| Title | Save a picture into a JPEG at a desired size in Visual Basic 2005 |
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| Description | This example shows how to save a picture into a JPEG at a desired size in Visual Basic 2005. |
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| Keywords | save picture, save image, size, jpeg, jpg, compress, VB .NET |
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| Categories | Algorithms, VB.NET, Graphics |
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Function SaveJpegIntoStream saves an image into a JPEG encoding in a memory stream. It makes an EncoderParameters object and sets its first parameter to indicate that the file should be compressed to a given level. The function then calls GetEncoderInfo to get the encoder needed for saving JPEG files. It calls the image's Save method, passing it parameters to make it save into a memory stream using the JPEG encoder with the desired compression level.
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' Save a JPEG in compressed form into a memory stream.
' The compression_level value
' should be between 10 and 100.
Public Function SaveJpegIntoStream(ByVal image As Image, _
ByVal compression_level As Long) As MemoryStream
If compression_level < 10 Then
Throw New ArgumentException("Compression level must " & _
"be between 10 and 100")
End If
' Get an encoder parameter array and set the
' compression level.
Dim encoder_params As EncoderParameters = New _
EncoderParameters(1)
encoder_params.Param(0) = New _
EncoderParameter(Encoder.Quality, compression_level)
' Prepare the codec to encode.
Dim image_codec_info As ImageCodecInfo = _
GetEncoderInfo("image/jpeg")
' Save the file.
Dim memory_stream As New MemoryStream()
image.Save(memory_stream, image_codec_info, _
encoder_params)
Return memory_stream
End Function
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Function GetEncoderInfo returns an encoder. It loops through the available encoders and returns the one that handles the desired type, in this example, JPEG files.
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' Get a codec's information.
Private Function GetEncoderInfo(ByVal mimeType As String) _
As ImageCodecInfo
Dim encoders As ImageCodecInfo()
encoders = ImageCodecInfo.GetImageEncoders()
For i As Integer = 0 To encoders.Length
If encoders(i).MimeType = mimeType Then
Return encoders(i)
End If
Next i
Return Nothing
End Function
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The main program uses the following code to compress the image to a desired size. It loops the compression_level variable from 100 to 10, compressing the image into a memory stream for each compression_level value. When it finds a stream that is less than or equal to the desired size, it stops and saves the stream into a JPEG file.
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Dim file_size As Long
For compression_level As Integer = 100 To 10 Step -1
' Save the file into a memory stream.
Dim memory_stream As MemoryStream = _
SaveJpegIntoStream(picOriginal.Image, _
compression_level)
' See how big it is.
file_size = memory_stream.Length
If file_size <= desired_size Then
' Display the final size and image.
' Save the file.
My.Computer.FileSystem.WriteAllBytes(file_name, _
memory_stream.ToArray(), False)
picNew.Image = LoadImage(file_name)
txtCompressionLevel.Text = compression_level
txtCompressedSize.Text = FormatBytes(file_size)
Exit For
End If
Next compression_level
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Note: Depending on the type of image you are compressing, the file may have the size you want even at a compression level of 100, which is the minimum compression. For example, you may be able to squeeze a lot of space out of a bitmap file without a lower compression level. The result can look very close to the original file but take up a lot less space.
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