[2005] Resizing a bitmap in memory

[Desolator144]

I know how to open a picture file and load it into memory as a bitmap, I know how to save it as a jpeg, and I even know how to crop them first but there doesn't seem to be a way to resize them to a certain size except for the GetThumbnailImage method but I have no idea what a callback or intptr is. Instead of selectively screenshotting a size changing picture box, I figured I'd ask you guys.

[ComputerJy]

The IntPtr is just like a Pointer (Memory Address) to something [AddressOf someMethod or Control.Handle]
The Callback is a method called after each fragment of the file is read (how many times? it depends on the size of the image, and the buffer located by the Framework). So, it's of no use in resizing

[Desolator144]

Okay so for the intptr, they're looking for a place to store the data in memory so I assume there's a way to generate an intptr from a bitmap variable that was declared without the new keyword, right? Also, does it toss that location when it's done? And if callbacks don't have anything to do with resizing and it's a built in thing that windows does to generate thumbnails, why is it asking me to supply one and how would I do so?

[Tinbeard]

To resize an image just use the graphics object of the control your drawing on (from within it's paint event) and use the DrawImage method and pass the image and a rectangle of the required size.

[Desolator144]

hmm well I spose I could stick it in a picturebox if I really have to. I'll try that

[DNA7433]

No no no, you simply want to resize the bitmap. Create a new Bitmap object with the desired size. Then call Graphics.CreateFromBitmap(), or CreateFromImage I can't remember, and pass in your new blank bitmap object. Then on the returned Graphics object call DrawImage() to copy your bitmap to the resized bitmap. You can forget about the old Bitmap reference if desired and simply store away the new bitmap object with no ill effects.

[Desolator144]

never mind this whole post, read the next one

[DNA7433]

You can't create a Graphics object instance by using new. You have to use the static methods in the Graphics class or a Controls CreateGraphics instance method. In this case you want the static methods. I already said I can't remember the exact name but the method does return a Graphics object just as I said above.

[Desolator144]

OOOOOOOOOH Fixed it! Okay, now it actually scales. Here's the real, actual, full, working version:

Private Sub btnChoose_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnChoose.Click
        Dim FolderChooser As New OpenFileDialog
        FolderChooser.Title = "Choose all the pictures to be formatted"
        FolderChooser.Multiselect = True
        FolderChooser.InitialDirectory = "C:\"
        FolderChooser.ShowDialog()
        Dim strDirectory() As String = FolderChooser.FileNames
        Dim intWidth As Integer = CInt(InputBox("How wide would you like each thumbnail to be?", "Width"))
        Dim intHeight As Integer = CInt(InputBox("How tall would you like each thumbnail to be?", "Height")) 
        Dim x As Integer
        For x = 0 To strDirectory.GetUpperBound(0)
            Dim bit As New Bitmap(strDirectory(x))
            pctBox.Image = bit
            me.update()
            Dim TheGraphics As Graphics
            Dim newbit As New Bitmap(intWidth, intHeight)
            TheGraphics = Graphics.FromImage(newbit)
            TheGraphics.DrawImage(bit, 0, 0, intWidth, intHeight)
            Dim strSavePath As String
            Dim intIndex As Integer
            intIndex = strDirectory(x).Length - 4
            strSavePath = strDirectory(x).Insert(intIndex, "Thumb")
            newbit.Save(strSavePath, System.Drawing.Imaging.ImageFormat.Jpeg)
        Next
    End Sub

[gigemboy]

Yep.. and its a common question, and theres a codebank submission over it as well...

http://www.vbforums.com/showthread.php?t=383136