RGB and HSL Colour Space Conversions.

This has been updated to include VB listings of the RGBHSL utility and the test programs.

The classic RGB colour space used in GDI+ is excellent for choosing or defining a specific colour as a mixture of  primary colour and intensity values but what happens if you want to take a particular colour and make it a bit lighter or a bit darker or change its saturation. For this you need to be able to use the HSL (Hue, Saturation and Luminance) colour space.

The .NET framework actually provides RGB-HSL conversion but for some reason, the conversion of HSL to RGB is not provided as a public method. The Color class has three methods, GetHue, GetSaturation and GetBrightness that provide the basic components of the HSL (or HSB) colour space. The conversion of HSL to RGB is a well known algorithm that you can find in numerous places on the web. Oddly enough, down in the bowels of the ControlPaint class, RGB to HSL is employed when you use the ControlPaint.Light(…) or ControlPaint.DarkDark(…) methods but also unfortunately the scaling of the Light, Dark and DarkDark settings are, to say the least, brutal and offer no finesse.

A specific use for RGB-HSL conversions I have used in the past, is to provide colours that enhance objects drawn in a psuedo-3D or isometric style. In such a drawing, a cube having a specific colour, say red, may be shown with a lighter red on top to simulate a shine on the object and a darker colour on one side to simulate a shadow. Figure1 shows two such cubes with and without shading yet still using red as the basic colour.

Figure 1

In addition to making a chosen colour lighter or darker, you may wish to choose a range of  colour values that all have a similar brightness. Not an easy task where RGB is concerned but trivial using the HSL colour space. Similarly, you may wish to choose random colours but ensure that they have a uniform level of saturation to ensure bright, non-pastel colours for ease of visibility. Once again, a task eminently possible if using HSL colour definitions.

If you're interested in further reading on the differences between colour spaces, this link will take you to an excellent article on the Apple site which explains them nicely.

To accompany this FAQ entry I have provided the source code shown in listing 1. It provides an RGB-HSL and HSL-RGB conversion plus a few methods that assist in setting or modifying a brightness.

Listing 1:

C#.

/* This tool is part of the xRay Toolkit and is provided free of charge by Bob Powell.

 * This code is not guaranteed to be free from defects or fit for merchantability in any way.

 * By using this tool in your own programs you agree to hold Robert W. Powell free from all

 * damages direct or incidental that arise from such use.

 * You may use this code free of charge in your own projects on condition that you place the

 * following paragraph (enclosed in quotes below) in your applications help or about dialog.

 * "Portions of this code provided by Bob Powell. http://www.bobpowell.net"

 */

using System;

using System.Drawing;

namespace xRay.Toolkit.Utilities

{

   public class RGBHSL

   {

      public class HSL

      {

         public HSL()

         {

            _h=0;

            _s=0;

            _l=0;

         }

         double _h;

         double _s;

         double _l;

         public double H

         {

            get{return _h;}

            set

            {

               _h=value;

               _h=_h>1 ? 1 : _h<0 ? 0 : _h;

            }

         }

         public double S

         {

            get{return _s;}

            set

            {

               _s=value;

               _s=_s>1 ? 1 : _s<0 ? 0 : _s;

            }

         }

         public double L

         {

            get{return _l;}

            set

            {

               _l=value;

               _l=_l>1 ? 1 : _l<0 ? 0 : _l;

            }

         }

      }

      public RGBHSL()

      {

      }

      /// <summary>

      /// Sets the absolute brightness of a colour

      /// </summary>

      /// <param name="c">Original colour</param>

      /// <param name="brightness">The luminance level to impose</param>

      /// <returns>an adjusted colour</returns>

      public static  Color SetBrightness(Color c, double brightness)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.L=brightness;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Modifies an existing brightness level

      /// </summary>

      /// <remarks>

      /// To reduce brightness use a number smaller than 1. To increase brightness use a number larger tnan 1

      /// </remarks>

      /// <param name="c">The original colour</param>

      /// <param name="brightness">The luminance delta</param>

      /// <returns>An adjusted colour</returns>

      public static  Color ModifyBrightness(Color c, double brightness)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.L*=brightness;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Sets the absolute saturation level

