Microsoft Windows Bitmap Format



Note: the constants BI_RGB, BI_RLE8, and BI_RLE4 have the values 0, 1, and 2,
	respectively.

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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.