How to generate thumbnails which preserve the aspect ratio

'                                                                                       
'   Name:                                                                               
'                                                                                       
'       ThumbnailBG                                                                     
'                                                                                       
'   Description:                                                                        
'                                                                                       
'       Demo program that shows how to generate thumbnails which preserve the aspect    
'       ratio of the original image as well as how to do it from a background thread.   
'                                                                                       
'   Notes:                                                                              
'                                                                                       
'       Thumbnail size is defined by THUMBW and THUMBH.                                 
'                                                                                       
'       The ListView and ImageList controls cannot be accessed directly by the back     
'       ground thread. That requires the use of the structure, FileList, to provide the 
'       names of the files, and the delegate, dlgUpdateThumbs, to handle the updating   
'       as thumbs are generated.                                                        
'                                                                                       
'       Generating the thumbnails requires creating a bitmap for each source file. If   
'       the space for these bitmaps is not quickly reclaimed it can quickly lead to an  
'       out-of0memory error. To guard against this, the background thread explicitly    
'       disposes of bitmaps as soon as they are no longer required, and forces the      
'       system to do a garbage collection after every tenth thumbnail is created.       
'                                                                                       
'   2014-01-07  rj  original code                                                       
'                                                                                       

Imports System.Drawing
Imports System.Drawing.Drawing2D

Public Class Form1

    Private Path As String = "D:\My Documents\My Pictures\Current"
    Private Filter() As String = {"*.jpg", "*.png", "*.gif"}

    Private FileList As New Dictionary(Of Integer, String)

    'When a ListView is in icon view, images for each item are displayed based  
    'on images in an ImageList. a ListView can reference separate ImageLists    
    'for both small and large icons. For this demo I use only LargeIcons        

    Private imgFiles As New ImageList

    Const THUMBW = 120      'thumbnail width    
    Const THUMBH = 90       'thumbnail height   

    Private Delegate Sub dlgUpdateThumbs(i As Integer, thumb As Bitmap)

    Private Sub Form1_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing

        'terminate the background worker thread

        bgwThumbs.CancelAsync()

    End Sub

    Private Sub Form1_Load(sender As Object, e As System.EventArgs) Handles Me.Load

        'set properties of image list

        imgFiles.ImageSize = New Size(THUMBW, THUMBH)
        imgFiles.ColorDepth = ColorDepth.Depth16Bit

        'load the file list then start the BG process to generate thumbs

        lvwFiles.Items.Clear()                          'clear out any old items        
        lvwFiles.View = View.Details                    'default view is Details        
        lvwFiles.Columns(0).Width = lvwFiles.Width      'size first column to full width
        lvwFiles.LargeImageList = imgFiles              'link to ImageList              

        Dim i As Integer = 0

        For Each file As String In My.Computer.FileSystem.GetFiles(Path, FileIO.SearchOption.SearchTopLevelOnly, Filter)
            lvwFiles.Items.Add(System.IO.Path.GetFileName(file))
            FileList.Add(i, file)
            i += 1
        Next

        'start the thumbnail generation

        bgwThumbs.WorkerSupportsCancellation = True
        bgwThumbs.RunWorkerAsync()

    End Sub

    Private Sub UpdateThumbs(i As Integer, thumb As Bitmap)

        'imgFiles and lvwFiles are updated with imformation coming from the background thread   
        'so we require a delegate. InvokeRequired will tell us if this sub is being called from 
        'the main thread or the background thread. The Try/Catch is required in case the user   
        'has decided to close the application. If this happens then Invoke will throw an error. 

        If lvwFiles.InvokeRequired Then
            'call was from the background thread - use the delegate
            Try
                Dim dlg As New dlgUpdateThumbs(AddressOf UpdateThumbs)
                Invoke(dlg, i, thumb)
            Catch
            End Try
        Else
            'call was from the foreground (main) thread
            imgFiles.Images.Add(thumb)
            lvwFiles.Items(i).ImageIndex = i
        End If

    End Sub

    Private Sub btnDetails_Click(sender As System.Object, e As System.EventArgs) Handles btnDetails.Click

        'Put the display into Details view. Because there is only one column, size it to the    
        'width of the control.                                                                  

        lvwFiles.View = View.Details
        lvwFiles.Columns(0).Width = lvwFiles.Width

    End Sub

    Private Sub btnThumbs_Click(sender As System.Object, e As System.EventArgs) Handles btnThumbs.Click

        'Puut the display into LargeIcon view. Thumbnails will appear on the display as they
        'are generated.                                                                     

        lvwFiles.View = View.LargeIcon

    End Sub

    'Everything from here down belongs to the background thread

    Private Sub bgwThumbs_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles bgwThumbs.DoWork

        'Generate a thumbnail for each file in the list

        For Each i As Integer In FileList.Keys

            'CancellationPending is triggered by code in the FormClosing event handler  

            If bgwThumbs.CancellationPending Then
                e.Cancel = True
                Exit For
            End If

            'Generate two bitmaps. srce is from the original file and dest is srce      
            'resized (proportionally) to fit within the desired thumbnail size.         

            Dim srce As New Bitmap(Image.FromFile(FileList(i)))
            Dim dest As Bitmap = ResizeImage(srce, imgFiles.ImageSize.Width, imgFiles.ImageSize.Height)

            UpdateThumbs(i, dest)

            'dispose of the bitmaps and do a garbage collection after every ten images  

            srce.Dispose()
            dest.Dispose()

            If imgFiles.Images.Count Mod 10 = 0 Then GC.Collect()

        Next

    End Sub

    Public Function ResizeImage(srce As Bitmap, destW As Int32, destH As Int32) As Drawing.Bitmap

        'Resize a bitmap to fit with a given width and height. Scaling is done in a     
        'manner that preserves the original image's aspect ratio.                       

        Dim dest As New Drawing.Bitmap(destW, destH, Drawing.Imaging.PixelFormat.Format32bppArgb)

        Dim srceAspect = srce.Width / srce.Height
        Dim destAspect = dest.Width / dest.Height

        Dim NewX As Integer = 0                 'X offset of image in new bitmap
        Dim NewY As Integer = 0                 'Y offset of image in new bitmap
        Dim NewW As Integer = dest.Width        'width of image in new bitmap   
        Dim NewH As Integer = dest.Height       'height of image in new bitmap  

        'calculate sizes and offsets for centering

        If destAspect > srceAspect Then
            NewW = Convert.ToInt32(Math.Floor(srceAspect * NewH))
            NewX = Convert.ToInt32(Math.Floor((dest.Width - NewW) / 2))
        Else
            NewH = Convert.ToInt32(Math.Floor((1 / srceAspect) * NewW))
            NewY = Convert.ToInt32(Math.Floor((dest.Height - NewH) / 2))
        End If

        'draw the srce image into the dest bitmap

        Using grDest As Graphics = Graphics.FromImage(dest)
            grDest.CompositingQuality = CompositingQuality.HighQuality
            grDest.InterpolationMode = InterpolationMode.HighQualityBicubic
            grDest.PixelOffsetMode = PixelOffsetMode.HighQuality
            grDest.SmoothingMode = SmoothingMode.AntiAlias
            grDest.CompositingMode = CompositingMode.SourceOver
            grDest.DrawImage(srce, NewX, NewY, NewW, NewH)
        End Using

        Return dest

    End Function

End Class