How to draw a rectangle around a blob?

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Sachin Dalvi
Sachin Dalvi 2017-12-13
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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回答（2 个）

Harish Ramachandran
Harish Ramachandran 2017-12-19
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
Sachin Dalvi 2018-1-22
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
Harish Ramachandran 2018-2-1
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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Sachin Dalvi
Sachin Dalvi 2018-2-6
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

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