      /// </summary>

      /// <remarks>Accepted values 0-1</remarks>

      /// <param name="c">An original colour</param>

      /// <param name="Saturation">The saturation value to impose</param>

      /// <returns>An adjusted colour</returns>

      public static  Color SetSaturation(Color c, double Saturation)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.S=Saturation;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Modifies an existing Saturation level

      /// </summary>

      /// <remarks>

      /// To reduce Saturation use a number smaller than 1. To increase Saturation use a number larger tnan 1

      /// </remarks>

      /// <param name="c">The original colour</param>

      /// <param name="Saturation">The saturation delta</param>

      /// <returns>An adjusted colour</returns>

      public static  Color ModifySaturation(Color c, double Saturation)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.S*=Saturation;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Sets the absolute Hue level

      /// </summary>

      /// <remarks>Accepted values 0-1</remarks>

      /// <param name="c">An original colour</param>

      /// <param name="Hue">The Hue value to impose</param>

      /// <returns>An adjusted colour</returns>

      public static  Color SetHue(Color c, double Hue)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.H=Hue;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Modifies an existing Hue level

      /// </summary>

      /// <remarks>

      /// To reduce Hue use a number smaller than 1. To increase Hue use a number larger tnan 1

      /// </remarks>

      /// <param name="c">The original colour</param>

      /// <param name="Hue">The Hue delta</param>

      /// <returns>An adjusted colour</returns>

      public static  Color ModifyHue(Color c, double Hue)

      {

         HSL hsl = RGB_to_HSL(c);

         hsl.H*=Hue;

         return HSL_to_RGB(hsl);

      }

      /// <summary>

      /// Converts a colour from HSL to RGB

      /// </summary>

      /// <remarks>Adapted from the algoritm in Foley and Van-Dam</remarks>

      /// <param name="hsl">The HSL value</param>

      /// <returns>A Color structure containing the equivalent RGB values</returns>

      public static Color HSL_to_RGB(HSL hsl)

      {

         double r=0,g=0,b=0;

         double temp1,temp2;

         if(hsl.L==0)

         {

            r=g=b=0;

         }

         else

         {

            if(hsl.S==0)

            {

               r=g=b=hsl.L;

            }

            else

            {

               temp2 = ((hsl.L<=0.5) ? hsl.L*(1.0+hsl.S) : hsl.L+hsl.S-(hsl.L*hsl.S));

               temp1 = 2.0*hsl.L-temp2;

               double[] t3=new double[]{hsl.H+1.0/3.0,hsl.H,hsl.H-1.0/3.0};

               double[] clr=new double[]{0,0,0};

               for(int i=0;i<3;i++)

               {

                  if(t3[i]<0)

                     t3[i]+=1.0;

                  if(t3[i]>1)

                     t3[i]-=1.0;

                  if(6.0*t3[i] < 1.0)

                     clr[i]=temp1+(temp2-temp1)*t3[i]*6.0;

                  else if(2.0*t3[i] < 1.0)

                     clr[i]=temp2;

                  else if(3.0*t3[i] < 2.0)

                     clr[i]=(temp1+(temp2-temp1)*((2.0/3.0)-t3[i])*6.0);

                  else

                     clr[i]=temp1;

               }

               r=clr[0];

               g=clr[1];

               b=clr[2];

            }

         }

         return Color.FromArgb((int)(255*r),(int)(255*g),(int)(255*b));

      }

      //

      /// <summary>

      /// Converts RGB to HSL

      /// </summary>

      /// <remarks>Takes advantage of whats already built in to .NET by using the Color.GetHue, Color.GetSaturation and Color.GetBrightness methods</remarks>

      /// <param name="c">A Color to convert</param>

      /// <returns>An HSL value</returns>

      public static HSL RGB_to_HSL (Color c)

      {

         HSL hsl =  new HSL();

         hsl.H=c.GetHue()/360.0; // we store hue as 0-1 as opposed to 0-360

         hsl.L=c.GetBrightness();

         hsl.S=c.GetSaturation();

         return hsl;

      }

   }

}

VB

' This tool is part of the xRay Toolkit and is provided free of charge by Bob Powell.

' This code is not guaranteed to be free from defects or fit for mechantability in any way.

' By using this tool in your own programs you agree to hold Robert W. Powell free from all

' damages direct or incidental that arise from such use.

' You may use this code free of charge in your own projects on condition that you place the

' following paragraph (enclosed in quotes below) in your applications help or about dialog.

