OpenCV : warpPerspective on whole image

edit

I'm detecting markers on images captured by my iPad. Because of that I want to calculate translations and rotations between them, I want to change change perspective on images these image, so it would look like I'm capturing them directly above markers.

Right now I'm using

points2D.push_back(cv::Point2f(-6, -6));
points2D.push_back(cv::Point2f(6, -6));
points2D.push_back(cv::Point2f(6, 6));
points2D.push_back(cv::Point2f(-6, 6));

Mat perspectiveMat = cv::getPerspectiveTransform(points2D, imagePoints);
cv::warpPerspective(*_image, *_undistortedImage, M, cv::Size(_image->cols, _image->rows));

Which gives my these results (look at the right-bottom corner for result of warpPerspective):

As you probably see result image contains recognized marker in left-top corner of the result image. My problem is that I want to capture whole image (without cropping) so I could detect other markers on that image later.

How can I do that? Maybe I should use rotation/translation vectors from solvePnP function?