一个Windows API SetROP2(int nDrawMode)的使用
该函数的主要的作用是根据nDrawMode设置的方式重新设定绘图的方式，下面就不同的nDrawMode值具体解释绘图模式是如何改变的。
首先就nDrawMode的取值有以下的情况：
R2_BLACK Pixel is always black. //所有绘制出来的像素为黑色
R2_WHITE Pixel is always white. //所有绘制出来的像素为白色
R2_NOP Pixel remains unchanged. //任何绘制将不改变当前的状态
R2_NOT Pixel is the inverse of the screen color. //当前绘制的像素值设为屏幕像素值的反，这样可以覆盖掉上次的绘图，（自动擦除上次绘制的图形）
R2_COPYPEN Pixel is the pen color. //使用当前的画笔的颜色
R2_NOTCOPYPEN Pixel is the inverse of the pen color. //当前画笔的反色
//下面是当前画笔的颜色和屏幕色的组合运算得到的绘图模式。
R2_MERGEPENNOT Pixel is a combination of the pen color and the inverse of the screen color (final pixel = (NOT screen pixel) OR pen).
R2_MASKPENNOT Pixel is a combination of the colors common to both the pen and the inverse of the screen (final pixel = (NOT screen pixel) AND pen).
R2_MERGENOTPEN Pixel is a combination of the screen color and the inverse of the pen color (final pixel = (NOT pen) OR screen pixel).
R2_MASKNOTPEN Pixel is a combination of the colors common to both the screen and the inverse of the pen (final pixel = (NOT pen) AND screen pixel).
R2_MERGEPEN Pixel is a combination of the pen color and the screen color (final pixel = pen OR screen pixel).
R2_NOTMERGEPEN Pixel is the inverse of the R2_MERGEPEN color (final pixel = NOT(pen OR screen pixel)).
R2_MASKPEN Pixel is a combination of the colors common to both the pen and the screen (final pixel = pen AND screen pixel).
R2_NOTMASKPEN Pixel is the inverse of the R2_MASKPEN color (final pixel = NOT(pen AND screen pixel)).
R2_XORPEN Pixel is a combination of the colors that are in the pen or in the screen, but not in both (final pixel = pen XOR screen pixel).
R2_NOTXORPEN Pixel is the inverse of the R2_XORPEN color (final pixel = NOT(pen XOR screen pixel)).
总之，上述api的一个作用是在需要改变绘图的模式时，不需要重新设置画笔，只需要设置不同的绘图的模式即可达到相应的目的。
程序橡皮筋绘图程序的使用实例：
void CXXXView::OnMouseMove(UINT nFlags, CPoint point)
{
// 按下左键移动开始画图
if (nFlags == MK_LBUTTON)
{
// 创建画笔RGB(0x00, 0x00, 0xFF)
HPEN hPen = CreatePen(PS_SOLID, m_PenWidth, m_PenColor);
// 选进DC
::SelectObject(m_hMemDC, hPen);
//设置系统色彩模式取反色
int oldRop=::SetROP2(m_hMemDC,R2_NOTXORPEN);
// 画线
::MoveToEx(m_hMemDC,m_pOrigin.x,m_pOrigin.y, NULL);
::LineTo(m_hMemDC, m_pPrev.x,m_pPrev.y);
//::SetROP2(m_hMemDC,oldRop);//回复系统默认色彩模式
::MoveToEx(m_hMemDC, m_pOrigin.x, m_pOrigin.y, NULL);
::LineTo(m_hMemDC, point.x, point.y);
m_pPrev = point;
Invalidate(FALSE);
}
}