Overview of GDI

Overview of GDI+

Before .NET, Windows programmers depended on the GDI system to draw pretty much anything on the screen, even if they didn't know that GDI existed. In addition to bitmap images, all controls, labels, window borders, and icons appeared on the screen thanks to GDI. It was a giant step forward from character graphics. GDI presented a basic set of drawing features from which you could potentially output any type of complex image. But it wasn't easy. The graphic primitives werewellprimitive, and you had to build up complex systems from the parts. Most programmers weren't into making things beautiful, so they tried to avoid the complexities of GDI. But sometimes you had to draw a line or a circle, and there was no way around it.

GDI+, new with .NET, builds on GDI, providing the basic primitives of GDI, but also supplying some more complex groupings of graphic features into easy-to-use functions. The simplicity has brought about a Renaissance of programmer-initiated graphic work. Take a look at Figure 17-1, which shows an image drawn using the older GDI, and that same image generated with just a few quick commands in GDI+.

Figure 17-1. The marvel that is GDI+

The GDI+ system makes its home in the System.Drawing namespace, and includes multitudes of classes that represent the drawing objects, surfaces, and embellishment features that enable display graphics. But it's not just about display. GDI+ generalizes bitmap and vector drawing on all available output surfaces: bitmaps or line drawings on the screen (including form and control surfaces), report output on a printer, graffiti on the back wall of your local supermarket, image content destined for a JPEG filethey are all the same to GDI+. All destinations use the same drawing methods and objects, making it easier for you to generalize your drawing code.

GDI+'s features include surfaces, drawing inks, drawing elements, and transformations.

• GDI+ generalizes drawing surfaces through the System.Drawing.Graphics class. This object represents a drawing canvas, with attributes for color depth and size (width and height). The canvas may link to a region of the workstation screen, an internal holding area for final output to the printer, or a general graphics canvas for manipulating content in-memory before outputting it to a display or a file. Another type of surface, the path (System.Drawing.Drawing2D.GraphicsPath), is like a macro recorder for vector (line) graphics. Drawing done within a path can be "replayed" back on a standard drawing surface, or used to supply boundaries for other drawing commands.

• Colors and inks appear in the form of colors (opaque or semi-transparent color values), brushes (bitmap-based pseudo-pens used for fills and tiling), and pens (colored line-drawing objects with a specific thickness).

• Drawing elements include rectangles, ellipses, lines, and other standard or custom-edge shapes. They also include fonts, both bitmapped and outline-based versions.

• Transformations let you resize, rotate, and skew drawings as you generate them. When a transformation is applied to a surface, you can draw objects as if there were no transformation applied, and the changes will happen in real time.

The Windows Forms controls that you use in desktop applications generally take care of their own display features. However, some controls let you take over some or all of the drawing responsibilities. For instance, the ListBox control displays simple single-color text for each list item. However, you can override the drawing of each list item, providing your own custom content, which may include multi-color text or graphics. This ability to supply some of the drawing code to a control is known as owner draw, and it works through the same generalized Graphics object used for other drawing. We'll include some owner draw code in the Library Project.

In the interest of full disclosure, you should know that this chapter will cover probably only one percent of the available GDI+ features, if even that. GDI+ is complex and vast, and you could spend three years delving into every little feature, just in time for the next major release of GDI+ (it will be called Windows Presentation Foundation and was formerly known as Avalon). I'll give you a brief overview of the system so you get a feel for some of the basics. If you need to manipulate images and text beyond what is listed here (and you probably will), try the MSDN documentation or another resource dedicated to deciphering GDI+.