I am working on an application which draws a simple dot grid. I would like the mouse to snap between the points on the grid,

I am working on an application which draws a simple dot grid. I would like the mouse to snap between the points on the grid, eventually to draw lines on the grid.

I have a method which takes in the current mouse location (X,Y) and calculates the nearest grid coordinate.

When I create an event and attempt to move the mouse to the new coordinate the whole system becomes jerky. The mouse doesn't snap smoothly between grid points.

Imports System
Imports System.Drawing
Imports System.Windows.Forms
Public Class Form1
    Public Const strApplicationTitle As String = "Snap Demo"
    Public frmSnap As Form1
    Public ptSnap, ptStart, ptEnd As Point
    Public Sub New()
        Me.Text = "Snap points"
        Me.ClientSize = New Size(800, 600)
        Me.FormBorderStyle = Windows.Forms.FormBorderStyle.FixedSingle
        Me.MaximizeBox = False
        Me.StartPosition = FormStartPosition.CenterScreen
        Me.DoubleBuffered = True
    End Sub
    Protected Overrides Sub OnPaint(ByVal e As System.Windows.Forms.PaintEventArgs)
        MyBase.OnPaint(e)
        e.Graphics.Clear(Color.Black)
        For row As Integer = 20 To 780 Step 20
            For col As Integer = 20 To 580 Step 20
                e.Graphics.DrawEllipse(Pens.Blue, New Rectangle(row - 2, col - 2, 4, 4))
            Next
        Next
        e.Graphics.DrawLine(Pens.Red, ptStart, ptEnd)
    End Sub
    Protected Overrides Sub OnMouseDown(ByVal e As System.Windows.Forms.MouseEventArgs)
        MyBase.OnMouseDown(e)
        Dim x As Integer = CInt(e.X / 20) * 20
        Dim y As Integer = CInt(e.Y / 20) * 20
        ptStart = New Point(x, y)
        ptSnap = New Point(x, y)
        Windows.Forms.Cursor.Position = Me.PointToScreen(ptSnap)
    End Sub
    Protected Overrides Sub OnMouseMove(ByVal e As System.Windows.Forms.MouseEventArgs)
        MyBase.OnMouseMove(e)
        If e.Button = Windows.Forms.MouseButtons.Left Then
            Dim x As Integer = CInt(e.X / 20) * 20
            Dim y As Integer = CInt(e.Y / 20) * 20
            ' must be some delta away from original snap point
            If (x < ptSnap.X - 15 Or x > ptSnap.X + 15) Or (y < ptSnap.Y - 15 Or y > ptSnap.Y + 15) Then
                ptSnap = New Point(x, y)
                ptEnd = New Point(x, y)
                Me.Invalidate(False)
                Windows.Forms.Cursor.Position = Me.PointToScreen(ptSnap)
            End If
        End If
    End Sub


    Public Sub main()
        Try
            frmSnap = New Form1
            Application.Run(frmSnap)
        Catch ex As Exception
            MessageBox.Show(ex.Message, strApplicationTitle, MessageBoxButtons.OK, MessageBoxIcon.Error)
        Finally
            frmSnap.Dispose()
        End Try
    End Sub
End Class