How to draw a rectangle around a blob?

Hello, I am very new to MATLAB programming

I have a binary blob which changes its position with each frame. I have managed to get lower and higher bounds for each frame (Horizontal and Vertical).

vertprofile = any(binaryImage>0,1);
        bbox_x_low = find(vertprofile, 1, 'first');
        box_x_high = find(vertprofile, 1, 'last');
        horzprofile = any(binaryImage>0,2);
        bbox_y_low = find(horzprofile, 1, 'first');
        bbox_y_high = find(horzprofile, 1, 'last');

My question is how can I draw a rectangle within these bounds?

also,

rectangle('Position',[1 2 5 6])

in this command how can I replace numbers with variables.

for example I have variables as, a=1;b=2;c=5;d=6 which changes with every frame.... and using the above command how can I draw rectangle by putting these variables instead numbers.

Thank you

Answer 1

[Harish Ramachandran]

Hi Sachin,

1. Using Rectangle: You can use variable names instead of values in the 'rectangle' command. You have the values for the following:

• x_low, y_low - First two arguments for the rectangle command
• (x_high - x_low), (y_high - y_low) - Final two arguments for the rectangle command

x_low = 1;
y_low = 2;
x_high = 6;
y_high = 8;
axis([0 10 0 10]);  
rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)])

is equivalent to

axis([0 10 0 10]);  
rectangle('Position',[1 2 5 6]);

2. Using plot: You can refer to this link for an alternate way.

[Sachin Dalvi]

Hello, thank you so much for the answer. but when I run the code, I get error at the function 'rectangle' as "Value must be a 4 element vector". Can you please help me with this ?

here is my code:

vertprofile = any(binaryImage>0,1);
horzprofile = any(binaryImage>0,2);

x_low = find(vertprofile, 1, 'first');
x_high = find(vertprofile, 1, 'last');

y_low = find(horzprofile, 1, 'first');
y_high = find(horzprofile, 1, 'last');

subplot(1, 2, 2);
imshow(binaryImage);
hold on;
axis([0 10 0 10]);  
rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)]);

[Harish Ramachandran]

I don't have any idea about the inputs you are providing.

However, take the inputs which I am providing:

x_low = 1;
y_low = 2;
x_high = 6;
y_high = 8;

The rectangle function works. The arguments to the rectangle function is a vector containing 4 values. In the event that I increase the arguments to the rectangle by 1 as in the case below:

x_low = [1 4];
y_low = 2;
x_high = 6;
y_high = 8;

rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)])
Error using rectangle
Value must be a 4 element vector

So please check the dimensions of x_low, y_low, x_high, y_high and make sure it is a scalar value.

[Sachin Dalvi]

Hi Harish, I have a video of cat walking which i am processing frame by frame, so the values of x_low,x_high,y_low,y_high are different for each frame. binary image(720 x 1280) of last frame is as this

and the corresponding values for bounds are :

Now i want to draw a rectangle within these bounds and crop out this part,but the function rectangle is somehow not working with this. I get error saying 'the value must be 4 element vector' but when I comment out the rectangle function, program runs and the scalar values of bounds can be seen in workspace.

rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)]);

I also tried performing arithmatic operation outside the function and rewrite the function just with single variables as

rectangle('Position',[x_low y_low a b);

but I am getting the same error.

[Harish Ramachandran]

Sorry for the delayed response. I am not sure what the issue is. Based on the values you provided -

x_high = 724;
y_high = 569;
x_low = 265;
y_low = 1;
rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)]);

I am able to draw a rectangle using those values. Can you provide a copy of the code / input? I will try my best to help you out.

Answer 2

[Sachin Dalvi]

Hello Harish, Below is my code and attached is the video file.

    clc;
        close all;
        imtool close all;
        clear;
        workspace;
        fontSize = 22;

        % Looking for video file

        folder = fileparts(which('cat1.mp4'));
        movieFullFileName = fullfile(folder, 'cat1.mp4');

        if ~exist(movieFullFileName, 'file')
            strErrorMessage = sprintf('File not found:\n%s\nYou can choose a new one, or cancel', movieFullFileName);
            response = questdlg(strErrorMessage, 'File not found', 'OK - choose a new movie.', 'Cancel', 'OK - choose a new movie.');
            if strcmpi(response, 'OK - choose a new movie.')
                [baseFileName, folderName, FilterIndex] = uigetfile('*.mp4');
                if ~isequal(baseFileName, 0)
                    movieFullFileName = fullfile(folderName, baseFileName);
                else
                    return;
                end
                elsearduino
                return;
            end
        end

      try

          %--------------Making video object

          videoObject = VideoReader(movieFullFileName);
          numberOfFrames = 85;    
          numberOfFramesWritten = 0;

          %-------------Reading frames

          for frame = 1 : numberOfFrames

              thisFrame = read(videoObject, frame);

              subplot(1, 2, 1);
              imshow(thisFrame);
              caption = sprintf('Frame %4d of %d.', frame, numberOfFrames);
              title(caption, 'FontSize', fontSize);
              drawnow;

              %-------------Differencing
              alpha = 0.4;

              if frame == 1
                  Background = thisFrame;
              else

                  Background = (1-alpha)* thisFrame + alpha * Background;
              end

              differenceImage = thisFrame - uint8(Background);        
              grayImage = rgb2gray(differenceImage);

              %--------------Get threshold.
              thresholdLevel = graythresh(grayImage);
              %-------------Do the binarization
              binaryImage = imbinarize(grayImage, thresholdLevel);

              %----getting vertical and horizontal bounds
              vertprofile = any(binaryImage>0,1);
              horzprofile = any(binaryImage>0,2);

              x_low = find(vertprofile, 1, 'first');
              x_high = find(vertprofile, 1, 'last');

              y_low = find(horzprofile, 1, 'first');
              y_high = find(horzprofile, 1, 'last');     

              %------Plot the binary image.
              subplot(1, 2, 2);
              imshow(binaryImage);
              hold on;       

              rectangle('Position',[x_low y_low (x_high-x_low) (y_high-y_low)]);   

              title('Binarized Difference Image', 'FontSize', fontSize);        

          end

      catch ME
          %--------Some error happened if you get here.
          strErrorMessage = sprintf('Error extracting movie frames from:\n\n%s\n\nError: %s\n\n)', movieFullFileName, ME.message);
          uiwait(msgbox(strErrorMessage));
      end