OpenCV draw an image over another image

Use Mat::rowRange() and Mat::colRange() to specify the area to which you want to draw in the destination Mat. Code:

Mat src( 5,  7, CV_8UC1, Scalar(1)); // 5x7
Mat dst(10, 10, CV_8UC1, Scalar(0)); // 10x10

src.copyTo(dst.rowRange(1, 6).colRange(3, 10));

Results in the following:

before copyTo():

dst:
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )

after copyTo():

dst:
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 1 1 1 1 1 1 1 )
    ( 0 0 0 1 1 1 1 1 1 1 )
    ( 0 0 0 1 1 1 1 1 1 1 )
    ( 0 0 0 1 1 1 1 1 1 1 )
    ( 0 0 0 1 1 1 1 1 1 1 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )
    ( 0 0 0 0 0 0 0 0 0 0 )