scrolling an offscreen bitmap

I am implementing a strip chart, a display that gradually scrolls itself horizontally.I find that I must use an offscreen bitmap, or else the display will scroll fragments of any window that overlies it. Now I'm looking for an operation to scroll an offscreen I am implementing a strip chart, a display that gradually scrolls itself horizontally.I find that I must use an offscreen bitmap, or else the display will scroll fragments of any window that overlies it. Now I'm looking for an operation to scroll an offscreen bitmap.

To scroll a window one uses CopyFromScreen. But this has the problem that it will copy material from an overlying window. So I try to scroll and bitmap using DrawImage to draw a part of my offscreen bitmap into a graphics context that refers to that bitmap itself. Nothing happens.

So what I want to know is, is there an operation that will allow me to, for example, copy the left half of a bitmap onto the right half?

Ken