How to zoom in a Picturebox with scrollwheel in vb.net

Question

I\'m using a set of graphics overlays to draw an image inside a picturebox control using the graphics object. I have placed the Picturebox inside a Panel and set the Panel t

Answer 1

You can try this code. It assumes there exist a Panel1 and PictureBox1 on the form (PictureBox1 inside the Panel1 with Panel1.AutoScroll = True) with an image set on the PictureBox.

The code doesn't calculate center point of the zoom, but you can use the e.Location (or e.X/e.Y) for that.

Update - here is the new code that is (should be) more robust than the previous (see bottom):

Public Class Form1

    Private _originalSize As Size = Nothing
    Private _scale As Single = 1
    Private _scaleDelta As Single = 0.0005

    Private Sub Form_MouseWheel(sender As System.Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseWheel

        'if very sensitive mouse, change 0.00005 to something even smaller   
        _scaleDelta = Math.Sqrt(PictureBox1.Width * PictureBox1.Height) * 0.00005

        If e.Delta < 0 Then
            _scale -= _scaleDelta
        ElseIf e.Delta > 0 Then
            _scale += _scaleDelta
        End If

        If e.Delta <> 0 Then _
        PictureBox1.Size = New Size(CInt(Math.Round(_originalSize.Width * _scale)), _
                                    CInt(Math.Round(_originalSize.Height * _scale)))

    End Sub

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage

        'init this from here or a method depending on your needs
        If PictureBox1.Image IsNot Nothing Then
            PictureBox1.Size = Panel1.Size
            _originalSize = Panel1.Size
        End If

    End Sub

End Class

Old code - works, but is unstable on large changes probably due to rounding errors in Scale():

Public Class Form1

    Private _scale As New SizeF(1, 1)
    Private _scaleDelta As New SizeF(0.01, 0.01) '1% for each wheel tick

    Private Sub Form_MouseWheel(sender As System.Object, 
                                e As MouseEventArgs) Handles Me.MouseWheel
'count incrementally 
        _scale.Height = 1
        _scale.Width = 1

        If e.Delta < 0 Then
            _scale += _scaleDelta
        ElseIf e.Delta > 0 Then
            _scale -= _scaleDelta
        End If

        If e.Delta <> 0 Then _
        PictureBox1.Scale(_scale)

    End Sub

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

        PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage

        'init picturebox size = image size
        If PictureBox1.Image IsNot Nothing Then
            PictureBox1.Scale(New SizeF(1, 1))
            PictureBox1.Size = PictureBox1.Image.Size
        End If

    End Sub

End Class