Reading Metadata of Images

If you have ever worked with mechanical and engineering drawings 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, date, 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 describes 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 objects encoded as ASCII
3	A 16-bit integer
4	A 32-bitinteger
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 void Form1_Load(object sender,
 System.EventArgs e)
{
 // Create an image from a file
 Graphics g = this.CreateGraphics();
 Image curImage = Image.FromFile("roses.jpg");
 Rectangle rect = new Rectangle(20, 20, 100, 100);
 g.DrawImage(curImage, rect);
 // Create an array of PropertyItem objects and read
 // items using PropertyItems
 PropertyItem[] propItems = curImage.PropertyItems;
 // Create values of PropertyItem members
 foreach (PropertyItem propItem in propItems)
 {
 System.Text.ASCIIEncoding encoder =
 new System.Text.ASCIIEncoding();
 string str = "ID ="+propItem.Id.ToString("x");
 str += ", Type ="+ propItem.Type.ToString();
 str += ", Length = "+ propItem.Len.ToString();
 str += ", Value ="
 + encoder.GetString(propItem.Value);
 MessageBox.Show(str);
 }
 // Dispose of object
 g.Dispose();
}

Figure 9.25 shows the output from Listing 9.17.

Figure 9.25. Reading the metadata of a bitmap