Selective Grayscale Filter

' Hue Saturation Lightness structure.
' 
' Copyright (c) 2007 Miran Uhan
' 
' This program is free software; you can redistribute it and/or modify
' it under the terms of the GNU General Public License as published by
' the Free Software Foundation; either version 2 of the License, or
' (at your option) any later version.
'
' This program is distributed in the hope that it will be useful,
' but WITHOUT ANY WARRANTY; without even the implied warranty of
' MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
' GNU General Public License for more details.
'
' You should have received a copy of the GNU General Public License
' along with this program; if not, write to the
'    Free Software Foundation, Inc.
'    51 Franklin Street, Fifth Floor
'    Boston, MA 02110-1301 USA
'
' For questions you can contact author at
'    miran.uhan@gmail.com
' Sugestions and bug reports are also welcome.
' 

Imports System.Drawing
Imports System.Drawing.Imaging

Public Structure HslColor

   Public H As Double
   Public S As Double
   Public L As Double

   ''' <summary>
   ''' Convert RGB to Hue, Saturation, Lightness.
   ''' </summary>
   ''' <param name="R">Value of red component in range [0..1].</param>
   ''' <param name="G">Value of green component in range [0..1].</param>
   ''' <param name="B">Value of blue component in range [0..1].</param>
   ''' <returns></returns>
   ''' <remarks></remarks>
   Public Shared Function FromRgb(ByVal R As Double, ByVal G As Double, ByVal B As Double) As HslColor
      Dim result As HslColor
      Dim min, max, dif, sum, f1, f2 As Double
      min = R
      If (G < min) Then min = G
      If (B < min) Then min = B
      max = R : f1 = 0.0 : f2 = G - B
      If (G > max) Then
         max = G : f1 = 1 / 3 : f2 = B - R
      End If
      If (B > max) Then
         max = B : f1 = 2 / 3 : f2 = R - G
      End If
      dif = max - min
      sum = max + min
      result.L = 0.5 * sum
      If (dif = 0) Then
         result.H = 0.0
         result.S = 0.0
      Else
         If (result.L < 0.5) Then
            result.S = dif / sum
         Else
            result.S = dif / (2 - sum)
         End If
         result.H = f1 + f2 / (6 * dif)
      End If
      If result.H < 0 Then result.H += 1
      If result.H > 1 Then result.H -= 1
      Return result
   End Function

   ''' <summary>
   ''' Convert RGB to Hue, Saturation, Lightness.
   ''' </summary>
   ''' <param name="C">Color structure.</param>
   ''' <returns></returns>
   ''' <remarks></remarks>
   Public Shared Function FromRgb(ByVal C As Color) As HslColor
      Return FromRgb(C.R / 255.0, C.G / 255.0, C.B / 255.0)
   End Function

   Public Shared Function ToRgb(ByVal H As Double, ByVal S As Double, ByVal L As Double) As Color
      Dim result(2) As Double
      If (S = 0) Then
         result(0) = L * 255
         result(1) = result(0)
         result(2) = result(0)
      Else
         Dim v2 As Double
         If (L < 0.5) Then
            v2 = L * (1 + S)
         Else
            v2 = (L + S) - (S * L)
         End If
         Dim v1 As Double = 2 * L - v2
         Dim dif As Double = v2 - v1
         Dim h1 As Double
         'Calc red component.
         h1 = H + 1 / 3
         If (h1 > 1) Then h1 -= 1
         If ((6 * h1) < 1) Then
            result(0) = 255 * (v1 + dif * 6 * h1)
         ElseIf ((2 * h1) < 1) Then
            result(0) = 255 * v2
         ElseIf ((3 * h1) < 2) Then
            result(0) = 255 * (v1 + dif * (4.0 - 6 * h1))
         Else
            result(0) = 255 * v1
         End If
         'Calc green component.
         h1 = H
         If ((6 * h1) < 1) Then
            result(1) = 255 * (v1 + dif * 6 * h1)
         ElseIf ((2 * h1) < 1) Then
            result(1) = 255 * v2
         ElseIf ((3 * h1) < 2) Then
            result(1) = 255 * (v1 + dif * (4.0 - 6 * h1))
         Else
            result(1) = 255 * v1
         End If
         'Calc blue component.
         h1 = H - 1 / 3
         If (h1 < 0) Then h1 += 1
         If ((6 * h1) < 1) Then
            result(2) = 255 * (v1 + dif * 6 * h1)
         ElseIf ((2 * h1) < 1) Then
            result(2) = 255 * v2
         ElseIf ((3 * h1) < 2) Then
            result(2) = 255 * (v1 + dif * (4.0 - 6 * h1))
         Else
            result(2) = 255 * v1
         End If
      End If
      Return Color.FromArgb(result(0), result(1), result(2))
   End Function

   Public Shared Function ToRgb(ByVal C As HslColor) As Color
      ToRgb(C.H, C.S, C.L)
   End Function

End Structure