C / C++ / MFC

if (IntersectRect(&rcIntersected, &ps.rcPaint, &rcImgScrolled))
{
	float rx = (float)ImgWidth / (float)ZoomedImgWidth;
	float ry = (float)ImgHeight / (float)ZoomedImgHeight;

	int dl = rcIntersected.left - ps.rcPaint.left;
	int dt = rcIntersected.top - ps.rcPaint.top;
	int dw = RECTWIDTH(rcIntersected);
	int dh = RECTHEIGHT(rcIntersected);
	int sl = int(float(rcIntersected.left - rcImgScrolled.left)*rx);
	int st = int(float(rcIntersected.top - rcImgScrolled.top)*ry);
	int sw = int(float(RECTWIDTH(rcIntersected))*rx);
	int sh = int(float(RECTHEIGHT(rcIntersected))*ry);

	StretchBlt(hDCBuffer, 
		dl, dt, dw, dh,
		hDCCurImg, 
		sl, st, sw, sh,
		SRCCOPY);
}