rotation matrix and regionprops

Question

michael scheinfeild 2014-10-1

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https://ww2.mathworks.cn/matlabcentral/answers/156952-rotation-matrix-and-regionprops

评论： Image Analyst 2014-10-1

hello i have some binary image i found the orientation and rotated the image

stats1 = regionprops(bw,'Orientation','Area');
bwRot=imrotate(bw,-stats1.Orientation);

after finding some interesting pixels i want to find their locations in the original image i create the matrix and try to rotate back but recive negative coordinates any help ,,

   stats2 = regionprops(bwRot,'BoundingBox');
      bx =  stats2.BoundingBox;
      xy =  [ bx(1:2) ; 
          bx(1)+bx(3) ,bx(2);
          bx(1)+bx(3) ,bx(2)+bx(4);
          bx(1) ,bx(2)+bx(4);];
      ct = cosd(stats1.Orientation);
      st = sind(stats1.Orientation);
      % rotation matrix
      MatrxRotInv = [ct -st 0 ;
                               st   ct 0;
                               0    0  1];
         xyz = MatrxRotInv* [xy ones(4,1)]'   ;

here the xy is negative why ?

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michael scheinfeild 2014-10-1

also somthing strange i have image 640*480 newimage=imrotate(image,60) oldimage=imrotate(newimage,-60)

the oldimage size is not the same as image

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Answer 1

Matt J 2014-10-1

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编辑：Matt J 2014-10-1

You need to call imrotate with the 'crop' option to preserve the size of the input image.

As for the negative xy, it is possible for a rotation to bring points in/out of the image field of view. That's the whole reason why imrotate has the 'crop' option to begin with.

Be mindful as well that imrotate's origin of rotation is at the center of the image. However, the BoundingBox coordinates are relative to an origin at the upper left corner of the image.