Gamma Correction

How to apply a gamma correction filter to an image

A Gamma Correction filter is primarily used for colour matching purposes on CRT based displays, but it can also be used for creative image processing. This article shows how the gamma filter is calculated and provides an implementation.

About the Gamma Filter

Gamma correction was developed to make it easier to adjust colours displayed on Cathode Ray Tube (CRT) displays. CRT displays produce a light intensity (luminance) proportional to the input voltage raised to a power. Since no two CRTs are exactly alike in their luminance characteristic, a way of adjusting the input image so the displayed colours match a reference is needed. This is done by adjusting the colours to be displayed by a power which is termed gamma.

Since gamma lightens or darkens the colours in an image it can be used for image processing effects as well as the normal colour profile adjustment. Gamma is normally restricted to the range ¹/₅ to 5, where a value less than 1 lightens the image, a value of 1 leaves the image unaffected and an value greater than 1 darkens the image:

Calculating Gamma

To apply a gamma filter, each colour is factored using the inverse power of the gamma value:

Since the gamma calculation involves powers, and for a given value of gamma and colour the output colour value is constant, it makes sense to pre-calculate the new colour values and store them in an array so the calculation doesn't need to be repeated each time. Then the new value can be looked up simply as the index of the entry in a one-dimensional array:

Private m_fGamma As Double
Private m_red(0 To 255) As Byte
Private m_green(0 To 255) As Byte
Private m_blue(0 To 255) As Byte

Private Sub createGammaTable()
Dim i As Long
Dim lValue As Long
   For i = 0 To 255
      lValue = (255# * ((i / 255#) ^ (1# / m_fGamma))) + 0.5
      If (lValue > 255) Then lValue = 255
      m_red(i) = lValue
      m_green(i) = lValue
      m_blue(i) = lValue
   Next i
End Sub

Note that the red, green and blue gamma values are all set to the same amount in this example. A different gamma could be used for each to achieve colourisation effects.

Applying The Filter

Once the gamma arrays have been calculated it is relatively simple to apply them to an image using a DIB Section. For each pixel in the image array, read the red, green and blue values and then replace them with their lookup within the gamma tables:

      Dim bDib() As Byte
      Dim bDibDst() As Byte
      Dim tSA As SAFEARRAY2D
      Dim tSADst As SAFEARRAY2D
    
      ' Get the bits in the from DIB section:
      With tSA
          .cbElements = 1
          .cDims = 2
          .Bounds(0).lLbound = 0
          .Bounds(0).cElements = cSrc.Height
          .Bounds(1).lLbound = 0
          .Bounds(1).cElements = cSrc.BytesPerScanLine
          .pvData = cSrc.DIBSectionBitsPtr
      End With
      CopyMemory ByVal VarPtrArray(bDib()), VarPtr(tSA), 4
   
      ' Get the bits in the from DIB section:
      With tSADst
          .cbElements = 1
          .cDims = 2
          .Bounds(0).lLbound = 0
          .Bounds(0).cElements = cDst.Height
          .Bounds(1).lLbound = 0
          .Bounds(1).cElements = cDst.BytesPerScanLine()
          .pvData = cDst.DIBSectionBitsPtr
      End With
      CopyMemory ByVal VarPtrArray(bDibDst()), VarPtr(tSADst), 4
      
      Dim x As Long
      Dim y As Long
      Dim xEnd As Long
      Dim yEnd As Long
      
      xEnd = cSrc.BytesPerScanLine() - 3
      yEnd = cSrc.Height - 1
      
      For x = 0 To xEnd Step 3
         For y = 0 To yEnd
            bDibDst(x + 2, y) = m_red(bDib(x + 2, y))
            bDibDst(x + 1, y) = m_green(bDib(x + 1, y))
            bDibDst(x, y) = m_blue(bDib(x, y))
         Next y
      Next x
      
      CopyMemory ByVal VarPtrArray(bDibDst), 0&, 4
      CopyMemory ByVal VarPtrArray(bDib), 0&, 4

Conclusion

The sample code demonstrates applying gamma to a True Colour DIB Section. This type of code can be used as a simple way to highlight, brighten or darken images.