Easiest way to rotate by 90 degrees an image using OpenCV

Question

What is the best way  in c c    to rotate an IplImage cv  Mat by 90 degrees  I would assume that there must be something better than transforming it using a matrix  but I can t seem to find anything other than that in the API and online

User · Answer

Here is my python cv2 implementation   import cv2  img cv2 imread  path to image jpg      rotate ccw out cv2 transpose img  out cv2 flip out flipCode 0     rotate cw out cv2 transpose img  out cv2 flip out flipCode 1   cv2 imwrite  rotated jpg   out

User · Answer

Well I was looking for some details and didn t find any example  So I am posting a transposeImage function which  I hope  will help others who are looking for a direct way to rotate 90   without losing data   IplImage  transposeImage IplImage  image         IplImage  rotated   cvCreateImage cvSize image- gt height image- gt width              IPL DEPTH 8U image- gt nChannels       CvPoint2D32f center      float center val    float   image- gt width -1    2      center x   center val      center y   center val      CvMat  mapMatrix   cvCreateMat  2  3  CV 32FC1                cv2DRotationMatrix center  90  1 0  mapMatrix       cvWarpAffine image  rotated  mapMatrix           CV INTER LINEAR   CV WARP FILL OUTLIERS           cvScalarAll 0              cvReleaseMat  amp mapMatrix        return rotated      Question   Why this   float center val    float   image- gt width -1    2     Answer   Because it works    The only center I found that doesn t translate image  Though if somebody has an explanation I would be interested

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Rotation is a composition of a transpose and a flip       Which in OpenCV can be written like this  Python example below    img   cv LoadImage  path to image jpg   timg   cv CreateImage  img height img width   img depth  img channels    transposed image    rotate counter-clockwise cv Transpose img timg  cv Flip timg timg flipMode 0  cv SaveImage  rotated counter clockwise jpg   timg     rotate clockwise cv Transpose img timg  cv Flip timg timg flipMode 1  cv SaveImage  rotated clockwise jpg   timg

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Here s my EmguCV  a C  port of OpenCV  solution   public static Image lt TColor  TDepth gt  Rotate90 lt TColor  TDepth gt  this Image lt TColor  TDepth gt  img      where TColor   struct  IColor     where TDepth   new         var rot   new Image lt TColor  TDepth gt  img Height  img Width       CvInvoke cvTranspose img Ptr  rot Ptr       rot  Flip FLIP HORIZONTAL       return rot     public static Image lt TColor  TDepth gt  Rotate180 lt TColor  TDepth gt  this Image lt TColor  TDepth gt  img      where TColor   struct  IColor     where TDepth   new         var rot   img CopyBlank        rot   img Flip FLIP VERTICAL       rot  Flip FLIP HORIZONTAL       return rot     public static void  Rotate180 lt TColor  TDepth gt  this Image lt TColor  TDepth gt  img      where TColor   struct  IColor     where TDepth   new         img  Flip FLIP VERTICAL       img  Flip FLIP HORIZONTAL      public static Image lt TColor  TDepth gt  Rotate270 lt TColor  TDepth gt  this Image lt TColor  TDepth gt  img      where TColor   struct  IColor     where TDepth   new         var rot   new Image lt TColor  TDepth gt  img Height  img Width       CvInvoke cvTranspose img Ptr  rot Ptr       rot  Flip FLIP VERTICAL       return rot      Shouldn t be too hard to translate it back into C

User · Answer

Update for transposition   You should use cvTranspose   or cv  transpose   because  as you rightly pointed out  it s more efficient  Again  I recommend upgrading to OpenCV2 0 since most of the cvXXX functions just convert IplImage  structures to Mat objects  no deep copies   If you stored the image in a Mat object  Mat t   would return the transpose   Any rotation   You should use cvWarpAffine by defining the rotation matrix in the general framework of the transformation matrix  I would highly recommend upgrading to OpenCV2 0 which has several features as well as a Mat class which encapsulates matrices and images  With 2 0 you can use warpAffine to the above

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As of OpenCV3 2  life just got a bit easier  you can now rotate an image in a single line of code   cv  rotate image  image  cv  ROTATE 90 CLOCKWISE     For the direction you can choose any of the following   ROTATE 90 CLOCKWISE ROTATE 180 ROTATE 90 COUNTERCLOCKWISE

User · Answer

This is an example without the new C   interface  works for 90  180 and 270 degrees  using param   1  2 and 3   Remember to call cvReleaseImage on the returned image after using it   IplImage  rotate image IplImage  image  int  90 degrees steps anti clockwise        IplImage  rotated       if  90 degrees steps anti clockwise    2          rotated   cvCreateImage cvSize image- gt height  image- gt width   image- gt depth  image- gt nChannels       else         rotated   cvCloneImage image        if  90 degrees steps anti clockwise    2          cvTranspose image  rotated        if  90 degrees steps anti clockwise    3          cvFlip rotated  NULL  1       else if  90 degrees steps anti clockwise    1          cvFlip rotated  NULL  0       else if  90 degrees steps anti clockwise    2          cvFlip rotated  NULL  -1        return rotated

[c++] Easiest way to rotate by 90 degrees an image using OpenCV?

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