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Compressed data        Expanded data
03 04        04 04 04
05 06        06 06 06 06 06
00 03 45 56 67 00        45 56 67
02 78        78 78
00 02 05 01        Move 5 right and 1 down
02 78        78 78
00 00        End of line
09 1E        1E 1E 1E 1E 1E 1E 1E 1E 1E
00 01        End of RLE bitmap
Compression of 4-Bits-per-Pixel Bitmaps
When the biCompression member of the BITMAPINFOHEADER structure is set
to BI_RLE4, the DIB is compressed using a run-length encoded format for
a 16-color bitmap. This format uses two modes: encoded mode and absolute
mode.
Encoded Mode
A unit of information in encoded mode consists of two bytes. The first
byte of the pair contains the number of pixels to be drawn using the
color indexes in the second byte. The second byte contains two color
indexes, one in its high-order nibble (that is, its low-order 4 bits)
and one in its low-order nibble. The first pixel is drawn using the
color specified by the high-order nibble, the second is drawn using the
color in the low-order nibble, the third is drawn with the color in the
high-order nibble, and so on, until all the pixels specified by the
first byte have been drawn. The first byte of the pair can be set to
zero to indicate an escape that denotes the end of a line, the end of
the bitmap, or a delta. The interpretation of the escape depends on the
value of the second byte of the pair. In encoded mode, the second byte
has a value in the range 0x00 through 0x02. The meaning of these values
is the same as for a DIB with 8 bits per pixel.
Absolute Mode
In absolute mode, the first byte contains zero, the second byte contains
the number of color indexes that follow, and subsequent bytes contain
color indexes in their high- and low-order nibbles, one color index for
each pixel. Each run must be aligned on a word boundary. Following is an
example of a 4-bit RLE bitmap (the one-digit hexadecimal values in the
second column represent a color index for a single pixel):
Compressed data        Expanded data
03 04        0 4 0
05 06        0 6 0 6 0
00 06 45 56 67 00        4 5 5 6 6 7
04 78        7 8 7 8
00 02 05 01        Move 5 right and 1 down
04 78        7 8 7 8
00 00        End of line
09 1E        1 E 1 E 1 E 1 E 1
00 01        End of RLE bitmap
Bitmap Example
The following example is a text dump of a 16-color bitmap (4 bits per
pixel):
Win3DIBFile
              BitmapFileHeader
                  Type       19778
                  Size       3118
                  Reserved1  0
                  Reserved2  0
                  OffsetBits 118
              BitmapInfoHeader
                  Size            40
                  Width           80
                  Height          75
                  Planes          1
                  BitCount        4
                  Compression     0
                  SizeImage       3000
                  XPelsPerMeter   0
                  YPelsPerMeter   0
                  ColorsUsed      16
                  ColorsImportant 16
              Win3ColorTable
                  Blue  Green  Red  Unused
[00000000]        84    252    84   0
[00000001]        252   252    84   0
[00000002]        84    84     252  0
[00000003]        252   84     252  0
[00000004]        84    252    252  0
[00000005]        252   252    252  0
[00000006]        0     0      0    0
[00000007]        168   0      0    0
[00000008]        0     168    0    0
[00000009]        168   168    0    0
[0000000A]        0     0      168  0
[0000000B]        168   0      168  0
[0000000C]        0     168    168  0
[0000000D]        168   168    168  0
[0000000E]        84    84     84   0
[0000000F]        252   84     84   0
              Image
    .
    .                                           Bitmap data
    .
Hope that helps!
PS The structures that are in the header are:
BITMAPFILEHEADER (3.0)
typedef struct tagBITMAPFILEHEADER {    /* bmfh */
    UINT    bfType;
    DWORD   bfSize;
    UINT    bfReserved1;
    UINT    bfReserved2;
    DWORD   bfOffBits;
} BITMAPFILEHEADER;
The BITMAPFILEHEADER structure contains information about the type,
size, and layout of a
device-independent bitmap (DIB) file.
Member        Description
bfType        Specifies the type of file. This member must be BM.
bfSize        Specifies the size of the file, in bytes.
bfReserved1        Reserved; must be set to zero.
bfReserved2        Reserved; must be set to zero.
bfOffBits        Specifies the byte offset from the BITMAPFILEHEADER
structure to the actual
bitmap data in the file.
Comments
A BITMAPINFO or BITMAPCOREINFO structure immediately follows the
BITMAPFILEHEADER
structure in the DIB file.
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 * SLMR 2.1a * We all live in a yellow subroutine.
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