I want to rotate 90 degrees counterclockwise, but it seems the rotation point is wrong. How do I find the rotation center of the source image?

    img=cvLoadImage(argv[1],-1); 
height    = img->height;
width     = img->width;
step      = img->widthStep;
channels  = img->nChannels;
data      = (uchar *)img->imageData;

    IplImage *rotatedImg = cvCreateImage(cvSize(height,width), IPL_DEPTH_8U,img->nChannels);

    CvPoint2D32f center;
    center.x = width/2;
    center.y = height/2;
    CvMat *mapMatrix = cvCreateMat( 2, 3, CV_32FC1 );

    cv2DRotationMatrix(center, 90, 1.0, mapMatrix);
    cvWarpAffine(img, rotatedImg, mapMatrix, CV_INTER_LINEAR + CV_WARP_FILL_OUTLIERS, cvScalarAll(0));


cvShowImage("My image", rotatedImg );