MATLAB Answers

Drawing rectangles using matrices.

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Anvinder  Singh
Anvinder Singh on 28 Feb 2016
Commented: Image Analyst on 1 Aug 2018
Hi, I need to draw a figure as shown in the picture using 256x256 matrix. There should be two rectangles and the rest of the matrix values should be zero. Can you please help ?
Image Analyst
Image Analyst on 1 Aug 2018
Use plot() with 3 x values and 3 y values.

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Answers (4)

Guillaume on 28 Feb 2016
Considering your rectangles are aligned with the side of the image, all you have to do is fill two rows and two columns with 1:
function img = drawrectangle(img, xbounds, ybounds)
img(ybounds, xbounds(1):xbounds(2)) = 1;
img(ybounds(1):ybounds(2), xbounds) = 1;
And in your case, you use it like this:
img = zeros(256);
img = drawrectangle(img, [40, 140], [60, 80]); %you didn't show the y coordinates for the 1st rectangle
img = drawrectangle(img, [120, 200], [120, 200]);

Stephen on 28 Feb 2016
Edited: Stephen on 28 Feb 2016
You can use MATLAB's array indexing quite effectively for this:
>> X = ones(256,256,3);
>> X(40:140,[40,140],:) = 0;
>> X([40,140],40:140,:) = 0;
and to view the rectangle:
>> image(X)
creates this:
Anvinder  Singh
Anvinder Singh on 29 Feb 2016
Guillaume you are right. I thought the rectangle function you defined was independent of the matrix and was it's values just based on the values given to it so it had nothing to do with the matrix (256*256) so got a little confused.

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Image Analyst
Image Analyst on 28 Feb 2016
If you want them on a graph, with the y increasing upwards, try this:
rectangle('Position', [40,40,100,100]);
hold on;
rectangle('Position', [120,120,80,80]);
axis([0,220, 0,220]);
Note, your rectangles will overlap/intersect given the coordinates you gave despite the fact that your picture incorrectly shows them not overlapping.
Anvinder  Singh
Anvinder Singh on 29 Feb 2016
how do i set a given points in a row of matrix as 1 ? example- set x coordinate 40 to 120 as 1 etc..

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Anvinder  Singh
Anvinder Singh on 1 Mar 2016
The y axis of the graph is still reversed. I have tried axis([0,280, 0,280]); and set(gca, 'ydir', 'reverse'); % or 'normal'
  1 Comment
Image Analyst
Image Analyst on 1 Mar 2016
Explain why 120 comes after 140 in your diagram.

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