C# と VB.NET の質問掲示板

多分こんな感じ？Zoomは設定したままで出来そうでした。
Disposeは省いてありますmm

    Private bmp As Bitmap
    Private grp As Graphics
    Private m_blnDown As Boolean = False

    Public Sub New()

        ' この呼び出しはデザイナーで必要です。
        InitializeComponent()

        ' InitializeComponent() 呼び出しの後で初期化を追加します。
        bmp = New Bitmap(PictureBox1.Width * 2, PictureBox1.Height * 2)   '<--- 分かりやすくするためPictureBoxの２ｘ２倍の大きさにしてみた
        grp = Graphics.FromImage(bmp)
        grp.Clear(Color.White)
        grp.ScaleTransform(2, 2)    '<--- ２×２倍なのでScaleTransformで座標変換を指定
        PictureBox1.Image = bmp
        PictureBox1.SizeMode = PictureBoxSizeMode.Zoom
    End Sub

    Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
        m_blnDown = True   '<--- MouseDownで描画開始
    End Sub

    Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove
        If m_blnDown Then
            '--- マウスポジションを中心に半径２の円、Scaleがきくので実際は４になると思う。
            grp.FillEllipse(Brushes.Black, e.X - 2, e.Y - 2, 4, 4)    
            PictureBox1.Refresh()
        End If
    End Sub

    Private Sub PictureBox1_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseUp
        m_blnDown = False   '<--- MouseUpで描画終了
    End Sub

■No57218 (ダメなネコさん さん) に返信

'Dim pos as Point    '<--- Pointとして使用するとこないので不要
Dim X as Integer
Dim X2 as Integer
Dim Y as Integer

・・・・

X2 = CInt((PictureBox1.Width - (img.Width * PictureBox1.Height / img.Height)) / 2)
X = CInt((e.X - X2) * img.Height / PictureBox1.Height)
Y = CInt(e.Y * img.Height / PictureBox1.Height)
If e.Button = MouseButtons.Left Then
    g.FillEllipse(Brushes.Red, X-5, Y-5, 10, 10)
else
    g.FillEllipse(Brushes.Blue, X-5, Y-5, 10, 10)
end if 


縦長の場合です。横長とかPictureBoxより小さい場合はまた処理が変わります。

-----

縦長、横長両対応

        Dim x As Integer
        Dim x2 As Integer
        Dim y As Integer
        Dim y2 As Integer

        ・・・・

        x2 = CInt((PictureBox1.Width - (img.Width * PictureBox1.Height / img.Height)) / 2)
        y2 = CInt((PictureBox1.Height - (img.Height * PictureBox1.Width / img.Width)) / 2)

        If x2 > 0 Then
            x = CInt((e.X - x2) * img.Height / PictureBox1.Height)
            y = CInt(e.Y * img.Height / PictureBox1.Height)
        Else
            x = CInt(e.X * img.Width / PictureBox1.Width)
            y = CInt((e.Y - y2) * img.Width / PictureBox1.Width)
        End If

        If e.Button = MouseButtons.Left Then
            g.FillEllipse(Brushes.Red, x, y, 10, 10)
        ElseIf e.Button = MouseButtons.Right Then
            g.FillEllipse(Brushes.Blue, x, y, 10, 10)
        End If