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 Download the Simple HLS Demo (11kb)
This is a supporting article describing in more detail the Hue Lightness and Saturation
colour model used in the vbAccelerator Image Processor.
The Hue Lightness Saturation Model
Normally in computers colours are described in terms of their Red, Green and Blue components.
Whilst you can specify all displayable colours this way, it leaves something to be desired
when it comes to picking a colour. For example, most people find it very difficult to
determine what RGB values you would use to create a Pink or Brown colour (try it!).
There are numerous colour models which attempt to work around this limitation and provide
a more intuitive way of getting to the colour you want. One of them is the Hue Lightness and
Saturation (HLS) Model.
This HLS model describes colours in the following terms:
- Hue, which is the horizontal axis of square box in the
picture above, and varies from magenta - red - yellow - green - cyan - blue - magenta.
- Saturation, which is the vertical axis of the square box, and describes how "grey" the colour.
- Lightness, which is the second picture box and varies from black through the colour to white.
With this model it is easier to pick the correct colour. The model also allows you to do some
things in code which you can't really achieve with RGB, such as determining what a lighter or
darker tone of a given RGB colour is.
Coding the HLS Model
Normally Hue is expressed as an angle between 0-360 to describe the colour and a value
between 0 and 1 to describe Hue and Saturation. I have missed out the conversion to an angle
in this implementation so the Hue works as follows:
| Hue Value |
Colour |
| -1 |
Magenta |
| 0 |
Red |
| 1 |
Yellow |
| 2 |
Green |
| 3 |
Aqua |
| 4 |
Blue |
| 5 |
Magenta |
Here is the code to do a conversion between HLS and RGB and vice-versa:
Public Sub RGBToHLS( _
    ByVal r As Long, ByVal g As Long, ByVal b As Long, _
    h As Single, s As Single, l As Single _
    )
Dim Max As Single
Dim Min As Single
Dim delta As Single
Dim rR As Single, rG As Single, rB As Single
    rR = r / 255: rG = g / 255: rB = b / 255
'{Given: rgb each in [0,1].
' Desired: h in [0,360] and s in [0,1], except if s=0, then h=UNDEFINED.}
        Max = Maximum(rR, rG, rB)
        Min = Minimum(rR, rG, rB)
            l = (Max + Min) / 2'{This is the lightness}
        '{Next calculate saturation}
        If Max = Min Then
            'begin {Acrhomatic case}
            s = 0
            h = 0
            'end {Acrhomatic case}
        Else
            'begin {Chromatic case}
                '{First calculate the saturation.}
            If l
                s = (Max - Min) / (Max + Min)
            Else
                s = (Max - Min) / (2 - Max - Min)
            End If
            '{Next calculate the hue.}
            delta = Max - Min
            If rR = Max Then
                    h = (rG - rB) / delta'{Resulting color is between yellow and magenta}
            ElseIf rG = Max Then
                h = 2 + (rB - rR) / delta '{Resulting color is between cyan and yellow}
            ElseIf rB = Max Then
                h = 4 + (rR - rG) / delta '{Resulting color is between magenta and cyan}
            End If
        'end {Chromatic Case}
    End If
End Sub
Public Sub HLSToRGB( _
    ByVal h As Single, ByVal s As Single, ByVal l As Single, _
    r As Long, g As Long, b As Long _
    )
Dim rR As Single, rG As Single, rB As Single
Dim Min As Single, Max As Single
    If s = 0 Then
   
' Achromatic case:
    rR = l: rG = l: rB = l
    Else
   
' Chromatic case:
   
' delta = Max-Min
    If l
        's = (Max - Min) / (Max + Min)
       
' Get Min value:
        Min = l * (1 - s)
    Else
        's = (Max - Min) / (2 - Max - Min)
       
' Get Min value:
        Min = l - s * (1 - l)
    End If
   
' Get the Max value:
    Max = 2 * l - Min
   
   
' Now depending on sector we can evaluate the h,l,s:
    If (h
        rR = Max
        If (h
            rG = Min
            rB = rG - h * (Max - Min)
        Else
            rB = Min
            rG = h * (Max - Min) + rB
        End If
    ElseIf (h
        rG = Max
        If (h
            rB = Min
            rR = rB - (h - 2) * (Max - Min)
        Else
            rR = Min
            rB = (h - 2) * (Max - Min) + rR
        End If
    Else
        rB = Max
        If (h
            rR = Min
            rG = rR - (h - 4) * (Max - Min)
        Else
            rG = Min
            rR = (h - 4) * (Max - Min) + rG
        End If
       
    End If
           
    End If
    r = rR * 255: g = rG * 255: b = rB * 255
End Sub
Private Function Maximum(rR As Single, rG As Single, rB As Single) As Single
    If (rR > rG) Then
    If (rR > rB) Then
        Maximum = rR
    Else
        Maximum = rB
    End If
    Else
    If (rB > rG) Then
        Maximum = rB
    Else
        Maximum = rG
    End If
    End If
End Function
Private Function Minimum(rR As Single, rG As Single, rB As Single) As Single
    If (rR
    If (rR
        Minimum = rR
    Else
        Minimum = rB
    End If
    Else
    If (rB
        Minimum = rB
    Else
        Minimum = rG
    End If
End If
End Function
Limitations of the HLS Model
Whilst the HLS Model is simple to implement, and gives a relatively easy colour picking interface,
it is not a physically accurate model. That is to say, linear changes in lightness, hue or
saturation do not correspond to the a linear change to the eye. More accurate models which
overcome these limitations exist, such as the CIE scheme, but I've yet to understand these
enough to work them out! Anyway, HLS works well enough unless you are interested in exact colour
matching and processing between different display devices.
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