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bmp_structure.txt

Graphics File Formats

This topic describes the graphics-file formats used by the Microsoft Windows
operating system. Graphics files include bitmap files, icon-resource files,
and cursor-resource files.

Bitmap-File Formats

Windows bitmap files are stored in a device-independent bitmap (DIB) format
that allows Windows to display the bitmap on any type of display device. The
term "device independent" means that the bitmap specifies pixel color in a
form independent of the method used by a display to represent color. The
default filename extension of a Windows DIB file is .BMP.

Bitmap-File Structures

Each bitmap file contains a bitmap-file header, a bitmap-information header,
a color table, and an array of bytes that defines the bitmap bits. The file
has the following form:

BITMAPFILEHEADER bmfh;
BITMAPINFOHEADER bmih;
RGBQUAD          aColors[];
BYTE             aBitmapBits[];

The bitmap-file header contains information about the type, size, and layout
of a device-independent bitmap file. The header is defined as a
BITMAPFILEHEADER structure.

The bitmap-information header, defined as a BITMAPINFOHEADER structure,
specifies the dimensions, compression type, and color format for the bitmap.

The color table, defined as an array of RGBQUAD structures, contains as many
elements as there are colors in the bitmap. The color table is not present
for bitmaps with 24 color bits because each pixel is represented by 24-bit
red-green-blue (RGB) values in the actual bitmap data area. The colors in the
table should appear in order of importance. This helps a display driver
render a bitmap on a device that cannot display as many colors as there are
in the bitmap. If the DIB is in Windows version 3.0 or later format, the
driver can use the biClrImportant member of the BITMAPINFOHEADER structure to
determine which colors are important.

The BITMAPINFO structure can be used to represent a combined
bitmap-information header and color table.  The bitmap bits, immediately
following the color table, consist of an array of BYTE values representing
consecutive rows, or "scan lines," of the bitmap. Each scan line consists of
consecutive bytes representing the pixels in the scan line, in left-to-right
order. The number of bytes representing a scan line depends on the color
format and the width, in pixels, of the bitmap. If necessary, a scan line
must be zero-padded to end on a 32-bit boundary. However, segment boundaries
can appear anywhere in the bitmap. The scan lines in the bitmap are stored
from bottom up. This means that the first byte in the array represents the
pixels in the lower-left corner of the bitmap and the last byte represents
the pixels in the upper-right corner.

The biBitCount member of the BITMAPINFOHEADER structure determines the number
of bits that define each pixel and the maximum number of colors in the
bitmap. These members can have any of the following values:

Value   Meaning

1       Bitmap is monochrome and the color table contains two entries. Each
bit in the bitmap array represents a pixel. If the bit is clear, the pixel is
displayed with the color of the first entry in the color table. If the bit is
set, the pixel has the color of the second entry in the table.

4       Bitmap has a maximum of 16 colors. Each pixel in the bitmap is
represented by a 4-bit index into the color table. For example, if the first
byte in the bitmap is 0x1F, the byte represents two pixels. The first pixel
contains the color in the second table entry, and the second pixel contains
the color in the sixteenth table entry.

8       Bitmap has a maximum of 256 colors. Each pixel in the bitmap is
represented by a 1-byte index into the color table. For example, if the first
byte in the bitmap is 0x1F, the first pixel has the color of the
thirty-second table entry.

24      Bitmap has a maximum of 2^24 colors. The bmiColors (or bmciColors)
member is NULL, and each 3-byte sequence in the bitmap array represents the
relative intensities of red, green, and blue, respectively, for a pixel.

The biClrUsed member of the BITMAPINFOHEADER structure specifies the number
of color indexes in the color table actually used by the bitmap. If the
biClrUsed member is set to zero, the bitmap uses the maximum number of colors
corresponding to the value of the biBitCount member.  An alternative form of
bitmap file uses the BITMAPCOREINFO, BITMAPCOREHEADER, and RGBTRIPLE
structures.

Bitmap Compression

Windows versions 3.0 and later support run-length encoded (RLE) formats for
compressing bitmaps that use 4 bits per pixel and 8 bits per pixel.
Compression reduces the disk and memory storage required for a bitmap.

Compression of 8-Bits-per-Pixel Bitmaps

When the biCompression member of the BITMAPINFOHEADER structure is set to
BI_RLE8, the DIB is compressed using a run-length encoded format for a
256-color bitmap. This format uses two modes: encoded mode and absolute mode.
Both modes can occur anywhere throughout a single bitmap.

