OpenCV: how to rotate IplImage?

Question

I need to rotate an image by very small angle, like 1-5 degrees. Does OpenCV provide simple way of doing that? From reading docs i can assume that getAf

Answer 1

Updating full answer for OpenCV 2.4 and up

// ROTATE p by R
/**
 * Rotate p according to rotation matrix (from getRotationMatrix2D()) R
 * @param R     Rotation matrix from getRotationMatrix2D()
 * @param p     Point2f to rotate
 * @return      Returns rotated coordinates in a Point2f
 */
Point2f rotPoint(const Mat &R, const Point2f &p)
{
    Point2f rp;
    rp.x = (float)(R.at(0,0)*p.x + R.at(0,1)*p.y + R.at(0,2));
    rp.y = (float)(R.at(1,0)*p.x + R.at(1,1)*p.y + R.at(1,2));
    return rp;
}

//COMPUTE THE SIZE NEEDED TO LOSSLESSLY STORE A ROTATED IMAGE
/**
 * Return the size needed to contain bounding box bb when rotated by R
 * @param R     Rotation matrix from getRotationMatrix2D()
 * @param bb    bounding box rectangle to be rotated by R
 * @return      Size of image(width,height) that will compleley contain bb when rotated by R
 */
Size rotatedImageBB(const Mat &R, const Rect &bb)
{
    //Rotate the rectangle coordinates
    vector rp;
    rp.push_back(rotPoint(R,Point2f(bb.x,bb.y)));
    rp.push_back(rotPoint(R,Point2f(bb.x + bb.width,bb.y)));
    rp.push_back(rotPoint(R,Point2f(bb.x + bb.width,bb.y+bb.height)));
    rp.push_back(rotPoint(R,Point2f(bb.x,bb.y+bb.height)));
    //Find float bounding box r
    float x = rp[0].x;
    float y = rp[0].y;
    float left = x, right = x, up = y, down = y;
    for(int i = 1; i<4; ++i)
    {
        x = rp[i].x;
        y = rp[i].y;
        if(left > x) left = x;
        if(right < x) right = x;
        if(up > y) up = y;
        if(down < y) down = y;
    }
    int w = (int)(right - left + 0.5);
    int h = (int)(down - up + 0.5);
    return Size(w,h);
}

/**
 * Rotate region "fromroi" in image "fromI" a total of "angle" degrees and put it in "toI" if toI exists.
 * If toI doesn't exist, create it such that it will hold the entire rotated region. Return toI, rotated imge
 *   This will put the rotated fromroi piece of fromI into the toI image
 *
 * @param fromI     Input image to be rotated
 * @param toI       Output image if provided, (else if &toI = 0, it will create a Mat fill it with the rotated image roi, and return it).
 * @param fromroi   roi region in fromI to be rotated.
 * @param angle     Angle in degrees to rotate
 * @return          Rotated image (you can ignore if you passed in toI
 */
Mat rotateImage(const Mat &fromI, Mat *toI, const Rect &fromroi, double angle)
{
    //CHECK STUFF
    // you should protect against bad parameters here ... omitted ...

    //MAKE OR GET THE "toI" MATRIX
    Point2f cx((float)fromroi.x + (float)fromroi.width/2.0,fromroi.y +
               (float)fromroi.height/2.0);
    Mat R = getRotationMatrix2D(cx,angle,1);
    Mat rotI;
    if(toI)
        rotI = *toI;
    else
    {
        Size rs = rotatedImageBB(R, fromroi);
        rotI.create(rs,fromI.type());
    }

    //ADJUST FOR SHIFTS
    double wdiff = (double)((cx.x - rotI.cols/2.0));
    double hdiff = (double)((cx.y - rotI.rows/2.0));
    R.at(0,2) -= wdiff; //Adjust the rotation point to the middle of the dst image
    R.at(1,2) -= hdiff;

    //ROTATE
    warpAffine(fromI, rotI, R, rotI.size(), INTER_CUBIC, BORDER_CONSTANT, Scalar::all(0)); 

    //& OUT
    return(rotI);
}