Victor Image Processing Library How-to Tips

Manipulate the Palette

• Copy the palette into a local array of type RGBQUAD.
• Read the red, green, and blue components of each palette entry by accessing each element of the array.
• Alter the RGB components and replace them in the local array.
• Replace the altered palette by copying the array back into the image palette.

Original

Altered Palette

In this example each palette entry is replaced with a gray tone representing the brightest of the RGB components. This is not a true grayscale, but gives the appearance of grayscale. To create a true grayscale image use the colortogray function in the Victor Library.

Manipulate the Palette - the Visual Basic Source Code

' ........... Defines and declarations ...........

Type RGBQUAD
   rgbBlue As Byte
   rgbGreen As Byte
   rgbRed As Byte
   rgbReserved As Byte
End Type

Declare Function updatebitmapcolortable Lib "VIC32.DLL" (image As imgdes) As Long
Declare Sub CopyPaletteFromImagetoRGBArray Lib "kernel32" Alias "RtlMoveMemory" (ByRef desarrayelement As RGBQUAD, ByVal paletteaddress As Long, ByVal cnt As Long)
Declare Sub CopyPaletteFromRGBArraytoImage Lib "kernel32" Alias "RtlMoveMemory" (ByVal paletteaddress As Long, ByRef srcarrayelement As RGBQUAD, ByVal cnt As Long)

' ........... The function ...........
Public Function changethepalette(ByRef myimage As imgdes) As Long
Dim rcode As Long
Dim j As Integer
Dim colours As Long
Dim mypalette(0 To 255) As RGBQUAD
Dim red As Byte
Dim grn As Byte
Dim blu As Byte
Dim newcolor As Byte

    colours = myimage.colors
    
    ' Copy the image palette into my local palette
    CopyPaletteFromImagetoRGBArray mypalette(0), myimage.palette, colours * 4
        
    For j = 0 To (colours - 1)
        blu = mypalette(j).rgbBlue
        grn = mypalette(j).rgbGreen
        red = mypalette(j).rgbRed
        
        ' ...............  manipulate the palette ...................
        
        ' Pick the brightest colors and create a palette of grays
        newcolor = blu
        If (red > newcolor) Then
            newcolor = red
        End If
        If (grn > newcolor) Then
            newcolor = grn
        End If
        
        blu = newcolor
        grn = newcolor
        red = newcolor
        ' ...........................................................
        
        ' insert the new palette entries
        mypalette(j).rgbBlue = blu
        mypalette(j).rgbGreen = grn
        mypalette(j).rgbRed = red

    Next j
        
    CopyPaletteFromRGBArraytoImage myimage.palette, mypalette(0), colours * 4
    rcode = updatebitmapcolortable(myimage)
        
    changethepalette = rcode
End Function

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