How to draw a rectangle around a blob?

Question

Sachin Dalvi 2017-12-13

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https://ww2.mathworks.cn/matlabcentral/answers/372754-how-to-draw-a-rectangle-around-a-blob

回答： Sachin Dalvi 2018-2-6

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

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Answer 1

Harish Ramachandran 2017-12-19

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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.

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Sachin Dalvi 2018-1-22

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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 2018-2-1

在 MATLAB Online 中打开

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.

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Answer 2

Sachin Dalvi 2018-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/372754-how-to-draw-a-rectangle-around-a-blob#answer_303737

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cat1.zip

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