void ViewStart(int *x, int * y)
Get the position at which the visible portion of the canvas starts. If either of the scrollbars is not at the home position, x and/or y will be greater than zero. Combined with wxWindow::GetClientSize, the application can use this function to efficiently redraw only the visible portion of the canvas. The positions are in logical scroll units, not pixels, so to convert to pixels you will have to multiply by the number of pixels per scroll increment.
See also wxCanvas::SetScrollbars.