Introduction to Bitmaps
A bitmap is a representation of a picture or another type of graphic on a window. For example, and as mentioned in the previous lesson, it can be used to show a regular picture on a form. Here is an example:
A bitmap can be used as a background for a window or a web page. A bitmap can also be used for an aesthetic purpose to decorate a dialog box. That’s how it is used on some dialog boxes. Another regular use of bitmaps is as small graphics on toolbars:
To support bitmaps, the GDI+ library provides the Bitmap class. In the .NET Framework, the bitmap is represented by a class called Bitmap. The Bitmap class is derived from the Image abstract class. Both classes are defined in the System.Drawing namespace of the System.Drawing.dll assembly. The Bitmap class is serializable.
There are two primary ways you can get a bitmap to use in your application: you can use an already created bitmap or you can design your own. To use an existing bitmap, you can open it as a file. To support this, the Bitmap class provides the following constructor:
Public Sub New(filename As String)
This constructor takes as argument the name of the file or the path to it. Here is an example of using it:
Imports System.Drawing Imports System.Windows.Forms Module Exercise Public Class Starter Inherits Form Friend WithEvents btnPicture As Button Dim components As System.ComponentModel.Container Public Sub New() InitializeComponent() End Sub Public Sub InitializeComponent() btnPicture = New Button btnPicture.Location = New Point(10, 10) btnPicture.Text = "Picture" Controls.Add(btnPicture) End Sub Private Sub btnPictureClicked(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles btnPicture.Click Dim bmpPicture As Bitmap bmpPicture = New Bitmap("woman.jpg") End Sub End Class Function Main() As Integer Dim frmStart As Starter = New Starter Application.Run(frmStart) Return 0 End Function End Module
Besides this constructor, Image, the parent of of the Bitmap class, provides the FromFile() method to its children. This method is overloaded with two versions. The first version uses the following syntax:
Public Shared Function FromFile(filename As String) As Image
As you can see, this is a static method that you call without instantiating the class. The method takes as argument the name of, or the path to, the file. It returns an Image object that you can then cast to a Bitmap. Here is an example:
Private Sub btnPictureClicked(ByVal sender As Object, _ ByVal e As EventArgs) _ Handles btnPicture.Click Dim bmpPicture As Bitmap bmpPicture = CType(Image.FromFile("vehicle.png"), Bitmap) End Sub
In our introduction to graphics, we saw various ways of getting a getting a Graphics object, either from the CreateGraphics() method of a Control-derived class or from the handle of the control. Besides these techniques, the Graphics class provides a method named FromImage. Its syntax is:
Public Shared Function FromImage(image As Image) As Graphics
This shared method takes as argument a variable of type Image. Therefore, when calling this method, pass an Image or an Image-based variable to it. After the method has been called, it produces a Graphics object.
In your application, you may also want the user to be able to open a picture as file. To assist you with this, you can use the Open File dialog box. As a reminder, to allow the user to open a picture in your application, add an OpenFileDialog control to your form or create one programmatically.
There are different types of graphic files with various extensions. The primary type of bitmap you will use in Microsoft Windows has the extension .bmp. There are many more graphic file extensions that Microsoft Windows supports. In our lessons, we cannot review all of them, their differences, advantages, or disadvantages. Simply know that most or all of the types of graphics you can think of are supported.
When providing an Open File dialog box to your application, if you will support various types of graphics, construct a filter accordingly. Here is an example from Microsoft Paint:
As you can see, the bitmap file is on top of the list.
As opposed to opening an existing picture, you can create your own, using the various classes and accessories of the GDI+ library. You can design a picture inside of Microsoft Visual Basic studio or you can use an external application.
To create a bitmap in Microsoft Visual Basic, on the main menu, you can click Project -> Add New Item... In the Templates section of the Add New Item dialog box, click Bitmap File, accept the suggested name or change the Name, and click Add. You would be presented with a window you can use. You either design a bitmap or you can paste an image from the clipboard. Here is an example:
There are many other more sophisticated applications used to create and manipulate graphics. As long as you can create an save a valid picture, you can use that picture in your application.
Once the picture is ready, to present it to the user, for example to display it in your application, you can call the Graphics.DrawImage() method that is overloaded with as many different versions as you can possibly need. One of the versions of this method has the following syntax:
Public Sub DrawImage(image As Image, x As Integer, y As Integer)
The first argument can be a bitmap that you may have previously initialized. The second argument specifies the location where the picture will be drawn.
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