getting an image matrix when I have the image.

My code to produce a white F on a black background:

    f_shape = [0,0;1,0;1,2;2,2;2,3;1,3;1,5;3,5;3,6;0,6;0,0]*[1,0;0,-1];
    r = ceil(max(range(f_shape))*sqrt(2));
    kappa =2;
    R = ceil(kappa*r);
    f_img = zeros(2*R);
    f_shape = f_shape + R*[1,1];
    imshow(f_img);
    hold;
    fill(f_shape(:,1),f_shape(:,2),'w');
    daspect = [1,1,1];

Now I want to apply imrotate to this 36 times 36 image matrix. The only way I can think of is to save it as a png, and then imread it. Do I really have to do that? Sound slow, and wrong in principle. It is also likely to result in lots of pointless files littering my hard disk.

Answer 1

[Image Analyst]

Simply call imrotate() on f_img. Why do you think you can't do that???

rotatedImage = imrotate(f_img, angle);

[David Epstein]

When I rotate a completely black image, I get a completely black image. I want to rotate the white F in my figure. The problem is that commands like `fill' change what one sees, but do not change the image matrix.

[Image Analyst]

That's because fill() goes into the overlay and is not affected by imrotate. Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
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;

% Make F image.
f_image = zeros(36, 36, 'uint8');
f_image(5:32, 12:16) = 255;
f_image(5:8, 17:29) = 255;
f_image(15:18, 17:25) = 255;

% imshow(f_img, []);
imshow(f_image, []);
grid on;
axis('on', 'image');
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0.04, 1, 0.96]);
impixelinfo();
grid on;

% Now rotate a few times.
figure;
plotNumber = 1;
for angle = 0 : 30 : 330
  thisImage = imrotate(f_image, angle);
  subplot(3, 4, plotNumber);
  imshow(thisImage, []);
  axis('on', 'image');
  impixelinfo();
  caption = sprintf('Rotated by %.1f degrees', angle);
  title(caption, 'FontSize', fontSize);
  plotNumber = plotNumber + 1;
end
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0.04, 1, 0.96]);

[David Epstein]

Thanks for the explanation. I had assumed that the graphics commands would change the matrix directly. This clarifies the issue for a naive user like me. I was interested to see how the shape changes when rotated, and I'm planning to look up and use an interpolation method with imrotate, in the hope that the change won't be so obvious.

[Image Analyst]

You need to understand the concept of an actual digital image, which has pixel values, and the concept of a graphical overlay, where you put computer graphics in over the image. The Image Processing Toolbox commands, starting with im or bw, work on the image itself, not with anything you may have put on top of them in the overlay.