int SetROP2(
HDC hdc, // handle to DC
int fnDrawMode // drawing mode
);
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。