Angles between 2 lines

[Paul_Hossler]

Try this

Option Explicit

'based on
'ref http://www.devx.com/tips/Tip/33124

Const cPI As Double = 3.14159265358979

Function getAngleBetweenLinesVect(commonX As Double, commonY As Double, _
    X1 As Double, Y1 As Double, _
    Y2 As Double, X2 As Double) As Double
    
    Dim aRadians As Double
    Dim diffX1 As Double, diffX2 As Double, diffY1 As Double, diffY2 As Double
    
    'check if both the lines are the same, if they are exit the function
    If (commonY = Y1 And commonX = X1) Or _
        (commonY = Y2 And commonX = X2) Or _
        (Y1 = Y2 And X1 = X2) Then
        
        getAngleBetweenLinesVect = 0#
        Exit Function
    End If
    
    'set the variables
    diffX1 = commonX - X1
    diffY1 = commonY - Y1
    diffX2 = commonX - X2
    diffY2 = commonY - Y2

    'applying the angle between two lines vector rule, calculate the angle in radians
    aRadians = invCos(((diffX1 * diffX2) + (diffY1 * diffY2)) / _
        (Sqr(diffX1 ^ 2 + diffY1 ^ 2) * Sqr(diffX2 ^ 2 + diffY2 ^ 2)))

    'convert to degrees
    getAngleBetweenLinesVect = aRadians * 180# / cPI
End Function

'the inverse cos function. Returns an angle.
Public Function invCos(X As Double) As Double
    If X > 1 Or X < -1 Then
    'if the ratio supplies is outside the acceptable range.
    ElseIf X = 1 Then
        invCos = 0
    ElseIf X = -1 Then
        invCos = cPI
    
    Else
        invCos = (Atn(-X / Sqr(-X * X + 1)) + 2 * Atn(1))
    End If
End Function