RotatedRect ROI in OpenCV

Question

I have a RotatedRect, I want to do some image processing in the rotated region (say extract the color histogram). How can I get the ROI? I mean get the region(pixels) so that I can do processing.

I find this, but it changes the region by using getRotationMatrix2D and warpAffine, so it doesn't work for my situation (I need to process the original image pixels).
Then I find this suggests using mask, which sounds reasonable, but can anyone teach me how to get the mask as the green RotatedRect below.

Excepts the mask, is there any other solutions ?
Thanks for any hint

Answer 1

Here is my solution, using mask:
The idea is construct a Mat mask by assigning 255 to my RotatedRect ROI.

How to know which point is in ROI (which should be assign to 255)?
I use the following function isInROI to address the problem.

/** decide whether point p is in the ROI.
*** The ROI is a rotated rectange whose 4 corners are stored in roi[] 
**/
bool isInROI(Point p, Point2f roi[])
{
    double pro[4];
    for(int i=0; i<4; ++i)
    {
        pro[i] = computeProduct(p, roi[i], roi[(i+1)%4]);
    }
    if(pro[0]*pro[2]<0 && pro[1]*pro[3]<0)
    {
        return true;
    }
    return false;
}

/** function pro = kx-y+j, take two points a and b,
*** compute the line argument k and j, then return the pro value
*** so that can be used to determine whether the point p is on the left or right
*** of the line ab
**/
double computeProduct(Point p, Point2f a, Point2f b)
{
    double k = (a.y-b.y) / (a.x-b.x);
    double j = a.y - k*a.x;
    return k*p.x - p.y + j;
}

How to construct the mask?
Using the following code.

Mat mask = Mat(image.size(), CV_8U, Scalar(0));
for(int i=0; i<image.rows; ++i)
{
    for(int j=0; j<image.cols; ++j)
    {
        Point p = Point(j,i);   // pay attention to the cordination
        if(isInROI(p,vertices))
        {
            mask.at<uchar>(i,j) = 255;
        }
    }
}

Done,
vancexu