Understanding the SetStyle Method in GDI+

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This article has been excerpted from book "Graphics Programming with GDI+".

Windows Forms and controls provide built-in support for double buffering, and the SetStyle method of the Control class plays a vital role in this process. Before we discuss how to use SetStyle, let's take a look at this method and its members.

The SetStyle method is defined in System.Windows.Forms.Controller, which sets the specified style of a control. This method takes two arguments. The first argument is of type ControlStyle enumeration, and it represents the style of the control. The second argument is true if we want to apply the specified style, false otherwise. The members of the ControlStyle enumeration are described in Table 13.1.

TABLE 13.1: ControlStyle members

Member	Description
AllPaintingInWmPaint	The WM_ERASEBKGND window message is sent to the message queue whenever a control needs to redraw its background. This method tells Windows to ignore the message, reducing clicker. Both OnPaint and OnPaintBackGround are called from the window message WM_Paint. AllPaintingInWmPaint should be used only if UserPaint is set to true.
CacheText	Applications can cache text using this option. The control keeps a copy of the text rather than getting it from the handle each time it is needed. This style default to false.
ContainerControl DoubleBuffer	The control is a container. This method provides built-in support for double buffering. When it is set to true, drawing is performed in a buffer and displayed only when complete. When using this option, you must also set the UserPaint and AllPaintingInWmPaint bits to true.
EnableNotifyMessage	If true, the OnNotifyMessage method is called for every message sent to the control's WindProc method. This style defaults to false.
FixedHeight	The control has a fixed height.
FixedWidth	The control has a fixed width.
Opaque	The control is drawn opaque, and the background is not painted.
ResizeRedraw	The control is redrawn when it is resized.
Selectable	The control can receive focus
StandardClick	The control implements standard click behavior.
StandardDoubleClick	The control implements standard double-click behavior. When using this option, you must also set StandardClick to true.
SupportsTransparentBackColor	The control accepts a Color object with alpha transparency for the background color. The UserPaint bit must be set to true, and the control must be derived from the Control class, like this: this.SetStyle(ControlStyle.UserPaint, true);
UserMouse	The control does its own mouse processing, and mouse events are not handled by the operating system.
UserPaint	The control paints itself rather than having the operating system do it. This option applies to classes derived from Control.

Let'sapply the SetStyle method to achieve double buffering. Double buffering can be enabled programmatically with the following code:

//Activate double buffering
this.SetStyle(ControlStyles.USerPaint, true);
this.SetStyle(ControlStyles.AllPaintingInWmPaint, true);
this.SetStyle(ControlStyles.DoubleBuffer, true);

We can also control the redrawing of controls when a control is resized. Setting ControlStyle.ResizeRedraw to true, as in the code snippet that follows, forces controls to be redrawn every time a control (or a form is resized.

SetStyle(ControlStyles.ResizeRedraw, true);

Sometimes we will not want a control to be redrawn when it is resized. In this can we can set ResizeRedraw to false.

NOTE

Many controls, such as PictureBox, are double-buffered automatically, which means we don't need to write any additional code when viewing images in a PictureBox control.

The Quality and Performance of Drawing

Drawing performance is inversely proportional to drawing quality. GDI+ provides several ways to set the quality of images and text. The SmoothingMode and TextRenderingHint properties are used to set image and text quality, respectively. The HighQuality and AntiAlias options provide slow drawing performance and better quality; the HighSpeed and None options provide poor quality and fast performance. Before using these options, we must decide if we really want to draw anti-aliased objects.

Sometimes anti-aliasing won't affect the quality of a drawing, and it is bad programming practice to use this processor-intensive feature when it is not required. In other cases we might need to set anti-aliasing for just one object out of 50. In these cases it is better to set the anti-alias option for that object only, instead of the entire canvas.

Section 13.4.1 through 13.4.6 describe some more tips and tricks that may help improve an application's performance.

Repaint Only the Required Area

Avoiding unwanted repainting is a good technique to increase painting performance. GDI+ provides many techniques for painting only required objects. Using regions and clipping rectangle may help in some cases. If you need to draw a single object with anti-aliasing on, just set anti-aliasing for that object instead of for the entire surface (form). Using regions is one of the best techniques for repainting only a required area. For better performance, you should know what area you need to redraw and invalidate only that area, thereby using regions instead of repainting the entire form.