Depending on your source data (which you havent mentioned) you can only output the lines you want.
inc cut = (720 - 640) / 2;
for(int y = cut; y < 640; y++ ) {
for(int x = 0; x 480; x++ ) {
draxPixel(x,y);
}
}
But that could be slow. Better is to use this logic with some of your internal libraries like openGL.
A
HACK could be to make the output control bigger, and plac some dummy controls over the unwanted parts. ;-)
Tip: if you dont want to display the pixels, dont copy that data around. Make that cut in or after the decoding.