MATLAB: Fast Image/Axes Rotation in GUI

gui image rotation

Hello,

I trying to to rotate an image in a GUI based on a transient input. Think of a Steering Wheel which should rotate in realtime (based on given input values). I have a solution but it takes up to 200 ms which is a bit slow. (Aiming for less to get it smoother) I am displaying the image with:

global Im_Theta 
Imshow(IMAGE, 'Parent', handles.ax_Image);

The Rotation is done by calling my rotation function:

function [Im_Rotated, Alpha_Rotated]=Im_rotate(Image, Alphamap, delta_theta)
global Im_Theta
Im_Theta=delta_theta+Im_Theta;
T = [cos(Im_Theta) -sin(Im_Theta) 0; 
       sin(Im_Theta) cos(Im_Theta) 0; 
     0 0 1];
form = maketform('affine', T);
Image_Alpha   = cat(3,Image, Alphamap);
Image_Alpha   = imtransform(Image_Alpha, tform);
Im_Rotated    = Image_Alpha(:,:,1:3);
Alpha_Rotated = Image_Alpha(:,:,4);

And then reploting the Image

h_Imshow=Imshow(Im_Rotated, 'Parent', handles.ax_Image);
set(h_Imshow, 'AlphaData', Alpha_Rotated);

The Problem is I dont need the rotated images' Information. All I need is the "spinning" Image on the GUI. I tried rotate and view but it doesn't seem to work with images.

Additional Infos:

– NO Image Processing Toolbox available

– It's a PNG Image with Alpha

– Using delta Theta and Theta to avoid fragments due to multiple rotations

– Summing up the Image with its Alpha in one matrix before imtransform helps to reduce by 50 ms.

Thanks in advance

Best Answer

If you only need to display the spinning image, then you would use a slightly different routine, makehgtform(), which you would apply it to a hgtransform() object that the image had been parented to.

Related Solutions

MATLAB: Is imrotate not rotating an image about its center point

The answer is everything here is working fine, imrotate IS rotating about the center of the input image, and imregtform is correctly recovering the registration that maps the moving image to the fixed image. The issues we have here are artifacts of coordinate system conventions and the way imshowpair is being used.

To start, take a look at the result of overlaying the registered image from your example with the original image:

I2D = zeros(256, 256);
i0 = 40;
for i = 71:230
    I2D(i-15,i0:i+10) = 1;
end
I2D(128:130,128:130)=0;
I2DT = imrotate(I2D, 13);  % or
%I2DT = imrotate(I2D, 13, 'crop');  
imshowpair(I2D, I2DT); 
title('original and transformed image');
[optimizer, metric] = imregconfig('monomodal');
tform = imregtform(I2DT,I2D,'affine',optimizer,metric);
Tmatrix = tform.T;
I2DT_reg = imwarp(I2DT,tform,'OutputView',imref2d(size(I2D)));
figure, imshowpair(I2DT_reg,I2D)

Looks good right?

Here are a couple of observations that might be helpful:

1) Rotation about the center of an image is NOT defined by an elemental affine matrix with a zero translation, except for the special case where the center of the input image is aligned with the origin (0,0). In all other orientations of the input image, including the default coordinate system assumed by MATLAB in IMROTATE in which the first pixel is centered at location 1,1 and the last pixel is centered at Colmax,Rowmax, this is not the case.

One way to define an equivalent affine operation to what imrotate is doing is to create a composite affine transformation in which you do three operations: 1) Translate the center of the image to the origin. 2) Rotate by desired angle. 3) Translate the center of the image back to it's original location.

Rdefault = imref2d(size(I2D)); % The default coordinate system used by imrotate
tX = mean(Rdefault.XWorldLimits);
tY = mean(Rdefault.YWorldLimits);
tTranslationToCenterAtOrigin = [1 0 0; 0 1 0; -tX -tY,1];
theta = 10;
tRotation = [cosd(theta) -sind(theta) 0; sind(theta) cosd(theta) 0; 0 0 1];
tTranslationBackToOriginalCenter = [1 0 0; 0 1 0; tX tY,1];
tformCenteredRotation = tTranslationToCenterAtOrigin*tRotation*tTranslationBackToOriginalCenter;
tformCenteredRotation = affine2d(tformCenteredRotation);
[out,Rout] = imwarp(I2D,tformCenteredRotation);
imshow(out,Rout)

Note that this transformation has a translation component, and that this is expected.

2) When you are testing the center of rotation and visualizing this using imshowpair, the reason the centers don't line up is that you aren't passing sufficient spatial referencing information to imshowpair. By default, imshowpair aligns the upper left corners of two input image unless the optional spatial referencing arguments are passed in which defined where each image is. That is why it appears that the center is not preserved.

MATLAB: How to get the new coordinates of a point after rotation of an image (2D)

P = [5;8];    % coordinates of point
alpha = 45;   % angle for rotation
RotatedIm = imrotate(im,alpha);   % rotation of the main image (im)
RotMatrix = [cosd(alpha) -sind(alpha); sind(alpha) cosd(alpha)]; 
ImCenterA = (size(im)/2)';         % Center of the main image
ImCenterB = (size(RotatedIm )/2)';  % Center of the transformed image
RotatedP = RotMatrix*(P-ImCenterA)+ImCenterB;

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