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CDC Attributes

A device context object has many attributes that can be set or retrieved through CDC member functions. Of these, we review attributes that relate to mapping modes and coordinate transformations in the next section. This section focuses on other attributes.

The background color, used to fill the gaps in styled lines, hatched brushes, and in character cells, is set by the SetBkColor member function. The current background color can be retrieved by calling GetBkColor. The background mode, which determines whether the background is transparent or opaque, is set by calling SetBkMode; GetBkMode retrieves the current background mode.

The SetROP2 member function can be used to set the drawing mode. The drawing mode determines how bits in the drawing tool and bits on the device surface are combined. The default drawing mode is R2_COPYPEN; in this mode, pixels from the drawing tool are copied over pixels in the device bitmap. This is what you would expect as normal behavior; as you draw with a specific pen or brush, the pen or brush will simply overwrite what may already be on the device context surface.

There are several other commonly used drawing modes that can be set with SetROP2. These include, for example, R2_BLACK (the target pixels turn always black), R2_NOTCOPYPEN (the target pixel acquires a color that is the inverse of the drawing tool's color), or R2_XORPEN (the target pixel's color is formed by performing an exclusive OR operation between the target pixel and the pixel in the drawing tool).

Drawing modes are not restricted to these preset values; the drawing mode setting can specify an arbitrary binary operation between pixels of the device surface and pixels of the drawing tool.

The current drawing mode setting can be acquired by calling GetROP2. Note that drawing mode settings are specific to raster devices and have no effect on vector devices, like plotters.

The SetPolyFillMode function determines the polygon filling mode. The difference between the ALTERNATE and WINDING filling modes is illustrated in Figure 2. The current filling mode can be retrieved by calling GetPolyFillMode.

MFC Example