Encoded Mode

A unit of information in encoded mode consists of two bytes. The first byte
specifies the number of consecutive pixels to be drawn using the color index
contained in the second byte.  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, which must be in the range 0x00 through 0x02.
Following are the meanings of the escape values that can be used in the
second byte:

Second byte     Meaning

0       End of line. 
1       End of bitmap. 
2       Delta. The two bytes following the escape contain unsigned values
indicating the horizontal and vertical offsets of the next pixel from the
current position.

Absolute Mode

Absolute mode is signaled by the first byte in the pair being set to zero and
the second byte to a value between 0x03 and 0xFF. The second byte represents
the number of bytes that follow, each of which contains the color index of a
single pixel. Each run must be aligned on a word boundary.  Following is an
example of an 8-bit RLE bitmap (the two-digit hexadecimal values in the
second column represent a color index for a single pixel):

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
    .

Icon-Resource File Format

An icon-resource file contains image data for icons used by Windows
applications. The file consists of an icon directory identifying the number
and types of icon images in the file, plus one or more icon images. The
default filename extension for an icon-resource file is .ICO.

Icon Directory

Each icon-resource file starts with an icon directory. The icon directory,
defined as an ICONDIR structure, specifies the number of icons in the
resource and the dimensions and color format of each icon image. The ICONDIR
structure has the following form:



typedef struct ICONDIR {
    WORD          idReserved;
    WORD          idType;
    WORD          idCount;
    ICONDIRENTRY  idEntries[1];
} ICONHEADER;

Following are the members in the ICONDIR structure:

idReserved      Reserved; must be zero. 
idType          Specifies the resource type. This member is set to 1. 
idCount         Specifies the number of entries in the directory. 
idEntries       Specifies an array of ICONDIRENTRY structures containing
information about individual icons. The idCount member specifies the number
of structures in the array.

The ICONDIRENTRY structure specifies the dimensions and color format for an
icon. The structure has the following form:



struct IconDirectoryEntry {
    BYTE  bWidth;
    BYTE  bHeight;
    BYTE  bColorCount;
    BYTE  bReserved;
    WORD  wPlanes;
    WORD  wBitCount;
    DWORD dwBytesInRes;
    DWORD dwImageOffset;
};

Following are the members in the ICONDIRENTRY structure: 

bWidth          Specifies the width of the icon, in pixels. Acceptable values
are 16, 32, and 64.

bHeight         Specifies the height of the icon, in pixels. Acceptable
values are 16, 32, and 64.

bColorCount     Specifies the number of colors in the icon. Acceptable values
are 2, 8, and 16.

bReserved       Reserved; must be zero. 
wPlanes         Specifies the number of color planes in the icon bitmap. 
wBitCount       Specifies the number of bits in the icon bitmap. 
dwBytesInRes    Specifies the size of the resource, in bytes. 
dwImageOffset   Specifies the offset, in bytes, from the beginning of the
file to the icon image.

Icon Image

Each icon-resource file contains one icon image for each image identified in
the icon directory. An icon image consists of an icon-image header, a color
table, an XOR mask, and an AND mask. The icon image has the following form:



BITMAPINFOHEADER    icHeader;
RGBQUAD             icColors[];
BYTE                icXOR[];
BYTE                icAND[];

The icon-image header, defined as a BITMAPINFOHEADER structure, specifies the
dimensions and color format of the icon bitmap. Only the biSize through
biBitCount members and the biSizeImage member are used. All other members
(such as biCompression and biClrImportant) must be set to zero.

The color table, defined as an array of RGBQUAD structures, specifies the
colors used in the XOR mask. As with the color table in a bitmap file, the
biBitCount member in the icon-image header determines the number of elements
in the array. For more information about the color table, see Section 1.1,
"Bitmap-File Formats."

The XOR mask, immediately following the color table, is an array of BYTE
values representing consecutive rows of a bitmap. The bitmap defines the
basic shape and color of the icon image. As with the bitmap bits in a bitmap
file, the bitmap data in an icon-resource file is organized in scan lines,
with each byte representing one or more pixels, as defined by the color
format. For more information about these bitmap bits, see Section 1.1,
"Bitmap-File Formats."

