Converting 16/24 bit image to 8 bit
Bitmap files start with a fileheader, then an infoheader, followed by
the palette entries (unless it's a 24-bit bmp), and finally the bitmap
bits themselves. The only complication is that the palette and bitmap
bits are Dword-aligned, which means that palette entries are RGBQUADs,
and each scan line of bitmap bits has to have an even multiple of 4
bytes (up to 3 padding bytes are added if necessary).
I've got a routine here to save a picturebox as a monochrome bitmap:
you should find it pretty easy to change it for 16- and 256-color
standard palettes (if not, just ask more questions here!). Custom
palettes are quite a bit more work...
Oh, and the picturebox must have AutoRedraw = True, ScaleMode = vbPixel
(3).
Good luck!
Jim Deutch
MS Dev MVP
Private Type BITMAPFILEHEADER '14 bytes
bfType As Integer
bfSize As Long
bfReserved1 As Integer
bfReserved2 As Integer
bfOffBits As Long
End Type
Private Type BITMAPINFOHEADER '40 bytes
biSize As Long
biWidth As Long
biHeight As Long
biPlanes As Integer
biBitCount As Integer
biCompression As Long
biSizeImage As Long
biXPel{*filter*}eter As Long
biYPel{*filter*}eter As Long
biClrUsed As Long
biClrImportant As Long
End Type
Private Type RGBQUAD
rgbBlue As Byte
rgbGreen As Byte
rgbRed As Byte
rgbReserved As Byte
End Type
Private Type BITMAPINFO_1
bmiHeader As BITMAPINFOHEADER
bmiColors(1) As RGBQUAD
End Type
Private Const PIXEL As Integer = 3
Private Const DIB_RGB_COLORS As Long = 0
Private Const PALVERSION = &H300
Private Declare Function GetDIBits1 Lib "gdi32" Alias "GetDIBits" _
(ByVal aHDC As Long, ByVal hBitmap As Long, ByVal nStartScan As Long,
_
ByVal nNumScans As Long, lpBits As Any, lpBI As BITMAPINFO_1, _
ByVal wUsage As Long) As Long
Private Sub Save1(pic As PictureBox)
Dim SaveBitmapInfo_1 As BITMAPINFO_1
Dim SaveFileHeader As BITMAPFILEHEADER
Dim SaveBits() As Byte
Dim BufferSize As Long
Dim fNum As Long
Dim Retval As Long
Dim nLen As Long
Const BitsPixel = 1
'size a buffer for the pixel data
BufferSize = ((pic.ScaleWidth / 8 + 3) And &HFFFC) *
pic.ScaleHeight
ReDim SaveBits(0 To BufferSize - 1)
'fill the header info for the save copy
With SaveBitmapInfo_1.bmiHeader
.biSize = 40
.biWidth = pic.ScaleWidth
.biHeight = pic.ScaleHeight
.biPlanes = 1
.biBitCount = BitsPixel
.biCompression = 0
.biClrUsed = 0
.biClrImportant = 0
.biSizeImage = BufferSize
End With
nLen = Len(SaveBitmapInfo_1)
'get the bitmap from the picturebox
Retval = GetDIBits1(pic.hdc, pic.Image, 0, _
SaveBitmapInfo_1.bmiHeader.biHeight, SaveBits(0),
SaveBitmapInfo_1, DIB_RGB_COLORS)
' create a header for the save file
With SaveFileHeader
.bfType = &H4D42
.bfSize = Len(SaveFileHeader) + nLen + BufferSize
.bfOffBits = Len(SaveFileHeader) + nLen
End With
' save it to disk
fNum = FreeFile
Open m_SaveFileName For Binary As fNum
Put fNum, , SaveFileHeader
Put fNum, , SaveBitmapInfo_1
Put fNum, , SaveBits()
Close fNum
End Sub
Quote:
> Howdy -
> I'm trying to create a customizable screen saver where my client can
enter
> various company names and the names become part of the graphic and
are
> manipulated (distorted etc.) along with the rest of the graphic
image.
> After trying a bunch of ways, I finally got the company name on the
image
> by using the PRINT method and using SAVEPICTURE to save the image.
That's
> fine. The problem I have now is that my app is expecting an 8 bit
image,
> but if the customization is done while the user is in a higher video
mode
> the image is saved as 24 bit.
> Is there any way of fairly quickly converting a 24 bit image to 8 bit
it
> vb? I'm using vb5 if that makes a difference.
> Thanks!
> george
