how can I draw a rectangle on a image?

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Hello friends I'm with a problem! I have a vector with x and y coordinates and I need to draw a rectangle over an image.A rectangle with the dimensions 3x3 with the red line from the centroid point how can I do this?

Accepted Answer

Walter Roberson
Walter Roberson on 19 Jun 2015
For any one (x, y) centroid pair:
linelen = 3;
pixbefore = floor((linelen - 1)/2);
pixafter = linelen - 1 - pixbefore;
paintcolor = [255, 0, 0];
for K = 1 : length(paintcolor)
YourImage(y - pixbefore : y + pixafter, [x - pixbefore, x + pixafter], K) = red(K); %left and right edge
YourImage([y - pixbefore + 1, y + pixafter - 1], x - pixbefore : x + pixafter, K) = red(K); %top and bottom edge
If I was doing a whole bunch of them, I would use a slightly different approach
Walter Roberson
Walter Roberson on 19 Jun 2015
For drawing on the graphics display, you should be using rectangle() unless you have the Computer Vision toolbox.
linelen = 3; %data units now, not pixels
paintcolor = [1 0 0]; %red
rectangle('Position', [x - linelen/2, y - linelen/2, linelen, linelen], 'EdgeColor', paintcolor);
This would be for a rectangle centered around the coordinates. If you want a rectangle whose bottom left is the given coordinates then
linelen = 3; %data units now, not pixels
paintcolor = [1 0 0]; %red
rectangle('Position', [x, y, linelen, linelen], 'EdgeColor', paintcolor);
Biza Ferreira
Biza Ferreira on 19 Jun 2015
Edited: Biza Ferreira on 19 Jun 2015
tanks for all

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More Answers (2)

Dima Lisin
Dima Lisin on 19 Jun 2015
Hi Biza,
Try using the insertShape function in the Computer Vision System Toolbox.

Edwards Ramirez
Edwards Ramirez on 22 Jan 2019
I had to write it on my own. Here the used function:
function imageReturn = drawRectangle(image, Xmin, Ymin, width, height)
imageReturn = image;
intensity = 255;
X1 = Ymin;
Y1 = Xmin;
X2 = Ymin + width;
Y2 = Xmin + height;
Xcenter = round((X1+X2)/2);
Ycenter = round((Y1+Y2)/2);
for i=X1:X2
for i=X1:X2
for i=Y1:Y2
for i=Y1:Y2
%Comment the following for cycles if you don't want the cross in the centroid
for i=Xcenter-3:Xcenter+3
for i=Ycenter-3:Ycenter+3
  1 Comment
Image Analyst
Image Analyst on 22 Jan 2019
Edited: Image Analyst on 22 Jan 2019
Well, okay. Your code burns the rectangle into the image (although it's very confusing because you're setting x equal to y and y equal to x instead of just doing it like you should). The poster said he wants to "draw a rectangle over an image", which I take to mean in the graphical overlay over the image, not INTO the image itself. You could put a rectangle into the graphical overlay over the image with the rectangle() function. If you want to burn the rectangle into the image, one could use insertShape like Dima said in his answer (though it requires the Computer Vision System Toolbox).
By the way, your for loops could be vectorized, so
for i=X1:X2
would become
imageReturn(Ymin, XMin:(Xmin + width - 1), 1) = intensity;
Notice that I corrected your confusing indexing. It's a very common beginner mistake and that is thinking arrays are indexed as (x,y) instead of (y, x). I know you tried to correct it with your swapping of x and y but I personally think that just made it more confusing. Arrays are indexed as (row, column), so the first index is row, or Y, not X as you had it.
You might want to edit your code and correct that lest anyone every copy it and try to use it and run into problems that confuse them.

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