About Device Contexts
One of the chief features of the Microsoft® Win32® application programming interface (API) is device independence. Win32-based
applications can draw and print output on a variety of devices. The software
that supports this device independence is contained in two dynamic-link libraries.
The first, GDI.DLL, is referred to as the graphics device interface (GDI); the
second is referred to as a device driver. The name of the second depends on
the device where the application draws output. For example, if the application
draws output in the client area of its window on a VGA display, this library is
VGA.DLL; if the application prints output on an Epson® FX-80 printer, this library is EPSON9.DLL.
An application must inform GDI to load a particular device driver and, once
the driver is loaded, to prepare the device for drawing operations (such as
selecting a line color and width, a brush pattern and color, a font typeface, a
clipping region, and so on). These tasks are accomplished by creating and
maintaining a device context. A
device context is a structure that defines a set of graphic objects and their associated
attributes, and the graphic modes that affect output. The
graphic objects include a pen for line drawing, a brush for painting and filling, a bitmap
for copying or scrolling parts of the screen, a palette for defining the set of
available colors, a region for clipping and other operations, and a path for
painting and drawing operations. Unlike most of the Win32 structures, an
application never has direct access to the device context; instead, it operates on the
structure indirectly by calling various functions.
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