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PackBits is a fast, simple lossless compression scheme for run-length encoding of data. Apple introduced the PackBits format with the release of MacPaint on the Macintosh computer. This compression scheme is one of the types of compression that can be used in TIFF-files. TGA-files also use this RLE compression scheme, but treats data stream as pixels instead of bytes. A PackBits data stream consists of packets with a one-byte header followed by data. The header is a signed byte; the data can be signed, unsigned, or packed (such as In the following table, ''n'' is the value of the header byte as a signed integer. Note that interpreting 0 as positive or negative makes no difference in the output. Runs of two bytes adjacent to non-runs are typically written as literal data. It should also be noted that there is no way based on the PackBits data to determine the end of the data stream; that is to say, one must already know the size of the compressed or uncompressed data before reading a PackBits data stream to know where it ends. Apple Computer (see the external link) provides this short example of packed data: FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AAThe following code, written in Microsoft VBA, unpacks the data: Sub UnpackBitsDemo() Dim File As Variant Dim MyOutput As String Dim Count As Long Dim i As Long, j As Long File = "FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AA" File = Split(File, " ") For i = LBound(File) To UBound(File) Count = Application.WorksheetFunction.Hex2Dec(File(i)) Select Case Count Case Is >= 128 Count = 256 - Count 'Two's Complement For j = 0 To Count 'zero-based MyOutput = MyOutput & File(i + 1) & " " Next j i = i + 1 'Adjust the pointer Case Else For j = 0 To Count 'zero-based MyOutput = MyOutput & File(i + j + 1) & " " Next j i = i + j 'Adjust the pointer End Select Next i Debug.Print MyOutput 'AA AA AA 80 00 2A AA AA AA AA 80 00 2A 22 AA AA AA AA AA AA AA AA AA AA' End Sub == External links== *(Apple webpage describing the PackBits format ) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「PackBits」の詳細全文を読む スポンサード リンク
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