Here is an answer to this:
The idea is to check whether every pixel is in the region respect to rectangle edges:

```
for(int i=0; i<myImage.rows; ++i)
{
for(int j=0; j<myImage.cols; ++j)
{
if(isInRotatedRect( Point(j,i), rotated_rect))
{
myImage.at<uchar>(i,j) = 0;
}
}
}
```

for computing isInRotatedRect:

```
bool isInRotatedRect(Point p, RotatedRect rRect)
{
double line_pro[4];
Point2f vertices[4];
rRect.points(vertices);
for(int i=0; i<4; ++i)
{
line_pro[i] = computeProduct(p, vertices[i], vertices[(i+1)%4]);
}
if(line_pro[1]*line_pro[3]<0 && line_pro[0]*line_pro[2]<0)
{
return true;
}
return false;
}
//finding whether point p is at the left or right side of rectangle edge ab.
double computeProduct(Point p, Point2f a, Point2f b)
{
double m = (a.y-b.y) / (a.x-b.x);
double c = a.y - k * a.x;
return m * p.x - p.y + c;
}
```

A better and more intuitive way would be rather than doing this for every image make a zero one mask
and the cross product original image! This may be more efficient specially in video.