The AND mask, immediately following the XOR mask, is an array of BYTE values,
representing a monochrome bitmap with the same width and height as the XOR
mask. The array is organized in scan lines, with each byte representing 8
pixels.

When Windows draws an icon, it uses the AND and XOR masks to combine the icon
image with the pixels already on the display surface. Windows first applies
the AND mask by using a bitwise AND operation; this preserves or removes
existing pixel color.  Windows then applies the XOR mask by using a bitwise
XOR operation. This sets the final color for each pixel.

The following illustration shows the XOR and AND masks that create a
monochrome icon (measuring 8 pixels by 8 pixels) in the form of an uppercase
K:

Windows Icon Selection

Windows detects the resolution of the current display and matches it against
the width and height specified for each version of the icon image. If Windows
determines that there is an exact match between an icon image and the current
device, it uses the matching image. Otherwise, it selects the closest match
and stretches the image to the proper size.

If an icon-resource file contains more than one image for a particular
resolution, Windows uses the icon image that most closely matches the color
capabilities of the current display. If no image matches the device
capabilities exactly, Windows selects the image that has the greatest number
of colors without exceeding the number of display colors. If all images
exceed the color capabilities of the current display, Windows uses the icon
image with the least number of colors.



Cursor-Resource File Format

A cursor-resource file contains image data for cursors used by Windows
applications. The file consists of a cursor directory identifying the number
and types of cursor images in the file, plus one or more cursor images. The
default filename extension for a cursor-resource file is .CUR.

Cursor Directory

Each cursor-resource file starts with a cursor directory. The cursor
directory, defined as a CURSORDIR structure, specifies the number of cursors
in the file and the dimensions and color format of each cursor image. The
CURSORDIR structure has the following form:


typedef struct _CURSORDIR {
    WORD           cdReserved;
    WORD           cdType;
    WORD           cdCount;
    CURSORDIRENTRY cdEntries[];
} CURSORDIR;

Following are the members in the CURSORDIR structure: 

cdReserved      Reserved; must be zero. 
cdType          Specifies the resource type. This member must be set to 2. 
cdCount         Specifies the number of cursors in the file. 
cdEntries       Specifies an array of CURSORDIRENTRY structures containing
information about individual cursors. The cdCount member specifies the number
of structures in the array.

A CURSORDIRENTRY structure specifies the dimensions and color format of a
cursor image. The structure has the following form:



typedef struct _CURSORDIRENTRY {
    BYTE  bWidth;
    BYTE  bHeight;
    BYTE  bColorCount;
    BYTE  bReserved;
    WORD  wXHotspot;
    WORD  wYHotspot;
    DWORD lBytesInRes;
    DWORD dwImageOffset;
} CURSORDIRENTRY;

Following are the members in the CURSORDIRENTRY structure: 

bWidth          Specifies the width of the cursor, in pixels. 
bHeight         Specifies the height of the cursor, in pixels. 
bColorCount     Reserved; must be zero. 
bReserved       Reserved; must be zero.
wXHotspot       Specifies the x-coordinate, in pixels, of the hot spot. 
wYHotspot       Specifies the y-coordinate, in pixels, of the hot spot. 
lBytesInRes     Specifies the size of the resource, in bytes. 
dwImageOffset   Specifies the offset, in bytes, from the start of the file to
the cursor image.

Cursor Image

Each cursor-resource file contains one cursor image for each image identified
in the cursor directory. A cursor image consists of a cursor-image header, a
color table, an XOR mask, and an AND mask. The cursor image has the following
form:



BITMAPINFOHEADER    crHeader;
RGBQUAD             crColors[];
BYTE                crXOR[];
BYTE                crAND[];

The cursor hot spot is a single pixel in the cursor bitmap that Windows uses
to track the cursor. The crXHotspot and crYHotspot members specify the x- and
y-coordinates of the cursor hot spot. These coordinates are 16-bit integers.

The cursor-image header, defined as a BITMAPINFOHEADER structure, specifies
the dimensions and color format of the cursor bitmap. Only the biSize through
biBitCount members and the biSizeImage member are used. The biHeight member
specifies the combined height of the XOR and AND masks for the cursor. This
value is twice the height of the XOR mask. The biPlanes and biBitCount
members must be 1. All other members (such as biCompression and
biClrImportant) must be set to zero.

The color table, defined as an array of RGBQUAD structures, specifies the
colors used in the XOR mask. For a cursor image, the table contains exactly
two structures, since the biBitCount member in the cursor-image header is
always 1.

