Collision between ball and curve as graphics objects - vb.net

Question

I am trying to make a game where I bounce a ball off of the ground which is drawn as a closed curve (The green part), I just don't know how I would go about calculating the collision between the ball and curve.
I've drawn both the ball and curve using graphics in a picturebox, I imagine I have to do it mathematically as I can't find any builtin functionality in Visual basic that lets me do it.
My code:
Public Class Form1
Dim BallSpeedY, BallSpeedX As Double
Dim BallLoc As Point
Dim Start As Boolean = False
Dim gameTime As Decimal = 60.1
Dim gameTimeFont As New Font("Arial", 24, FontStyle.Bold)
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Me.WindowState = FormWindowState.Maximized
'Size of the picturebox that is being drawn on
PictureBox1.Width = Me.Width
PictureBox1.Height = Me.Height - 24
DrawGame(False, False, True)
End Sub
Private Sub DrawGame(refreshMap As Boolean, drawBall As Boolean, drawTime As Boolean)
Dim g As Graphics = Graphics.FromImage(PictureBox1.Image)
Static startcornerpt As PointF
Static firstpt As PointF
Static pt2 As PointF
Static pt3 As PointF
Static pt4 As PointF
Static pt5 As PointF
Static lastpt As PointF
Static Endcornerpt As PointF
' Clears the window
g.Clear(Color.White)
If refreshMap Then
Randomize() ' The rnd seed would always be the same if this is not done
' Creates 5 points chosen at random positions on the window within certain parameters
startcornerpt = New PointF(0, Me.Height)
pt2 = New PointF(Me.Width * 0.2, Me.Height * ((39 * Rnd() + 30) / 100))
firstpt = New PointF(0, pt2.Y * 0.5 * 2)
pt3 = New PointF(Me.Width * 0.4, Me.Height * ((39 * Rnd() + 30) / 100))
pt4 = New PointF(Me.Width * 0.6, Me.Height * ((39 * Rnd() + 30) / 100))
pt5 = New PointF(Me.Width * 0.8, Me.Height * ((39 * Rnd() + 30) / 100))
lastpt = New PointF(Me.Width, pt5.Y * 0.5 * 2)
Endcornerpt = New PointF(Me.Width, Me.Height)
End If
' Draws the map with the 5 points
Dim curvepoints As PointF() = {startcornerpt, firstpt, pt2, pt3, pt4, pt5, lastpt, Endcornerpt}
g.FillClosedCurve(Brushes.PaleGreen, curvepoints)
If drawBall Then
' Draws the ball
g.DrawEllipse(Pens.Blue, BallLoc.X, BallLoc.Y, 20, 20)
g.FillEllipse(Brushes.Blue, BallLoc.X, BallLoc.Y, 20, 20)
' Accelerates speed
BallSpeedY = BallSpeedY - 1
' Update position
BallLoc = New Point(BallLoc.X + BallSpeedX, BallLoc.Y - BallSpeedY)
End If
If drawTime Then
gameTime = gameTime - 0.1
gameTime.ToString()
g.DrawString(gameTime, gameTimeFont, Brushes.Black, Me.Width / 2 - 120, Me.Height * 0.025) ' Draws the gametime on screen,
' Width is set to be around the middle of the screen while height is just under the strip menu.
End If
PictureBox1.Refresh()
End Sub
Private Sub StartToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles StartToolStripMenuItem.Click
If Start = False Then
gameTime = 60.1
'Resets ball for testing
BallLoc.X = 800
BallLoc.Y = 300
BallSpeedY = 0
'Starts the timers
Gravity.Start()
Movement.Start()
Start = True
ElseIf Start = True Then
' Resets the ball
Dim ballloc As New Point(800, 300)
Gravity.Stop()
Movement.Stop()
Start = False
End If
End Sub
Private Sub NewMapToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles NewMapToolStripMenuItem.Click
' Creates a new map
DrawGame(True, False, False)
Start = False
End Sub
Private Sub Movement_Tick(sender As Object, e As EventArgs) Handles Movement.Tick
' Starts movement of the ball
DrawGame(False, True, True)
End Sub
End Class
EDIT: Collision now works with these additions:
Dim wider = CType(ground.Clone(), GraphicsPath)
Using widenizer As Pen = New Pen(Color.Black, ballDiameter)
wider.Widen(widenizer)
End Using
And:
If ground.IsVisible(BallLoc) OrElse wider.IsVisible(BallLoc) Then
BallSpeedY = BallSpeedY + 50 ' rebound on collision
End If

Answer 1

The code below illustrates the comment I've added about using Widen() so you can test using the center of the ball.
It has been ages since I've done any VB.NET, but it does show the idea:
Imports System.Drawing.Drawing2D
Partial Public Class Form1
Inherits Form
Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)
Dim diameter As Integer = 10
Dim path As GraphicsPath = New GraphicsPath()
path.AddCurve(New Point() {New Point(0, 100), New Point(200, 200), New Point(400, 100)})
path.AddLines(New Point() {New Point(400, 400), New Point(0, 400)})
path.CloseAllFigures()
e.Graphics.FillPath(Brushes.Green, path)
Dim wider = CType(path.Clone(), GraphicsPath)
Using widenizer As Pen = New Pen(Color.Black, diameter)
wider.Widen(widenizer)
End Using
For x As Integer = 0 To 400 Step diameter
For y As Integer = 0 To 400 Step diameter
If path.IsVisible(x, y) OrElse wider.IsVisible(x, y) Then
e.Graphics.DrawEllipse(Pens.Red, CType(x - diameter / 2, Single), CType(y - diameter / 2, Single), diameter, diameter)
Else
e.Graphics.DrawEllipse(Pens.Blue, CType(x - diameter / 2, Single), CType(y - diameter / 2, Single), diameter, diameter)
End If
Next y
Next x
End Sub
End Class
It fills the area with circles, where the red ones are "touching" the ground: