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：

1、R2_BLACK//Pixel is always black. 所有绘制出来的像素为黑色

2、R2_WHITE//Pixel is always white. 所有绘制出来的像素为白色

3、R2_NOP//Pixel remains unchanged. 任何绘制将不改变当前的状态

4、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）

5、R2_COPYPEN//Pixel is the pen color. 使用当前的画笔的颜色

6、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。

7、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_COPYPEN和R2_NOT的并集

8、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_COPYPEN和R2_NOT的交集

9、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_NOTCOPYPEN和屏幕像素值的并集

10、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_NOTCOPYPEN和屏幕像素值的交集

11、R2_MERGEPEN//Pixel is a combination of the pen color and the screen color (final pixel = pen OR screen pixel). R2_COPYPEN和屏幕像素值的并集

12、R2_NOTMERGEPEN//Pixel is the inverse of the R2_MERGEPEN color (final pixel = NOT(pen OR screen pixel)). R2_MERGEPEN的反色

13、R2_MASKPEN//Pixel is a combination of the colors common to both the pen and the screen (final pixel = pen AND screen pixel). R2_COPYPEN和屏幕像素值的交集

14、R2_NOTMASKPEN//Pixel is the inverse of the R2_MASKPEN color (final pixel = NOT(pen AND screen pixel)). R2_MASKPEN的反色

15、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_COPYPEN和屏幕像素值的异或

16、R2_NOTXORPEN//Pixel is the inverse of the R2_XORPEN color (final pixel = NOT(pen XOR screen pixel)). R2_XORPEN的反色

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。