VB.NET Bitmap Trimming linked by plot data

VB Sub Bmp1_FixTrim()

'****************************************************************************************************************
    '先端データを読んで切り取りXY座標を制御しながら先端固定モードで描画
    '****************************************************************************************************************
    Private Sub Bmp1_FixTrim(ByVal SlideVal As Integer, ByVal rNlin As Integer)
        '------------------------------------------------------------------------------------------------
        '-----HscrollBar値で切り取り前進動画-------------------------------------------------------------
        '------------------------------------------------------------------------------------------------
        Dim BP1 As New Pen(Color.Red, 1)
        Dim BP2 As New Pen(Color.LawnGreen, 1)
        Dim BP3 As New Pen(Color.Black)
        Dim scrollN As Integer = Math.Truncate(SlideVal / PictureBox1.Width)
        Dim cWidth As Integer = SlideVal Mod PictureBox1.Width
        Dim sY As Integer
        Dim dY As Integer = 0 'desRectのオフセットY位置
        Dim rN As Integer = rNlin - Val(TextBox4.Text)
        If Fy(SlideVal) > 300 Then
            'sY = RY(rN) - 300
            sY = CInt(Fy(SlideVal)) - 300
            dY = 0
        Else
            sY = 0
            ' dY = (300 - RY(rN))
            dY = 300 - CInt(Fy(SlideVal))
        End If
        currentX = CInt(PictureBox1.Width * scrollN)
        currentY = sY
        ' Dim srcRect As New Rectangle(PictureBox1.Width * scrollN,0, cWidth, rangeY) 'Y原点スタート
        Dim srcRect As New Rectangle(PictureBox1.Width * scrollN, sY, cWidth, 600) '切り取り部分定義
        Dim desRect As New Rectangle(0, dY, srcRect.Width, srcRect.Height)
        Debug.Print("StartNo=" + CStr(PictureBox1.Width * scrollN) + ",cWidth=" + CStr(cWidth) + ",SlideVal=" + CStr(SlideVal) + ",rNlin=" + CStr(rNlin) + ",RY=" + CStr(RY(rN)) + ",sY=" + CStr(sY))
        Label43.Text = "StartNo=" + CStr(PictureBox1.Width * scrollN) + ",cWidth=" + CStr(cWidth) + ",SlideVal=" + CStr(SlideVal) + ",rNlin=" + CStr(rNlin) + ",RY=" + CStr(RY(rN)) + ",sY=" + CStr(sY) + ",dY=" + CStr(dY)
        'Dim desRect As New Rectangle(0, 0, PictureBox1.Width, PictureBox1.Height)
        bmap2 = New Bitmap(PictureBox1.Width, PictureBox1.Height)
        g2 = Graphics.FromImage(bmap2)
        g2.DrawImage(bmap1, desRect, srcRect, GraphicsUnit.Pixel) 'bmap1から指定サイズで切り取ってg2:bmap2へ書き込む
        'g2.DrawRectangle(BP1, 0, 0, srcRect.Width, srcRect.Height) '切り取った矩形の外枠を赤線で描画

        'g1.Dispose()
        g2.Dispose()
        'Y軸を１８０度回転させて上下反転して上を北に変更
        bmap2.RotateFlip(RotateFlipType.Rotate180FlipX)
        PictureBox1.Image = bmap2 'bmap2を表示
        '================================================================




    End Sub