' Using s and t values, return the coordinates of a
' point in a quadrilateral.
Private Sub STToPoints(ByRef X As Single, ByRef Y As Single, ByVal s As Single, ByVal t As Single, ByVal x1 As Single, ByVal y1 As Single, ByVal x2 As Single, ByVal y2 As Single, ByVal x3 As Single, ByVal y3 As Single, ByVal x4 As Single, ByVal y4 As Single)
Dim xa As Single
Dim ya As Single
Dim xb As Single
Dim yb As Single
xa = x1 + t * (x2 - x1)
ya = y1 + t * (y2 - y1)
xb = x3 + t * (x4 - x3)
yb = y3 + t * (y4 - y3)
X = xa + s * (xb - xa)
Y = ya + s * (yb - ya)
End Sub
Private Sub Form_Load()
Dim i As Integer
ScaleMode = vbPixels
AutoRedraw = True
x1(1) = 20
x2(1) = 120
x3(1) = 10
x4(1) = 150
y1(1) = 50
y2(1) = 30
y3(1) = 130
y4(1) = 110
x1(2) = 120
x2(2) = 210
x3(2) = 100
x4(2) = 250
y1(2) = 150
y2(2) = 170
y3(2) = 240
y4(2) = 260
x1(3) = 200
x2(3) = 300
x3(3) = 200
x4(3) = 300
y1(3) = 20
y2(3) = 20
y3(3) = 120
y4(3) = 120
For i = 1 To 3
Line (x1(i), y1(i))-(x2(i), y2(i))
Line -(x4(i), y4(i))
Line -(x3(i), y3(i))
Line -(x1(i), y1(i))
Next i
Picture = Image
DrawWidth = 3
End Sub
' Find S and T for the point (X, Y) in the
' quadrilateral with points (x1, y1), (x2, y2),
' (x3, y3), and (x4, y4). Return True if the point
' lies within the quadrilateral and False otherwise.
Private Function PointsToST(ByVal X As Single, ByVal Y As Single, ByRef s As Single, ByRef t As Single, ByVal x1 As Single, ByVal y1 As Single, ByVal x2 As Single, ByVal y2 As Single, ByVal x3 As Single, ByVal y3 As Single, ByVal x4 As Single, ByVal y4 As Single) As Boolean
Dim Ax As Single
Dim Bx As Single
Dim Cx As Single
Dim Dx As Single
Dim Ex As Single
Dim Ay As Single
Dim By As Single
Dim Cy As Single
Dim Dy As Single
Dim Ey As Single
Dim a As Single
Dim b As Single
Dim c As Single
Dim det As Single
Dim denom As Single
Ax = x2 - x1: Ay = y2 - y1
Bx = x4 - x3: By = y4 - y3
Cx = x3 - x1: Cy = y3 - y1
Dx = X - x1: Dy = Y - y1
Ex = Bx - Ax: Ey = By - Ay
a = -Ax * Ey + Ay * Ex
b = Ey * Dx - Dy * Ex + Ay * Cx - Ax * Cy
c = Dx * Cy - Dy * Cx
det = b * b - 4 * a * c
If det >= 0 Then
If Abs(a) < 0.001 Then
t = -c / b
Else
t = (-b - Sqr(det)) / (2 * a)
End If
denom = (Cx + Ex * t)
If denom > 0.01 Then
s = (Dx - Ax * t) / denom
Else
s = (Dy - Ay * t) / (Cy + Ey * t)
End If
PointsToST = (t >= 0# And t <= 1# And _
s >= 0# And s <= 1#)
Else
PointsToST = False
End If
End Function
Private Sub Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
(.txt)