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eviously then the pixels in 8-bit bitmaps are not color values, but instead indexes into a color table. The color table holds 256 different 24-bit colors. So what we have to do in order to obtain the effects (all beside the ripple algorithm) is to manipulate this color table, instead of the pixels.
A Color Table in 8-bit bitmaps is an array of 256 RGBQUAD structures. The RGBQUAD structure is defined as such:
Private Type RGBQUAD
rgbBlue As Byte
rgbGreen As Byte
rgbRed As Byte
rgbReserved As Byte
End Type
The rgbRed, rgbBlue and rgbGreen members combine to make the 24-bit color value of the given index. The rgbReserved member is reserved and must therefore always be 0.
To get the color table of an 8-bit bitmap, we will use the GetDIBColorTable API function. This function is declared as such:
Private Declare Function GetDIBColorTable Lib "gdi32" (ByVal hDC As Long, _
ByVal un1 As Long, ByVal un2 As Long, pRGBQuad As RGBQUAD) As Long
The first parameter, hDC, is the device context to retrieve the color table from. The second parameter un1, is the start index of the color table you wish to get. The third parameter un2, is the number of indexes you want to retrieve. The last parameter pRGBQuad is the first index into an array of RGBQUAD structures, which will be filled with the color table.
The function to set a color table back to a device context is the SetDIBColorTable API function. It looks like this:
Private Declare Function SetDIBColorTable Lib "gdi32" (ByVal hDC As Long, _
ByVal un1 As Long, ByVal un2 As Long, pcRGBQuad As RGBQUAD) As Long
The parameters are the same as with the GetDIBColorTable.
Now that we know how to set and get the color table of an 8-bit bitmap, let抯 get to work on applying the effects. The sample project in BIT8BITMAPS.ZIP , demonstrates the same effects as the BITMAP project did with the 24-bit bitmaps.
Since we are now working with color tables instead of pixels, we can do much of the hard work in the initializing phase of the application. This means that we can build the color tables for the Gray, Red, Blue, Green and Invert tables, before applying any of the effects.
The tables are generated in the CreateColorTables() procedure:
Private Sub CreateColorTables()
Dim I As Long
Dim TempValue As Long
For I = LBound(GrayTable) To UBound(GrayTable)
''''Create Gray Color table
''''Add the values together
TempValue = OriginalTable(I).rgbBlue
TempValue = TempValue + OriginalTable(I).rgbGreen
TempValue = TempValue + OriginalTable(I).rgbRed
''''Get the medium value
TempValue = TempValue / 3
''''Set the color in the gray table
GrayTable(I).rgbBlue = TempValue
GrayTable(I).rgbGreen = TempValue
GrayTable(I).rgbRed = TempValue
''''Create the rest of the color tables
RedTable(I).rgbBlue = 0
RedTable(I).rgbGreen = 0
RedTable(I).rgbRed = OriginalTable(I).rgbRed
GreenTable(I).rgbBlue = 0
GreenTable(I).rgbRed = 0
GreenTable(I).rgbGreen = OriginalTable(I).rgbGreen
BlueTable(I).rgbBlue = OriginalTable(I).rgbBlue
BlueTable(I).rgbGreen = 0
BlueTable(I).rgbRed = 0
InvertTable(I).rgbBlue = 255 - OriginalTable(I).rgbBlue
InvertTable(I).rgbGreen = 255 - OriginalTable(I).rgbGreen
InvertTable(I).rgbRed = 255 - OriginalTable(I).rgbRed
Next I
End Sub
As you can see the actual algorithms are the same. The gray effect is still implemented by adding the three color-values and dividing the result by three. The same thing goes for the color effects. The color, which should be dominant, is kept, while the rest is set to 0.
The ripple effect is implemented in the same way as for 24-bit bitmaps, by manipulating the actual positions of the pixels instead of the color of the pixels. But since each pixel is only 8-bits we do not have to implement any special iteration when we set the pixel positions.
The brightness effect is generated in the same fashion as with the 24-bit bitmaps, by first making a lookup table and then applying these effects into the color table.
This concludes our little expedition into bitmapped country. There is much more to it than what is written here, but you should now have a general idea on how to manipulate with bitmap pixels in order to get some special effects into your game.
End of Part III of the three part Drawing and Animation Tutorial by Burt Abreu & S鴕en Skov.
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