Drawing a rectangle on top of an image

Say I have a rectangle with some dimensions. If I want to draw a rectangle over an image using for instance `plot`, if I start as follows:

    hold on
    imshow(img)

Then, regarding `plot` and my rectangle. How would they fit in this process?

Thanks.

Answer 1

[Image Analyst]

imshow(img);
hold on;
% Then, from the help:
rectangle('Position',[0.59,0.35,3.75,1.37],...
          'Curvature',[0.8,0.4],...
         'LineWidth',2,'LineStyle','--')

[med-sweng]

Thanks for your reply. I did as you mentioned, but couldn't see any rectangle drawn on the image? should it may have another colour?

[Image Analyst]

You probably didn't adjust your position values properly.

Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures if you have the Image Processing Toolbox.
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;

% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'cameraman.tif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
grayImage = imread(fullFileName);
% Get the dimensions of the image.
% numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(grayImage);
if numberOfColorBands > 1
  % It's not really gray scale like we expected - it's color.
  % Convert it to gray scale by taking only the green channel.
  grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the original gray scale image.
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
hold on;
axis on;
% Then, from the help:
rectangle('Position',[95,35,40,50],...
  'Curvature',[0.8,0.4],...
  'EdgeColor', 'r',...
  'LineWidth', 3,...
  'LineStyle','-')

[Katia-Renae Purnell]

I see you calculate the size of the image, but I don't see where it is used. How do you calculate the offset so that the rectangle is actually drawn over the image?

[DGM]

As to how you find the correct offset, that certainly might be a function of the image height and width as you suspect. The problem with writing a generalized example is that there are multiple ways to find an offset for an arbitrary location in space. In some case, you might want to draw a rectangle over some text to censor it. In some example, you might want to draw a rectangle around a peak value. In the above example, the rectangle is drawn around a face. Programmatic solutions to these three tasks would be significantly different and some are individually involved enough that it's arguable that they're beyond the scope of a simple rectangle() demo.

I don't know for certain how Image Analyst found the face, but I imagine that if I needed to draw a rectangle over one such image, I would probably just use a datatip or impixelinfo() to fetch an approximate set of coordinates.

If your're implicitly asking how you can programmatically get the offsets to place a rectangle() object in a certain spot on various images, you'll need to start with a good description of what your images look like and what your goals are. Depending on the scope of the problem, it may be best to open a new question so as to get more eyes on it.