How to obtain an angular segment with its vertex on the center from a circular image?

I have a circular image. I need to extract eight equal angular segment from it. Can anyone help me with the matlab code for this?

Answer 1

[Image Analyst]

See the third example in the FAQ http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

xCenter = 12;
yCenter = 10;
theta = 0 : 0.01 : 2*pi;
radius = 5;
x = radius * cos(theta) + xCenter;
y = radius * sin(theta) + yCenter;
plot(x, y);
axis square;
xlim([0 20]);
ylim([0 20]);
grid on;

Adjust the starting and ending theta to be what you need them to be, then pass x and y into poly2mask() to get a binary image which can then multiply by your image.

mask = poly2mask(x,y,size(rgbImage, 1), size(rgbImage, 2));
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, class(rgbImage)));

[Jyothis Jose]

I tried this one. But for 0 to pi/4 I am getting only a small segment not an angular segment with its vertex on the center of the circular image. Please suggest me a solution. Thank You

[Image Analyst]

How about you post your code and I fix it?

[Image Analyst]

On second thought, that might be more work than doing it myself. Here, run this code:

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.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 22;

%===============================================================================
% Read in a standard MATLAB color demo image.
baseFileName = 'peppers.png';
folder = fileparts(which(baseFileName));
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % 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.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
rgbImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows, columns, numberOfColorBands] = size(rgbImage);
% Display the original color image.
subplot(2, 2, 1);
imshow(rgbImage);
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
drawnow;

% Make pie sector.
xCenter = columns/2;
yCenter = rows/2;
theta = 0 : 0.01 : pi/2;
radius = min([rows/2, columns/2]);
x = radius * cos(theta) + xCenter;
y = radius * sin(theta) + yCenter;
% Add point at center to beginning and end of array and shift to center.
x = [xCenter, x, xCenter];
y = [yCenter, y, yCenter];
subplot(2, 2, 2);
plot(x, y);
title('Sector Plot', 'FontSize', fontSize);
axis square;
grid on;

mask = poly2mask(x,y, size(rgbImage, 1), size(rgbImage, 2));
% Display the mask image.
subplot(2, 2, 3);
imshow(mask);
title('Mask Image', 'FontSize', fontSize);

% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, class(rgbImage)));
% Display the mask image.
subplot(2, 2, 4);
imshow(maskedRgbImage);
title('Masked Color Image', 'FontSize', fontSize);

[Jyothis Jose]

Thank You so much sir. It worked well on my image. It helped me a lot in doing my project. Thank You once again........

[Image Analyst]

Please mark the answer as "Accepted" then. Thanks in advance.

[NUR LYANA SHAHFIQA ALBASHAH]

How to apply it for grayscale Image?

[Image Analyst]

It's the same code, using bsxfun. Change the ending angle if you don't want a full circle.

xCenter = 128;
yCenter = 128;
theta = 0 : 0.01 : 2*pi/3;
radius = 100;
x = radius * cos(theta) + xCenter;
y = radius * sin(theta) + yCenter;
plot(x, y);
axis square;
xlim([0 20]);
ylim([0 20]);
grid on;
% Tack on the center
x = [xCenter, x, xCenter];
y = [yCenter, y, yCenter];

% Read in gray scale image.   Actually can be anything - RGB or gray scale.
grayImage = imread('cameraman.tif');
subplot(2,2,1);
imshow(grayImage, []);
axis on;

% Create mask
mask = poly2mask(x,y,size(grayImage, 1), size(grayImage, 2));
subplot(2,2,2);
imshow(mask, []);
axis on;

% Mask the image using bsxfun() function
maskedGrayImage = bsxfun(@times, grayImage, cast(mask, class(grayImage)));
subplot(2,2,3:4);
imshow(maskedGrayImage, []);
axis on;