Advanced MFC Programming

Chapter 8. DC, Pen, Brush and Palette
Chapter 8
DC, Pen, Brush and Palette
8.0 Device Context & GDI Objects
Starting from this chapter, we are going to study topics on GDI (Graphics Device Interface)
programming.
Situation
GDI is a standard interface between the programmer and physical devices. It provides many functions
that can be used to output various objects to the hardware (e.g. a display or a printer). GDI is very
important because, as a programmer, we may want our applications to be compatible with as many
peripherals as possible. For example, almost every application need to write to display, and many
applications also support printer output. The problem here is that since a program should be able to run on
different types of devices (low resolution displays, high resolution displays with different color depth, etc.),
it is impossible to let the programmer know the details of every device and write code to support it
beforehand.
The solution is to introduce GDI between the hardware and the programmer. Because it is a standard
interface, the programmer doesn’t have to have any knowledge on the hardware in order to operate it. As
long as the hardware supports standard GDI, the application should be able to execute correctly.
Device Context
As a programmer, we do not output directly to hardware such as display or printer. Instead, we output
to an object that will further realize our intention. This object is called device context (DC), it is a
Windows object that contains the detailed information about hardware. When we call a standard GDI
function, the DC implements it according to hardware attributes and configuration.
Suppose we want to put a pixel at specific logical coordinates on the display. If we do not have GDI,
we need the following information of the display in order to implement this simple operation:
1) Video memory configuration. We need this information in order to convert logical coordinates to
physical buffer address.
1) Device type. If the device is a palette device, we need to convert a RGB combination to an index to the
color table and use it to specify a color. If the device is a non-palette device, we can use the RGB
combination directly to specify a color.
Because the actual devices are different form one type to another, it is impossible for us to gather
enough information to support all the devices in the world. So instead of handling it by the programmer,
GDI functions let us use logical coordinates and RGB color directly, the conversion will be implemented
by the device driver.
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