The XOR mask, immediately following the color table, is an array of BYTE
values representing consecutive rows of a bitmap. The bitmap defines the
basic shape and color of the cursor image. As with the bitmap bits in a
bitmap file, the bitmap data in a cursor-resource file is organized in scan
lines, with each byte representing one or more pixels, as defined by the
color format. For more information about these bitmap bits, see Section 1.1,
"Bitmap-File Formats."

The AND mask, immediately following the XOR mask, is an array of BYTE values
representing a monochrome bitmap with the same width and height as the XOR
mask. The array is organized in scan lines, with each byte representing 8
pixels.

When Windows draws a cursor, it uses the AND and XOR masks to combine the
cursor image with the pixels already on the display surface. Windows first
applies the AND mask by using a bitwise AND operation; this preserves or
removes existing pixel color.  Window then applies the XOR mask by using a
bitwise XOR operation. This sets the final color for each pixel.

The following illustration shows the XOR and the AND masks that create a
cursor (measuring 8 pixels by 8 pixels) in the form of an arrow:

Following are the bit-mask values necessary to produce black, white,
inverted, and transparent results:

Pixel result    AND maskXOR mask

Black           0               0 
White           0               1 
Transparent     1               0 
Inverted1               1 

Windows Cursor Selection

If a cursor-resource file contains more than one cursor image, Windows
determines the best match for a particular display by examining the width and
height of the cursor images.


==============================================================================


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.

See Also

BITMAPCOREINFO, BITMAPINFO 


==============================================================================
BITMAPINFO (3.0)



typedef struct tagBITMAPINFO {  /* bmi */
    BITMAPINFOHEADER    bmiHeader;
    RGBQUAD             bmiColors[1];
} BITMAPINFO;

The BITMAPINFO structure fully defines the dimensions and color information
for a Windows 3.0 or later device-independent bitmap (DIB).

Member          Description

bmiHeader       Specifies a BITMAPINFOHEADER structure that contains
information about the dimensions and color format of a DIB.

bmiColors       Specifies an array of RGBQUAD structures that define the
colors in the bitmap.

Comments

A Windows 3.0 or later DIB consists of two distinct parts: a BITMAPINFO
structure, which describes the dimensions and colors of the bitmap, and an
array of bytes defining the pixels of the bitmap. The bits in the array are
packed together, but each scan line must be zero-padded to end on a LONG
boundary. Segment boundaries, however, can appear anywhere in the bitmap. The
origin of the bitmap is the lower-left corner.

The biBitCount member of the BITMAPINFOHEADER structure determines the number
of bits which define each pixel and the maximum number of colors in the
bitmap. This member may be set to any of the following values:

Value   Meaning

1       The bitmap is monochrome, and the bmciColors member must contain two
entries. Each bit in the bitmap array represents a pixel. If the bit is
clear, the pixel is displayed with the color of the first entry in the
bmciColors table. If the bit is set, the pixel has the color of the second
entry in the table.

4       The bitmap has a maximum of 16 colors, and the bmciColors member
contains 16 entries. Each pixel in the bitmap is represented by a four-bit
index into the color table.

For example, if the first byte in the bitmap is 0x1F, the byte represents two
pixels. The first pixel contains the color in the second table entry, and the
second pixel contains the color in the sixteenth table entry.

8       The bitmap has a maximum of 256 colors, and the bmciColors member
contains 256 entries. In this case, each byte in the array represents a
single pixel.

24      The bitmap has a maximum of 2^24 colors. The bmciColors member is
NULL, and each 3-byte sequence in the bitmap array represents the relative
intensities of red, green, and blue, respectively, of a pixel.

The biClrUsed member of the BITMAPINFOHEADER structure specifies the number
of color indexes in the color table actually used by the bitmap. If the
biClrUsed member is set to zero, the bitmap uses the maximum number of colors
corresponding to the value of the biBitCount member.

The colors in the bmiColors table should appear in order of importance.
Alternatively, for functions that use DIBs, the bmiColors member can be an
array of 16-bit unsigned integers that specify an index into the currently
realized logical palette instead of explicit RGB values. In this case, an
application using the bitmap must call DIB functions with the wUsage
parameter set to DIB_PAL_COLORS.

Note:   The bmiColors member should not contain palette indexes if the bitmap
is to be stored in a file or transferred to another application. Unless the
application uses the bitmap exclusively and under its complete control, the
bitmap color table should contain explicit RGB values.

