Reading GDI+ Metadata of Images in VB.NET
In this article I will explain about reading Metadata of Images in GDI+.
If you have ever worked with mechanical and
engineering
drawing or digital images, you are probably aware of metadata. Metadata is information about the image, that's not part of the image itself. When an engineer draws an image, metadata is often added, such as the following information: last updated, updated by, data, place, and names. A photograph might include metadata such as image title, manufacturer, and model.
In the .NET Framework library, the PropertyItem object is used as a placeholder for metadata. The PropertyItem class provides four properties: Id, Len, Type, and Value. All of these properties have both read and write access.
The Id property is a tag, which identifies the metadata item. Table 9.10 describes Id tag values.
The Value property is an array of values whose format is determined by the Type property. The Len property represents the length of the array of values in bytes. The Type property represents the data type of values stored in the array. Table 9.11 described the format of the Type property values.
TABLE 9.10: Id values
|
Hexadecimal Value
|
Description
|
|
0x0320
|
Image title
|
|
0x010F
|
Equipment manufacturer
|
|
0x0110
|
Equipment model
|
|
0x9003
|
ExifDTOriginal
|
|
0x829A
|
EXIF exposure time
|
|
0x5090
|
Luminance table
|
|
0x5091
|
Chrominance table
|
TABLE 9.11: Format of Type property values
|
Numeric Value
|
Description
|
|
1
|
A Byte object
|
|
2
|
An array of Byte object encoded as ASCII
|
|
3
|
A 16-bit integer
|
|
4
|
A 32-bit integer
|
|
5
|
An array of two Byte objects that represent a rational number
|
|
6
|
Not used
|
|
7
|
Undefined
|
|
8
|
Not used
|
|
9
|
Slong
|
|
10
|
Srational
|
An Image object may contain more than one PropertyItem object. The PropertyItems property of the Image class represents an array of PropertyItem objects corresponding to an image. The PropertyIdList property of the Image class returns an array of property IDs stored in an image object. Listing 9.17 uses the PropertyItems property of the Image class and reads all property items of an image.
LISTING 9.17: Reading the metadata of a bitmap
Private Sub Form1_Load(ByVal sender As Object,ByVal e As System.EventArgs)
' Create an image from a file
Dim gAs Graphics = Me.CreateGraphics()
Dim curImageAs Image = Image.FromFile("C:/Documents and Settings/Manu/Desktop/16864z.Bmp")
Dim rect As New Rectangle(20, 20, 100, 100)
g.DrawImage(curImage, rect)
' Create an array of PropertyItem objects and read items using PropertyItems
Dim propItemsAs PropertyItem() = curImage.PropertyItems
' Create values of PropertyItem members
For Each propItem As PropertyItem In propItems
Dim encoderAs New System.Text.ASCIIEncoding()
Dim str As String ="ID =" + propItem.Id.ToString("x")
str += ", Type =" + propItem.Type.ToString()
str += ", Length =" + propItem.Len.ToString()
str += ", Value =" + encoder.GetString(propItem.Value)
MessageBox.Show(str)
Next
' Dispose of object
g.Dispose()
End Sub
Figure 9.25 shows the output from Listing 9.17.
FIGURE 9.25: Reading the metadata of a bitmap
Conclusion
Hope the article would have helped you in understanding reading Metadata of Images in GDI+. Read other articles on GDI+ on the website.