ah, yes of course. Well, the application does not care about how they are partition.

Sign in to comment.

Accepted Answer

Image Analyst
Image Analyst on 15 Sep 2022
What I'd do first is to get the four perimeter blobs and store those, then fill in that border. Then I'd threshold for black then label the image. Then use ismember to get each blob in turn. Then I'd scan down each image until I hit the blob. Then use find() to find the first and last point in the blob and note it's width and top row. Then scan down until the width changes, which means you're now in a new rectangle so the last one had its bottom row on the prior line. I believe this should work. Give it a try. If you're still unable to do it, post the original image (with no graphics or JPG blurring) in a .PNG file.
Niklas Persson
Niklas Persson on 16 Sep 2022
Hi again,
Thanks for the help! I manage to solve the problem in the end with inspiration of your code.
It is not the most beautiful thing ever written, but it does the work and runs quite smooth, anyway here it is if someone have the same question.
[rows, columns, numbreOfcolorchannels] = size(binaryImage);
%Find out the max number of regions so we can initlaise a matrix for the
%columns among other things..
numRegions = zeros(rows,1);
for row = 1 : rows
thisRow = binaryImage(row,:);
[~, numRegions(row)] = bwlabel(thisRow);
maxNrRegions = max(numRegions);
nrOfCols = maxNrRegions * 2;
cols = zeros(rows, nrOfCols);
lastCol = zeros (1, nrOfCols);
heights = repmat([0 1], rows, nrOfCols/2);
%go through the image and get all the start and end columns for each label
for row = 1 : rows
thisRow = binaryImage(row,:);
[regions, nR] = bwlabel(thisRow);
for regionID = 1 : nR
col1 = find(regions == regionID, 1, 'first');
if ~isempty(col1) %just making sure
col2 = find(regions == regionID, 1, 'last');
cols(row, regionID*2-1:regionID*2) = [col1 col2]; %save the index of the columns
%find out if the columns in the current row was also present in the last
%row, if so we add to the height
for row = 1 :rows
currentCols = cols(row,:);
for i = 2: 2: nrOfCols
cc = currentCols (1, i-1:i);
if isequal(cc, [0 0]) %[0 0] happens if nrOfRegions in the row is less than the maximum number of regions
heights(row,i) = 0;
for j = 2: 2: length(lastCol)
if isequal (cc, lastCol(1,j-1:j))
heights(row,i) = heights(row-1,j)+1; %get the last height and increase it by one
heights(row-1,j) = 0; %set the last height to zero, makes it easier to find the correct indecies later on
lastCol = currentCols;
%find all heights > 0 and retrive their row & columns
linIdx = find(heights > 0);
[r,c] = ind2sub (size(heights), linIdx);
rect = zeros(length(r),4);
rectPlot = zeros(length(r),4);
%Finally, we extract the rectangles positions
for i = 1 :length(r)
rowStart = r(i) - heights(r(i),c(i)) +1;
rowEnd = r(i);
colStart = cols(r(i),c(i)-1);
colEnd = cols( r(i),c(i));
rect(i,:) = [colStart, colEnd, rowStart,rowEnd];
rectPlot (i,:) = [colStart, rowStart, colEnd-colStart, rowEnd-rowStart+1]; %just for plotting
%compare the results
hold on
for i = 1:length(r)
rectangle('Position', rectPlot(i,:), 'FaceColor',[0 .5 .5])
hold off
set(gca, 'YDir','reverse')
axis([1 120 1 120])

Sign in to comment.

More Answers (0)

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by