MATLAB: How can i make automatic rotation using PCA to set the galaxies oriantation to be horizontal

cropgalaxies rotationimageorientationrotaterotate image

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How can i make automatic rotation using PCA to set the galaxies oriantation to be horizontal and cropping the image?

Knowing that the galaxies in the original images have different directions

I searched a lot and could not find any code

Best Answer

This approach uses the Orientation property of the regionprops() function to get the angle between the x-axis and the major axis of the galaxy ellipse. It uses the orientation angle to rotate the image so the galaxy is horizontal.

There are several plots produced to show the steps of this process. They can be commented out in your final function/script. The image file I'm analyzing in this script is attached.

See the inline comments for detail.

% Identify file
file = 'PGC0001862.png'; % full path is better; see fullfile()
% Binarize the image
I = imread(file); 
BW = imbinarize(rgb2gray(I)); 
% Filter out noise by isolating the object with the largest perimeter
BWfilter = bwpropfilt(BW,'perimeter',1);
% Show orginal and binary images for a sanity check
clf()
imshowpair(I,BWfilter,'montage')


% Get the orientation of the main object
stats = regionprops(BWfilter,'Orientation');  % 56.981 for this image
% rotate the image
IRot = imrotate(I, -stats.Orientation);
% Show image before/after rotation for sanity check
clf()
subplot(1,2,1)
imshow(I)
subplot(1,2,2)
imshow(IRot)
linkaxes(); % to match aspect ratios

% Crop the rotated image based on the limits of the binary image
BWfilterRot = imrotate(BWfilter, -stats.Orientation);
[rows, cols] = find(BWfilterRot);
BWcrop = BWfilterRot(min(rows):max(rows), min(cols):max(cols)); % only needed for sanity-check
Icrop = IRot(min(rows):max(rows), min(cols):max(cols),:); 
% Show the cropped RGB and BW images
clf()
subplot(1,2,1)
imshow(Icrop)
subplot(1,2,2)
imshow(BWcrop)
linkaxes(); % to match aspect ratios

% Show original and final image
clf()
subplot(1,2,1)
imshow(I)
subplot(1,2,2)
imshow(Icrop)
linkaxes(); % to match aspect ratios

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 J = imrotate(rgb,barAngle,'bilinear','crop');

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To apply PCA to an image, see this link: http://www.mathworks.com/matlabcentral/answers/167938#answer_163231 I don't think it will work, though you can try. Make sure you pass it the location of the black pixels, not the image gray levels.

I'd probably use a radon transform. See my attached demo where I use the radon transform to figure out the angle and rotate the image.

% Demo to use the radon transform to determine the angle to rotate an image to straighten it.
% Initialization / clean-up code.
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 = 20;
% Read in the color demo image.
[rgbImage, colorMap] = imread('football.jpg');
subplot(2, 3, 1);
imshow(rgbImage, colorMap);
axis on;
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Extract the red channel and display it.
grayImage = rgbImage(:, :, 1);
subplot(2, 3, 2);
imshow(grayImage, colorMap);
axis on;
title('Red Channel Image', 'FontSize', fontSize);
% Do the Radon transform.
theta = 0:180;
[R,xp] = radon(grayImage,theta);
% Find the location of the peak of the radon transform image.
maxR = max(R(:));
[rowOfMax, columnOfMax] = find(R == maxR)
% Display the Radon Transform image.
h3 = subplot(2, 3, [3,6]);
imshow(R,[],'Xdata',theta,'Ydata',xp,...
            'InitialMagnification','fit')
axis on;
% Plot a blue circle over the max.
hold on;
plot(h3, columnOfMax, xp(rowOfMax), 'bo', 'MarkerSize', 30, 'LineWidth', 3);
line([columnOfMax, columnOfMax], [xp(end), xp(rowOfMax)+15], 'Color', 'b', 'LineWidth', 3);
caption = sprintf('Radon Transform.  Max at angle %.1f', columnOfMax);
title(caption, 'FontSize', fontSize);
xlabel('\theta (degrees)', 'FontSize', fontSize)
ylabel('x''', 'FontSize', fontSize)
colormap(h3, hot(256));
colorbar;
% The column of the max is the angle of the football --
% the angle that the projected sum (profile) will have the highest sum.
% Rotate it by minus that angle to straighten it.
rotatedImage = imrotate(rgbImage, -columnOfMax);
% Display the rotated image.
subplot(2, 3, 4);
imshow(rotatedImage);
axis on;
title('Rotated Color Image', 'FontSize', fontSize);
% Rotate perpendicular to that angle and display that rotation.
rotatedImage = imrotate(rgbImage, -columnOfMax+90);
subplot(2, 3, 5);
imshow(rotatedImage);
axis on;
title('Rotated Color Image', 'FontSize', fontSize);

Best Answer

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