Visual Basic.NET How to Program (PDF)

Chapter 16 Graphics and Multimedia 687
Calling the OnPaint method raises the Paint event. Instead of overriding the
OnPaint method, programmers can add an event handler for the Paint event. First,
write the code for the Paint event handler in this form:
Public Sub MyEventHandler_Paint( _
ByVal sender As Object, ByVal e As PaintEventArgs) _
Handles MyBase.Paint
Programmers seldom call the OnPaint method directly, because the drawing of
graphics is an event-driven process. An event—such as the covering, uncovering or resizing
of a window—calls the OnPaint method of that form. Similarly, when any control (such as
a TextBox or Label) is displayed, the program calls that control’s Paint method.
If programmers need to invoke method OnPaint explicitly, they can call the
Invalidate method (inherited from Control). This method refreshes a control’s
client area and repaints all graphical components. Visual Basic contains several overloaded
Invalidate methods that allow programmers to update portions of the client area.
Performance Tip 16.1
Calling the Invalidate method to refresh the Control often is inefficient. Instead, call
Invalidate with a Rectangle parameter to refresh only the area designated by the
rectangle. This improves program performance. 16.1
Controls, such as Labels and Buttons, also have their own graphics contexts. To
draw on a control, first obtain its graphics object by invoking the CreateGraphics
method:
Dim graphicsObject As Graphics = label1.CreateGraphics()
Then, you can use the methods provided in class Graphics to draw on the control.
16.3 Color Control
Colors can enhance a program’s appearance and help convey meaning. For example, a red
traffic light indicates stop, yellow indicates caution and green indicates go.
Structure Color defines methods and constants used to manipulate colors. Because it
is a lightweight object that performs only a handful of operations and stores Shared fields,
Color is implemented as a structure, rather than as a class.
Every color can be created from a combination of alpha, red, green and blue components.
Together, these components are called ARGB values. All four ARGB components
are Bytes that represent integer values in the range from 0 to 255. The alpha value determines
the intensity of the color. For example, the alpha value 0 results in a transparent
color, whereas the value 255 results in an opaque color. Alpha values between 0 and 255
result in a weighted blending effect of the color’s RGB value with that of any background
color, causing a semi-transparent effect. The first number in the RGB value defines the
amount of red in the color, the second defines the amount of green and the third defines the
amount of blue. The larger the value, the greater the amount of that particular color. Visual
Basic enables programmers to choose from almost 17 million colors. If a particular computer
cannot display all these colors, it will display the color closest to the one specified.
Figure 16.3 summarizes some predefined color constants, and Fig. 16.4 describes several
Color methods and properties.