Raster operation codes define how GDI combines colors in output operations that involve a current brush, a possible source bitmap, and a destination bitmap.
光栅操作模式说明了GDI是如何组合输出操作中的颜色，包括当前画刷、可能的源位图和目标位图。
When a memory device context is created, GDI automatically selects a 1-by-1 monochrome stock bitmap for it.
当内存设备环境创建后，GDI会自动为它装载一个黑白位图。
GDI output functions can be used with a memory device context only if a bitmap has been created and selected into that context.
GDI的输出函数只有当内存设备环境上有了位图画布之后，才能进行输出。
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