How can I find out where the centriod is present in a gridded image in MATLAB

Question

I hope this image helps. I have taken an image from the camera and generated grids on it. How can I know in which grid my centroid is present? I need to generate a variable or a number which I can use further. For example the centroid is in the 3rd box it generates 3 and I can send it to Arduino using serial communication for further action. I hope you get the idea. Can ROI be helpful in this case if yes then how can i use it?

Centroid and bounding box are already generated in the image using this code.

    stat = regionprops(Ibw1, 'Centroid', 'BoundingBox')
hold on
for x=1:numel(stat)

  rectangle('Position',stat(x).BoundingBox,'EdgeColor','r','LineWidth',1);

    plot(stat(x).Centroid(1),stat(x).Centroid(2),'r*');

end
hold off

After this I used this code to generate grids to the image.

[rows, columns, ~] = size(I);
for row = 1 : 52 : rows
  line([1, columns], [row, row], 'Color', 'r');
end
for col = 1 : 53 : columns
  line([col, col], [1, rows], 'Color', 'r');
end

Answer 1

If you want to get the grid row and column numbers, you should plot the grid with predefined grid row and column width:

rowWidth = 52; % Grig row width in pixels
colWidth = 53; % Grig column width in pixels
[rows, columns, ~] = size(I);
for row = 1 : rowWidth : rows
    line([1, columns], [row, row], 'Color', 'r');
end
for col = 1 : colWidth : columns
    line([col, col], [1, rows], 'Color', 'r');
end

To get the the grid row and column numbers (assuming you have only one centroid):

% Get the row and column of the centroid (in grid units) 
centroidGridCol = ceil(stat(1).Centroid(1) / colWidth);
centroidGridRow = ceil(stat(1).Centroid(2) / rowWidth);

To see that it is correct, you can calculate the bounding box of the grid an plot it:

% Get the bounding box of the grid containing the centroid (in pixel units)
centroidGridBox(1) = (centroidGridCol - 1) * colWidth;
centroidGridBox(2) = (centroidGridRow - 1) * rowWidth;
centroidGridBox(3) = colWidth;
centroidGridBox(4) = rowWidth;

% Plot the bounding box of the grid containing the centroid
hold on
rectangle('Position',centroidGridBox ,'EdgeColor','g','LineWidth',1);
hold off

You can plot the grid row and column as text next to the centroid:

% Plot the grid row and column next to the centroid
text(stat(1).Centroid(1),stat(1).Centroid(2),...
    ['(' num2str(centroidGridCol), ', ', num2str(centroidGridRow) ')'],...
    'color','b')