Rotate Image & Video

Rotate Image

Rotating images by a given angle is a common image processing task. Although it seems little bit complicated, OpenCV provides some built-in functions making it easy to do it. Here is a simple OpenCV C++ example code to rotate an image. Here I use a track bar to change the rotating angle dynamically.

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

using namespace cv;

int main( int argc, char** argv )
{
// Load the image
Mat imgOriginal = imread( "MyPic.JPG", 1 );

//show the original image
const char* pzOriginalImage = "Original Image";
namedWindow( pzOriginalImage, CV_WINDOW_AUTOSIZE );
imshow( pzOriginalImage, imgOriginal );

const char* pzRotatedImage = "Rotated Image";
namedWindow( pzRotatedImage, CV_WINDOW_AUTOSIZE );

int iAngle = 180;
createTrackbar("Angle", pzRotatedImage, &iAngle, 360);

int iImageHieght = imgOriginal.rows / 2;
int iImageWidth = imgOriginal.cols / 2;

while (true)
{
Mat matRotation = getRotationMatrix2D( Point(iImageWidth, iImageHieght), (iAngle - 180), 1 );

// Rotate the image
Mat imgRotated;
warpAffine( imgOriginal, imgRotated, matRotation, imgOriginal.size() );

imshow( pzRotatedImage, imgRotated );

int iRet = waitKey(30);
if ( iRet == 27 )
{
break;
}
}

return 0;
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
You can download this visual C++ project from here.

Original Image

Rotated Image

Explanation

Here is the explanation for new OpenCV functions which are not found in previous lessons.

Mat getRotationMatrix2D( Point2f center, double angle, double scale )

This function returns 2x3 affine transformation matrix for the 2D rotation.

Arguments -

center - The center of the rotation of the the source image.
angle - Angle of rotation in degrees (Positive values for counter-clockwise direction and negative values for clockwise rotation)
scale - The scaling factor of the image. (Scaling factor of 1 means its original size)

Try different values for center, angle and scale and observe the output image.

void warpAffine( InputArray src, OutputArray dst, InputArray M, Size dsize, int flags = INTER_LINEAR, int bordreMode=BORDER_CONSTANT, const Scalar& borderValue=Scalar() )

This OpenCV function applies affine transformation to an image.

Arguments -

src - Source Image
dst - Destination image which should have the same type as the source image(The transformed image is stored in this location)
M - 2x3 affine transformation matrix
dsize - Size of the destination image
flags - Interpolation methods
borderMode - pixel extrapolation method. (Try these values; BORDER_REPLICATE, BORDER_CONSTANT, BORDER_REFLECT, BORDER_WRAP, BORDER_REFLECT_101, BORDER_TRANSPARENT and BORDER_ISOLATED)
borderValue - If you use BORDER_CONSTANT for borderMode, this argument define the value used for the border

Another OpenCV Example to Rotate an Image

Here is another way to rotate an image. Here I use a callback function to apply the rotation instead of using a infinite while loop. Other than the rotation, you can change the scale of the image and border extrapolation method dynamically.

Here is the OpenCV C++ code.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

using namespace cv;

int iAngle = 180;
int iScale = 50;
int iBorderMode = 0;
Mat imgOriginal ;
int iImageCenterY = 0;
int iImageCenterX = 0;
const char* pzRotatedImage = "Rotated Image";

void CallbackForTrackBar(int, void*)
{
Mat matRotation = getRotationMatrix2D( Point( iImageCenterX, iImageCenterY ), (iAngle - 180), iScale / 50.0 );

// Rotate the image
Mat imgRotated;
warpAffine( imgOriginal, imgRotated, matRotation, imgOriginal.size(), INTER_LINEAR, iBorderMode, Scalar() );

imshow( pzRotatedImage, imgRotated );

}

int main( int argc, char** argv )
{
// Load the image
imgOriginal = imread( "MyPic.JPG", 1 );

iImageCenterY = imgOriginal.rows / 2;
iImageCenterX = imgOriginal.cols / 2;

//show the original image
const char* pzOriginalImage = "Original Image";
namedWindow( pzOriginalImage, CV_WINDOW_AUTOSIZE );
imshow( pzOriginalImage, imgOriginal );

//create the "Rotated Image" window and 3 trackbars in it
namedWindow( pzRotatedImage, CV_WINDOW_AUTOSIZE );
createTrackbar("Angle", pzRotatedImage, &iAngle, 360, CallbackForTrackBar);
createTrackbar("Scale", pzRotatedImage, &iScale, 100, CallbackForTrackBar);
createTrackbar("Border Mode", pzRotatedImage, &iBorderMode, 5, CallbackForTrackBar);

int iDummy = 0;

CallbackForTrackBar(iDummy, &iDummy);

waitKey(0);

return 0;
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

You can download this visual C++ project from here.

Rotated Image which is down-scaled and whose border values have been extrapolated

All the OpenCV functions have been discussed previously.

Rotate a Video

Rotating a video is also simple. The code is just like the 1st example in this lesson. Here is the OpenCV C++ code.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, char* argv[])
{
// open the video file for reading
VideoCapture cap("C:/Users/SHERMAL/Desktop/MyVideo.mp4");

// if not success, exit program
if ( !cap.isOpened() )
{
cout << "Cannot open the video file" << endl;
return -1;
}

const char* pzOriginalWindowName = "Original Video";
namedWindow(pzOriginalWindowName, CV_WINDOW_AUTOSIZE);

const char* pzRotatingWindowName = "Rotated Video";
namedWindow( pzRotatingWindowName, CV_WINDOW_AUTOSIZE );

int iAngle = 180;
createTrackbar("Angle", pzRotatingWindowName, &iAngle, 360);

while (true)
{
Mat matOriginalFrame;

// read a new frame from video
bool bSuccess = cap.read(matOriginalFrame);

//if not success, break loop
if (!bSuccess)
{
cout << "Cannot read the frame from video file" << endl;
break;
}

imshow(pzOriginalWindowName, matOriginalFrame);

//get the affine transformation matrix
Mat matRotation = getRotationMatrix2D( Point(matOriginalFrame.cols / 2, matOriginalFrame.rows / 2), (iAngle - 180), 1 );

// Rotate the image
Mat matRotatedFrame;
warpAffine( matOriginalFrame, matRotatedFrame, matRotation, matOriginalFrame.size() );

imshow( pzRotatingWindowName, matRotatedFrame );

//wait for 'esc' key press for 30 ms. If 'esc' key is pressed, break loop
if (waitKey(30) == 27)
{
cout << "esc key is pressed by user" << endl;
break;
}
}

return 0;
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
You can download this visual C++ project from here.

All the OpenCV functions, found in the above example have been discussed earlier.

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