overlay smaller image over bigger image in a particular area

edit

if smallImg and bigImg have the same format, and you're sure it 'fits' at the given position, try:

 smallImg.copyTo( bigImg.submat( y, smallImg.rows(), x, smallImg.cols() ) );

copyTo is the correct approach, however be mindful that the function will fail without warning in many cases. A common pitfall is if you try to copy an RGBa image into GBR or vice versa

var selectedArea = RGBaImgToDisplay.submat(resultRect)
sampleImageColor.copyTo(selectedArea)

will not work, unless you first make sure that sampleImageColor is also an RGBa like so:

cvtColor(bgrImage, sampleImageColor, Imgproc.COLOR_BGR2RGBA);

Then it works as expected