See Also

BITMAPINFOHEADER, RGBQUAD 

==============================================================================
BITMAPINFOHEADER (3.0)



typedef struct tagBITMAPINFOHEADER {    /* bmih */
    DWORD   biSize;
    LONG    biWidth;
    LONG    biHeight;
    WORD    biPlanes;
    WORD    biBitCount;
    DWORD   biCompression;
    DWORD   biSizeImage;
    LONG    biXPelsPerMeter;
    LONG    biYPelsPerMeter;
    DWORD   biClrUsed;
    DWORD   biClrImportant;
} BITMAPINFOHEADER;

The BITMAPINFOHEADER structure contains information about the dimensions and
color format of a Windows 3.0 or later device-independent bitmap (DIB).

Member          Description

biSize          Specifies the number of bytes required by the
BITMAPINFOHEADER structure.

biWidth         Specifies the width of the bitmap, in pixels. 
biHeightSpecifies the height of the bitmap, in pixels. 

biPlanesSpecifies the number of planes for the target device. This
member must be set to 1.

biBitCount      Specifies the number of bits per pixel. This value must be 1,
4, 8, or 24.

biCompression   Specifies the type of compression for a compressed bitmap. It
can be one of the following values:

Value           Meaning

BI_RGB          Specifies that the bitmap is not compressed. 

BI_RLE8         Specifies a run-length encoded format for bitmaps with 8 bits
per pixel. The compression format is a 2-byte format consisting of a count
byte followed by a byte containing a color index.  For more information, see
the following Comments section.

BI_RLE4         Specifies a run-length encoded format for bitmaps with 4 bits
per pixel. The compression format is a 2-byte format consisting of a count
byte followed by two word-length color indexes.  For more information, see
the following Comments section.

biSizeImage     Specifies the size, in bytes, of the image. It is valid to
set this member to zero if the bitmap is in the BI_RGB format.

biXPelsPerMeter Specifies the horizontal resolution, in pixels per meter, of
the target device for the bitmap. An application can use this value to select
a bitmap from a resource group that best matches the characteristics of the
current device.

biYPelsPerMeter Specifies the vertical resolution, in pixels per meter, of
the target device for the bitmap.

biClrUsed       Specifies the number of color indexes in the color table
actually used by the bitmap. If this value is zero, the bitmap uses the
maximum number of colors corresponding to the value of the biBitCount member.
For more information on the maximum sizes of the color table, see the
description of the BITMAPINFO structure earlier in this topic.

If the biClrUsed member is nonzero, it specifies the actual number of colors
that the graphics engine or device driver will access if the biBitCount
member is less than 24. If biBitCount is set to 24, biClrUsed specifies the
size of the reference color table used to optimize performance of Windows
color palettes.  If the bitmap is a packed bitmap (that is, a bitmap in which
the bitmap array immediately follows the BITMAPINFO header and which is
referenced by a single pointer), the biClrUsed member must be set to zero or
to the actual size of the color table.

biClrImportant  Specifies the number of color indexes that are considered
important for displaying the bitmap. If this value is zero, all colors are
important.

Comments

The BITMAPINFO structure combines the BITMAPINFOHEADER structure and a color
table to provide a complete definition of the dimensions and colors of a
Windows 3.0 or later DIB. For more information about specifying a Windows 3.0
DIB, see the description of the BITMAPINFO structure.

An application should use the information stored in the biSize member to
locate the color table in a BITMAPINFO structure as follows:

pColor = ((LPSTR) pBitmapInfo + (WORD) (pBitmapInfo->bmiHeader.biSize))

Windows supports formats for compressing bitmaps that define their colors
with 8 bits per pixel and with 4 bits per pixel. Compression reduces the disk
and memory storage required for the bitmap. The following paragraphs describe
these formats.

BI_RLE8

When the biCompression member is set to BI_RLE8, the bitmap is compressed
using a run-length encoding format for an 8-bit bitmap. This format may be
compressed in either of two modes: encoded and absolute. Both modes can occur
anywhere throughout a single bitmap.

Encoded mode consists of two bytes: the first byte specifies the number of
consecutive pixels to be drawn using the color index contained in the second
byte. In addition, the first byte of the pair can be set to zero to indicate
an escape that denotes an end of line, end of bitmap, or