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Description of BMP bitmap file formats.
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Description of BMP bitmap file formats.
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1.1 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.

1.1.1 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 224 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.


BITMAPCOREHEADER

3.0



typedef struct tagBITMAPCOREHEADER { /* bmch */
DWORD bcSize;
short bcWidth;
short bcHeight;
WORD bcPlanes;
WORD bcBitCount;
} BITMAPCOREHEADER;

The BITMAPCOREHEADER structure contains information about the
dimensions and color format of a device-independent bitmap (DIB).
Windows applications should use the BITMAPINFOHEADER structure instead
of BITMAPCOREHEADER whenever possible.

Members

bcSize

Specifies the number of bytes required by the BITMAPCOREHEADER
structure.

bcWidth

Specifies the width of the bitmap, in pixels.

bcHeight

Specifies the height of the bitmap, in pixels.

bcPlanes

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

bcBitCount

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

Comments

The BITMAPCOREINFO structure combines the BITMAPCOREHEADER structure
and a color table to provide a complete definition of the dimensions
and colors of a DIB. See the description of the BITMAPCOREINFO
structure for more information about specifying a DIB.
An application should use the information stored in the bcSize member
to locate the color table in a BITMAPCOREINFO structure with a method
such as the following:



pColor = ((LPSTR) pBitmapCoreInfo + (WORD) (pBitmapCoreInfo ->
bcSize))



BITMAPCOREINFO

3.0

typedef struct tagBITMAPCOREINFO { /* bmci */
BITMAPCOREHEADER bmciHeader;
RGBTRIPLE bmciColors[1];
} BITMAPCOREINFO;

The BITMAPCOREINFO structure fully defines the dimensions and color
information for a device-independent bitmap (DIB). Windows
applications should use the BITMAPINFO structure instead of
BITMAPCOREINFO whenever possible.

Members

bmciHeader

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

bmciColors

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

Comments

The BITMAPCOREINFO structure describes the dimensions and colors of a
bitmap. It is followed immediately in memory by an array of bytes
which define 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 bcBitCount member of the BITMAPCOREHEADER structure determines the
number of bits that 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 224 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 colors in the bmciColors table should appear in order of
importance.
Alternatively, for functions that use DIBs, the bmciColors 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 bmciColors 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.


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.

Members

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.

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).

Members

biSize

Specifies the number of bytes required by the BITMAPINFOHEADER
structure.

biWidth

Specifies the width of the bitmap, in pixels.

biHeight

Specifies the height of the bitmap, in pixels.

biPlanes

Specifies 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 chapter.

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 a delta. The interpretation of the escape depends on
the value of the second byte of the pair. The following list shows the
meaning of the second byte:

Value 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 offset of the next pixel from
the current position.
Absolute mode is signaled by the first byte set to zero and the second
byte set to a value between 0x03 and 0xFF. In absolute mode, the
second byte represents the number of bytes that follow, each of which
contains the color index of a single pixel. When the second byte is
set to 2 or less, the escape has the same meaning as in encoded mode.
In absolute mode, each run must be aligned on a word boundary.
The following example shows the hexadecimal values of an 8-bit
compressed bitmap:



03 04 05 06 00 03 45 56 67 00 02 78 00 02 05 01
02 78 00 00 09 1E 00 01

This bitmap would expand as follows (two-digit values represent a
color index for a single pixel):



04 04 04
06 06 06 06 06
45 56 67
78 78
move current position 5 right and 1 down
78 78
end of line
1E 1E 1E 1E 1E 1E 1E 1E 1E
end of RLE bitmap

BI_RLE4

When the biCompression member is set to BI_RLE4, the bitmap is
compressed using a run-length encoding (RLE) format for a 4-bit
bitmap, which also uses encoded and absolute modes. In encoded mode,
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 four bits) and one in its low-order nibble. The first of the
pixels 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.

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. In absolute mode, each run must be aligned on a
word boundary. The end-of-line, end-of-bitmap, and delta escapes also
apply to BI_RLE4.
The following example shows the hexadecimal values of a 4-bit
compressed bitmap:



03 04 05 06 00 06 45 56 67 00 04 78 00 02 05 01
04 78 00 00 09 1E 00 01

This bitmap would expand as follows (single-digit values represent a
color index for a single pixel):



0 4 0
0 6 0 6 0
4 5 5 6 6 7
7 8 7 8
move current position 5 right and 1 down
7 8 7 8
end of line
1 E 1 E 1 E 1 E 1
end of RLE bitmap

RGBQUAD

3.0



typedef struct tagRGBQUAD { /* rgbq */
BYTE rgbBlue;
BYTE rgbGreen;
BYTE rgbRed;
BYTE rgbReserved;
} RGBQUAD;

The RGBQUAD structure describes a color consisting of relative
intensities of red, green, and blue. The bmiColors member of the
BITMAPINFO structure consists of an array of RGBQUAD structures.

Members

rgbBlue

Specifies the intensity of blue in the color.

rgbGreen

Specifies the intensity of green in the color.

rgbRed

Specifies the intensity of red in the color.

rgbReserved

Not used; must be set to zero.



1.1.2 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.

1.1.2.1 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


1.1.2.2 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

1.1.3 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
.



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