copy small part of mat to another

edit

Check Mat doc for details

BTW, you have to be sure that there is enough space(size and data type) in destination.

Check for data type

if (srcMat.type() != dstMat.type())
    return;

for a single row you can use assignment like below (this dstMat.row(5) = srcMat.row(1) will not work)

if (srcMat.cols == dstMat.cols)
    srcMat.row(1).copyTo(dstMat.row(5)); //copy the 2nd row to the 6th row

In general you can use ROI like here

Rect srcRect(Point(0, 1), Size(srcMat.cols, 1)); //select the 2nd row
Rect dstRect(Point(3, 5), srcRect.size() ); //destination in (3,5), size same as srcRect

dstRect = dstRect & Rect(Point(0, 0), dstMat.size()); //intersection to avoid out of range
srcRect = Rect(srcRect.tl(), dstRect.size()); //adjust source size same as safe destination
srcMat(srcRect).copyTo(dstMat(dstRect)); //copy from (0,1) to (3,5) max allowed cols

As alternative you can use rowRange

if (srcMat.cols == dstMat.cols)
    srcMat.rowRange(1, 2).copyTo(dstMat.rowRange(5, 6)); //copy row: 2nd->6th

Or more general range

Range colRange = Range(0, min(srcMat.cols, dstMat.cols)); //select maximum allowed cols
//copy the 2nd row to the 6th row  max allowed cols
srcMat(Range(1, 2), colRange).copyTo(dstMat(Range(5, 6), colRange));