Option Explicit

Public Function ProofLineCollision(ByVal L1_X1 As Single, _
            ByVal L1_Y1 As Single, ByVal L1_X2 As Single, _
            ByVal L1_Y2 As Single, ByVal L2_X1 As Single, _
            ByVal L2_Y1 As Single, ByVal L2_X2 As Single, _
            ByVal L2_Y2 As Single) As Boolean

  'Anstiege
  Dim L1_m As Single, L2_m As Single
  'Schnittpunkte mit der Ordinatenachse
  Dim L1_n As Single, L2_n As Single
  'Schnittpunkt der Linien
  Dim s    As Single
  'Hilfsvariablen für X- und Y- Werte
  Dim h_Y  As Single, h_X  As Single

  'X- und Y- Werte vertauschen, falls X2 < X1
  '(nur ein Problem mathematischer Natur)
  If L1_X2 < L1_X1 Then
    SwitchVar L1_X1, L1_X2
    SwitchVar L1_Y1, L1_Y2
  End If

  If L2_X2 < L2_X1 Then
    SwitchVar L2_X1, L2_X2
    SwitchVar L2_Y1, L2_Y2
  End If

  'X- und Y- Werte vertauschen, falls Y2 < Y1 und X1 = X2
  If L1_X1 = L1_X2 And L1_Y2 < L1_Y1 Then
    SwitchVar L1_X1, L1_X2
    SwitchVar L1_Y1, L1_Y2
  End If

  If L2_X1 = L2_X2 And L2_Y2 < L2_Y1 Then
    SwitchVar L2_X1, L2_X2
    SwitchVar L2_Y1, L2_Y2
  End If

  'Beide Linien senkrecht
  If L1_X1 = L1_X2 And L2_X1 = L2_X2 Then
    'Prüfen, ob Linien aufeinanderliegen
    ProofLineCollision = (L1_X1 = L2_X1 And _
          ((L1_Y1 >= L2_Y1 And L1_Y1 <= L2_Y2) Or _
          (L2_Y1 >= L1_Y1 And L2_Y1 <= L1_Y2)))
    Exit Function
  End If

  'Beide Linien waagerecht
  If L1_Y1 = L1_Y2 And L2_Y1 = L2_Y2 Then
    'Prüfen, ob Linien aufeinanderliegen
    ProofLineCollision = (L1_Y1 = L2_Y1 And _
          ((L1_X1 >= L2_X1 And L1_X1 <= L2_X2) Or _
          (L2_X1 >= L1_X1 And L2_X1 <= L1_X2)))
    Exit Function
  End If

  'Nur eine der Linien ist senkrecht, die andere waagerecht
  If L1_X1 = L1_X2 And L2_Y1 = L2_Y2 Then
    ProofLineCollision = (L1_X1 >= L2_X1 And L1_X1 <= L2_X2 _
                      And L2_Y1 >= L1_Y1 And L2_Y1 <= L1_Y2)
    Exit Function

  'nochmal umgekehrt
  ElseIf L1_Y1 = L1_Y2 And L2_X1 = L2_X2 Then
    ProofLineCollision = (L1_Y1 >= L2_Y1 And L1_Y1 <= L2_Y2 _
                      And L2_X1 >= L1_X1 And L2_X1 <= L1_X2)
    Exit Function
  End If

  'Eine Linie ist senkrecht, die andere ist diagonal
  If L1_X1 = L1_X2 Then
    'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie2
    L2_m = (L2_Y2 - L2_Y1) / (L2_X2 - L2_X1)
    L2_n = L2_Y2 - L2_m * L2_X2

    h_Y = L2_m * L1_X1 + L2_n

    ProofLineCollision = (L1_X1 >= L2_X1 And L1_X1 <= L2_X2 _
                      And h_Y >= L1_Y1 And h_Y <= L1_Y2)
    Exit Function

  'nochmal umgekehrt
  ElseIf L2_X1 = L2_X2 Then
    'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie1
    L1_m = (L1_Y2 - L1_Y1) / (L1_X2 - L1_X1)
    L1_n = L1_Y2 - L1_m * L1_X2

    h_Y = L1_m * L2_X1 + L1_n

    ProofLineCollision = (L2_X1 >= L1_X1 And L2_X1 <= L1_X2 And _
                          h_Y >= L2_Y1 And h_Y <= L2_Y2)
    Exit Function
  End If

  'Eine Linie ist waagerecht, die andere ist diagonal
  If L1_Y1 = L1_Y2 Then
    'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie2
    L2_m = (L2_Y2 - L2_Y1) / (L2_X2 - L2_X1)
    L2_n = L2_Y2 - L2_m * L2_X2

    h_X = (L1_Y1 - L2_n) / L2_m

    ProofLineCollision = (h_X >= L1_X1 And h_X <= L1_X2 And _
                          h_X >= L2_X1 And h_X <= L2_X2)
    Exit Function

  'nochmal umgekehrt
  ElseIf L2_Y1 = L2_Y2 Then
    'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie1
    L1_m = (L1_Y2 - L1_Y1) / (L1_X2 - L1_X1)
    L1_n = L1_Y2 - L1_m * L1_X2

    h_X = (L2_Y1 - L1_n) / L1_m

    ProofLineCollision = (h_X >= L2_X1 And h_X <= L2_X2 And _
                          h_X >= L1_X1 And h_X <= L1_X2)
    Exit Function
  End If

  'Wenn beide Linien diagonal sind...
  'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie1
  L1_m = (L1_Y2 - L1_Y1) / (L1_X2 - L1_X1)
  L1_n = L1_Y2 - L1_m * L1_X2

  'Anstieg und Schnittpunkt mit der Ordinatenachse für Linie2
  L2_m = (L2_Y2 - L2_Y1) / (L2_X2 - L2_X1)
  L2_n = L2_Y2 - L2_m * L2_X2

  'Linien sind eventuell parallel zueinander
  If L2_m = L1_m Then
    ProofLineCollision = _
          (L1_m * L1_X1 + L1_n = L2_m * L1_X1 + L2_n And _
           L1_X1 >= L2_X1 And L1_X1 <= L2_X2)
    Exit Function

  'Wenn nicht, Schnittpunkt errechnen
  Else
    s = (L1_n - L2_n) / (L2_m - L1_m)
  End If

  'Überprüfen, ob der Schnittpunkt innerhalb beider Linien liegt
  ProofLineCollision = (s >= L1_X1 And s <= L1_X2 And _
                        s >= L2_X1 And s <= L2_X2)
End Function

'Dient zum Vertauschen von zwei Variablen
Public Sub SwitchVar(ByRef a As Single, ByRef b As Single)
  Dim dummy As Long

  dummy = a
  a = b
  b = dummy
End Sub

Option Explicit

Private Sub Form_MouseMove(Button As Integer, Shift As Integer, _
        X As Single, Y As Single)

  Line1.X1 = X
  Line1.Y1 = Y

  If ProofLineCollision(Line1.X1, Line1.Y1, Line1.X2, Line1.Y2, _
          Line2.X1, Line2.Y1, Line2.X2, Line2.Y2) Then
    Me.Caption = "  »»»  Kollision !!!!  «««"
  Else
    Me.Caption = "Kollision zwischen zwei Linien"
  End If
End Sub