multithreading - Parallelizing GDI+ Image Resizing .net

Question

I've tried to parallelize the resizing of jpegs using .Net. All my tries failed, because the Graphics.DrawImage-func seems to lock while active. Try the following snipped:

Sub Main()
    Dim files As String() = IO.Directory.GetFiles("D:TEMP")
    Dim imgs(25) As Image
    For i As Integer = 0 To 25
      imgs(i) = Image.FromFile(files(i))
    Next

    Console.WriteLine("Ready to proceed ")
    Console.ReadLine()

    pRuns = 1
    For i As Integer = 0 To 25
      Threading.Interlocked.Increment(pRuns)
      Threading.ThreadPool.QueueUserWorkItem(New Threading.WaitCallback(AddressOf LongTerm), imgs(i))
    Next
    Threading.Interlocked.Decrement(pRuns)

    pSema.WaitOne()
    Console.WriteLine("Fin")
    Console.ReadLine()
  End Sub

  Sub LongTerm(ByVal state As Object)
    Dim newImageHeight As Integer
    Dim oldImage As Image = CType(state, Image)
    Dim newImage As Image
    Dim graph As Graphics
    Dim rect As Rectangle
    Dim stream As New IO.MemoryStream

    Try
      newImageHeight = Convert.ToInt32(850 * oldImage.Height / oldImage.Width)
      newImage = New Bitmap(850, newImageHeight, oldImage.PixelFormat)
      graph = Graphics.FromImage(newImage)
      rect = New Rectangle(0, 0, 850, newImageHeight)

      With graph
        .CompositingQuality = Drawing2D.CompositingQuality.HighQuality
        .SmoothingMode = Drawing2D.SmoothingMode.HighQuality
        .InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
      End With

      'Save image to memory stream
      graph.DrawImage(oldImage, rect)
      newImage.Save(stream, Imaging.ImageFormat.Jpeg)
    Catch ex As Exception

    Finally
      If graph IsNot Nothing Then
        graph.Dispose()
      End If
      If newImage IsNot Nothing Then
        newImage.Dispose()
      End If
      oldImage.Dispose()
      stream.Dispose()

      Console.WriteLine("JobDone {0} {1}", pRuns, Threading.Thread.CurrentThread.ManagedThreadId)
      Threading.Interlocked.Decrement(pRuns)
      If pRuns = 0 Then
        pSema.Set()
      End If
    End Try

  End Sub

All threads wait at graph.DrawImage(). Is there a way to speed up code performance using other functions? Is it impossible to use Graphics.Draw with multiple threads? In the real application multiple images should be resized at the same time (on a quad-core pc), not always the same. The posted code is only for testing purposes...

Thanks in advance

Edit: Updated the code according to comments

Answer 1

Use Processes.

GDI+ blocks per process a lot of ways. Yep, a pain, but there's no way around it. Fortunately with tasks like this one (and any one that processes files on the filesystem), it's too easy to just split the workload up between multiple processes. Fortunately it looks like GDI+ is using locks, not mutex, so it is concurrent!

We have some graphics programs where I work to do image processing. One programmer starts 6-7 copies at once of a conversion program, in production. So it is not messy, trust me. Hack? You're not getting paid to look pretty. Get the job done!

Cheap Example (note this will NOT work in the ide, build it and run the EXE):

Imports System.Drawing
Module Module1
    Dim CPUs As Integer = Environment.ProcessorCount

    Dim pRuns As New System.Collections.Generic.List(Of Process)

    Sub Main()
        Dim ts As Date = Now
        Try
            If Environment.GetCommandLineArgs.Length > 1 Then
                LongTerm(Environment.GetCommandLineArgs(1))
                Exit Sub
            End If

            Dim i As Integer = 0
            Dim files As String() = IO.Directory.GetFiles("D:TEMP", "*.jpg")
            Dim MAX As Integer = Math.Min(26, files.Count)
            While pRuns.Count > 0 Or i < MAX

                System.Threading.Thread.Sleep(100)

                If pRuns.Count < CInt(CPUs * 1.5) And i < MAX Then ''// x2 = assume I/O has low CPU load
                    Console.WriteLine("Starting process pRuns.count = " & pRuns.Count & " for " & files(i) & " path " & _
                                        Environment.GetCommandLineArgs(0))
                    Dim p As Process = Process.Start(Environment.GetCommandLineArgs(0), """" & files(i) & """")
                    pRuns.Add(p)
                    i += 1
                End If

                Dim i2 As Integer
                i2 = 0
                While i2 < pRuns.Count
                    If pRuns(i2).HasExited Then
                        pRuns.RemoveAt(i2)
                    End If
                    i2 += 1
                End While


            End While
        Catch ex As Exception
            Console.WriteLine("Blew up." & ex.ToString)
        End Try
        Console.WriteLine("Done, press enter. " & Now.Subtract(ts).TotalMilliseconds)
        Console.ReadLine()
    End Sub

    
    Sub LongTerm(ByVal file As String)
        Try
            Dim newImageHeight As Integer
            Dim oldImage As Image
            Console.WriteLine("Reading " & CStr(file))
            oldImage = Image.FromFile(CStr(file))
            Dim rect As Rectangle

            newImageHeight = Convert.ToInt32(850 * oldImage.Height / oldImage.Width)
            Using newImage As New Bitmap(850, newImageHeight, oldImage.PixelFormat)
                Using graph As Graphics = Graphics.FromImage(newImage)
                    rect = New Rectangle(0, 0, 850, newImageHeight)

                    With graph
                        .CompositingQuality = Drawing2D.CompositingQuality.HighQuality
                        .SmoothingMode = Drawing2D.SmoothingMode.HighQuality
                        .InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
                    End With

                    Console.WriteLine("Converting " & CStr(file))
                    graph.DrawImage(oldImage, rect)

                    Console.WriteLine("Saving " & CStr(file))
                    newImage.Save("d:empResized" & _
                                  IO.Path.GetFileNameWithoutExtension(CStr(file)) & ".JPG", _
                                   System.Drawing.Imaging.ImageFormat.Jpeg)
                End Using
            End Using
        Catch ex As Exception
            Console.WriteLine("Blew up on  " & CStr(file) & vbCrLf & ex.ToString)
            Console.WriteLine("Press enter")
            Console.ReadLine()
        End Try
    End Sub

End Module