Hi Frank,
Double buffering is a technique used to make drawing faster and appear
smoother by reducing flicker. The basic idea is to take the drawing
operations used to paint your control and apply them to an off-screen
buffer. Once all of the drawing operations have finished, this buffer is
drawn as a single image onto the control. This usually reduces flicker and
makes the application seem faster.
This behavior can be achieved in the .NET Framework 2.0 by setting a
control style to OptimizedDoubleBuffer, which is equivalent to setting the
DoubleBuffer and UserPaint styles in previous versions of the Framework.
To fully enable double buffering, you must also set AllPaintingInWmPaint to
true. When it is set, the window message WM_ERASEBKGRND is ignored, and
both OnPaintBackground and OnPaint methods are called directly from the
WM_PAINT message. If you set DoubleBuffering and AllPaintingInWmPaint to
true, then OnPaintBackground and OnPaint will be called with the same
buffered graphics object and everything will paint off-screen together and
update all at once.
To benefit from double buffering you have to set both these styles to true
or set the DoubleBuffered property on your control, like this:
SetStyle(ControlStyles.OptimizedDoubleBuffer |
ControlStyles.AllPaintingInWmPaint, true);
// OR
DoubleBufferred = true; // sets both flags
I am not quite sure why it's needed (particularly when DoubleBuffered
property is set to true). Does it help? Does it hurt?
If you have set the DoubleBuffered property to true, you needn't set the
AllPaintingInWmPaint and OptimizedDoubleBuffer styles.
It is important to mention here that setting the DoubleBuffered property is
not always the optimal solution to painting problems. This property should
be used with care, depending on your application and targeted scenarios.
The main drawback of setting DoubleBuffered is that it results in a
significant amount of memory being allocated for painting.
In addition, The quick and dirty way to avoid most flicker is to use a
transparency key. Pick a color that won't affect your program such as lime
green, or some other ugly color that is not used by anything on your form.
The reason a form will sometimes flicker, often has to do with a bug in
WinForms and how it deals with transparent windows. WinForms is in fact
just a wrapper to the Windows API. When dealing with opacity, WinForms will
use a Windows API to convert your form to what is known as a layered window.
"Using a layered window can significantly improve performance and visual
effects for a window that has a complex shape, animates its shape, or
wishes to use alpha blending effects." ¨C MSDN
However, WinForms does not make a window a layered window until it feels it
is necessary. The flicker you are seeing is WinForms trying to convert a
non layered window into a layered one.
The reason why setting a transparency key works is because WinForms will
force any window with a transparency key to be of the type layered. Set the
transparency key at design time, and at runtime, the window will always be
a layered window. With no need to convert, the flicker goes away.
Hope this helps.
Sincerely,
Linda Liu
Microsoft Online Community Support
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