The Graphical Device Interface

The Device Context

A device context is an ensemble of the tools needed to draw lines, shapes, and other graphics. It includes the platform you draw on, the dimensioning of the platform, the orientation and other variations of your drawing, the tools you need to draw on the platform, the colors, and various other accessories that can complete your imagination. To provide support for drawing on the Windows operating system, Microsoft created the Graphical Device Interface, abbreviated as GDI. It is a set of classes, functions, variables, and constants that group all or most of everything you need to draw on an application. The GDI is provided as a library called Gdi.dll and is already installed on your computer.

In a Win32 application, in order to draw, you must create a device context. This can be taken care of by declaring a variable of type HDC. To keep track of the various drawings, the device context uses a coordinate system that has its origin (0, 0) on the top-left corner of the desktop:

Anything that is positioned on the screen is based on this origin. This coordinate system can get the location of an object using a horizontal and a vertical measurement. The horizontal measures are based on an x axis that moves from the origin to the right direction. The vertical measures use a y axis that moves from the origin to the bottom direction:

This means that, if you start drawing something such as a line, it would start on the origin and continue where you want it to stop. To significantly simplify drawing and make it compatible with the VCL, Borland created a class called TCanvas. TCanvas is based on TPersistent, which is derived from TObject. To further make it easy to draw, every control that needs drawing already has a TCanvas variable available. This means that you usually will not have to declare a TCanvas variable before drawing.

In a VCL application, a canvas is the object on which you draw but the TCanvas class actually encompasses everything that can be used to draw. This includes the platform and the tools. Because TCanvas does not implement everything that is possibly available on Win32 drawing, this class provides a handle that allows TCanvas to use an HDC variable to.