' "Portions of this code provided by Bob Powell. Http://www.bobpowell.net"

Imports System

Imports System.Drawing

Namespace xRay.Toolkit.Utilities

   Public Class RGBHSL

    Public Class HSL

     Public Sub New()

      _h = 0

      _s = 0

      _l = 0

     End Sub 'New

     Private _h As Double

     Private _s As Double

     Private _l As Double

     Public Property H() As Double

      Get

         Return _h

      End Get

      Set

         _h = value

         _h = IIf(_h > 1, 1, IIf(_h < 0, 0, _h))

      End Set

     End Property

     Public Property S() As Double

      Get

         Return _s

      End Get

      Set

         _s = value

         _s = IIf(_s > 1, 1, IIf(_s < 0, 0, _s))

      End Set

     End Property

     Public Property L() As Double

      Get

         Return _l

      End Get

      Set

         _l = value

         _l = IIf(_l > 1, 1, IIf(_l < 0, 0, _l))

      End Set

     End Property

    End Class 'HSL

    Public Sub New()

    End Sub 'New

    '/ <summary>

    '/ Sets the absolute brightness of a colour

    '/ </summary>

    '/ <param name="c">Original colour</param>

    '/ <param name="brightness">The luminance level to impose</param>

    '/ <returns>an adjusted colour</returns>

    Public Shared Function SetBrightness(c As Color, brightness As Double) As Color

     Dim hsl As HSL = RGB_to_HSL(c)

     hsl.L = brightness

     Return HSL_to_RGB(hsl)

    End Function 'SetBrightness

    '/ <summary>

    '/ Modifies an existing brightness level

    '/ </summary>

    '/ <remarks>

    '/ To reduce brightness use a number smaller than 1. To increase brightness use a number larger tnan 1

    '/ </remarks>

    '/ <param name="c">The original colour</param>

    '/ <param name="brightness">The luminance delta</param>

    '/ <returns>An adjusted colour</returns>

    Public Shared Function ModifyBrightness(c As Color, brightness As Double) As Color

     Dim hsl As HSL = RGB_to_HSL(c)

     hsl.L *= brightness

     Return HSL_to_RGB(hsl)

    End Function 'ModifyBrightness

    '/ <summary>

    '/ Sets the absolute saturation level

    '/ </summary>

    '/ <remarks>Accepted values 0-1</remarks>

    '/ <param name="c">An original colour</param>

    '/ <param name="Saturation">The saturation value to impose</param>

    '/ <returns>An adjusted colour</returns>

    Public Shared Function SetSaturation(c As Color, Saturation As Double) As Color

     Dim hsl As HSL = RGB_to_HSL(c)

     hsl.S = Saturation

     Return HSL_to_RGB(hsl)

    End Function 'SetSaturation

    '/ <summary>

    '/ Modifies an existing Saturation level

    '/ </summary>

    '/ <remarks>

    '/ To reduce Saturation use a number smaller than 1. To increase Saturation use a number larger tnan 1

    '/ </remarks>

    '/ <param name="c">The original colour</param>

    '/ <param name="Saturation">The saturation delta</param>

    '/ <returns>An adjusted colour</returns>

    Public Shared Function ModifySaturation(c As Color, Saturation As Double) As Color

     Dim hsl As HSL = RGB_to_HSL(c)

     hsl.S *= Saturation

     Return HSL_to_RGB(hsl)

    End Function 'ModifySaturation

    '/ <summary>

    '/ Sets the absolute Hue level

    '/ </summary>

    '/ <remarks>Accepted values 0-1</remarks>

    '/ <param name="c">An original colour</param>

    '/ <param name="Hue">The Hue value to impose</param>

    '/ <returns>An adjusted colour</returns>

    Public Shared Function SetHue(c As Color, Hue As Double) As Color

     Dim hsl As HSL = RGB_to_HSL(c)

     hsl.H = Hue

     Return HSL_to_RGB(hsl)

    End Function 'SetHue

    '/ <summary>

    '/ Modifies an existing Hue level

    '/ </summary>

    '/ <remarks>

    '/ To reduce Hue use a number smaller than 1. To increase Hue use a number larger tnan 1

    '/ </remarks>

    '/ <param name="c">The original colour</param>

    '/ <param name="Hue">The Hue delta</param>

    '/ <returns>An adjusted colour</returns>

    Public