insert a frame to mat , instead of coloring the whole Mat

edit

I know that to retrieve a ROI from the image you would usually do something like

Mat image(Rect(x,y,sx,sy))

And to color at the rectangle you would just

Mat image(Rect(x,y,sx,sy)) = 255

But if only want to create a frame with that pixelvalue such that some of the center elements isn't touched.. what shall i do?