For the cv::warpPerspective, is the picture zero point in the upper left corner of the pixel(0,0) or in the center of the pixel(0,0)
the coordinate (0,0) is the center of the top left pixel.
conceptually, the warp matrix is applied to every source point and maps it to a destination point.
practically, the warp matrix is inverted and then every destination point is warped backwards to a source point, for interpolation.
relevant to cv::resize's behavior: the image is considered to start at the top left corner of the top left pixel, and it extends to the bottom right corner of the bottom right pixel.