Microsoft KB Archive/324788

= Determine the JPEG quality factor by using Visual Basic .NET =

Article ID: 324788

Article Last Modified on 5/10/2006

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APPLIES TO


 * Microsoft .NET Framework Software Development Kit 1.0 Service Pack 2
 * Microsoft Windows XP Professional
 * Microsoft Windows XP Professional for Itanium-based systems

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This article was previously published under Q324788



For a Microsoft Visual C# .NET version of this article, see 324790.

This article refers to the following Microsoft .NET Framework Class Library namespace:
 * System.Drawing.Imaging



SUMMARY
Compression in a Joint Photographic Experts Group (JPEG) file is controlled by a set of quantization tables. Typically, there are two such tables in each file: one for the luminance (brightness) information and the other for the chrominance (color) information. These tables are 8x8 matrices that determine how the 8x8 blocks of discrete cosine coefficients are quantized.

Most applications allow you to specify a number, called the JPEG quality factor, to control the compression level. This number typically has a range from 0 or 1, which results in more compression and a smaller file, to 100, which results in almost no compression and a larger file. However, this number is not stored in the file.

The JPEG quality factor is used to generate a pair of quantization tables. Many applications that are based on the Independent JPEG Group (IJG) code generate the same quantization tables as the IJG reference code. However, some applications may use custom quantization tables instead.

For more information about how the quality factor is used to influence the quantization tables, see the JPEG specification and the IJG reference code, both of which are available through the &quot;References&quot; section of this article.



Set the JPEG Quality Factor
The JPEG encoder in System.Drawing gives an encoder parameter for setting the JPEG quality factor when you save a JPEG file. This encoder parameter complies with the convention of using an integer with a range of 1 to 100. The value of this encoder parameter influences the creation and use of quantization tables related to the suggested quality factor.

The following code sample shows how this encoder parameter is used to save a JPEG file with a specified quality factor: Private Function GetEncoderInfo(ByVal mimeType As String) As ImageCodecInfo Dim j As Integer Dim encoders As ImageCodecInfo encoders = ImageCodecInfo.GetImageEncoders For j = 0 To encoders.Length If encoders(j).MimeType = mimeType Then Return encoders(j) End If       Next j        Return Nothing End Function

Private Sub SaveJPGWithCompressionSetting(ByVal image As Image, ByVal szFileName As String, ByVal lCompression As Long) Dim eps As EncoderParameters = New EncoderParameters(1) eps.Param(0) = New EncoderParameter(Encoder.Quality, lCompression) Dim ici As ImageCodecInfo = GetEncoderInfo(&quot;image/jpeg&quot;) image.Save(szFileName, ici, eps) End Sub

Retrieve the Quality Factor from a JPEG File
The quality factor is not stored directly in the JPEG file, so you cannot read the quality factor from the file. However, you can read the quantization tables from the JPEG file by using the PropertyItems property on the Image class. But even with the quantization tables, you cannot always determine a quality factor.

You might be able to determine the quality factor by comparing the quantization tables against the &quot;standard&quot; IJG-generated tables. However, because some applications may use custom tables, you will not always find a match. For more information about the quantization tables, see the &quot;References&quot; section.

