BMP file format


BMP file format
Windows Bitmap
Filename extension .bmp or .dib
Internet media type

image/x-ms-bmp (unofficial) or

image/x-bmp (unofficial)
Type code 'BMP '
'BMPf'
'BMPp'
Uniform Type Identifier com.microsoft.bmp
Type of format Raster graphics

The BMP File Format, also known as Bitmap Image File or Device Independent Bitmap (DIB) file format or simply a Bitmap, is a Raster graphics image file format used to store bitmap digital images, independently of the display device (such as a graphics adapter), especially on Microsoft Windows and OS/2 operating systems.

The BMP File Format is capable of storing 2D digital images of arbitrary width, height, and resolution, both monochrome and color, in various color depths, and optionally with data compression, alpha channels, and color profiles.

Contents

Device-independent bitmaps and BMP file format

Microsoft has defined a particular representation of color bitmaps of different color depths, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps or DIBs, and the file format for them is called DIB file format or BMP image file format. According to Microsoft support:[1]

A device-independent bitmap (DIB) is a format used to define device-independent bitmaps in various color resolutions. The main purpose of DIBs is to allow bitmaps to be moved from one device to another (hence, the device-independent part of the name). A DIB is an external format, in contrast to a device-dependent bitmap, which appears in the system as a bitmap object (created by an application...). A DIB is normally transported in metafiles (usually using the StretchDIBits() function), BMP files, and the Clipboard (CF_DIB data format).

The following sections discuss the data stored in the BMP file or DIB in details. This is the standard BMP file format.[1] Some applications create bitmap image files which are not compliant with the current Microsoft documentation. Also, not all fields are used; a value of 0 will be found in these unused fields.

File structure

Diag.1 - The Structure of the Bitmap Image File (BMP)

The Bitmap Image File consists of fixed-size structures (headers) as well as variable-size structures appearing in a predetermined sequence. Many different versions of some of these structures can appear in the file, due to the long evolution of this file format.

Referring to the Diagram 1, the Bitmap Image File is composed of structures in the following order:

Structure Name Optional Size Purpose Comments
Bitmap File Header No 14 Bytes To store general information about the Bitmap Image File Not needed after the file is loaded in memory
DIB Header No Fixed-size
(however 7 different versions exist)
To store detailed information about the bitmap image and define the pixel format Immediately follows the Bitmap File Header
Extra bit masks Yes 3 or 4 DWORDs[2]
(12 or 16 Bytes)
To define the pixel format Present only in case the DIB Header is the BITMAPINFOHEADER
Color Table Semi-optional Variable-size To define colors used by the bitmap image data (Pixel Array) Mandatory for color depths <= 8
Gap1 Yes Variable-size Structure alignment An artifact of the File Offset to PixelArray in the Bitmap File Header
Pixel Array No Variable-size To define the actual values of the pixels The pixel format is defined by the DIB Header or Extra bit masks. Each row in the Pixel Array is padded to a multiple of 4 bytes in size
Gap2 Yes Variable-size Structure alignment An artifact of the ICC Profile Data offset field in the DIB Header
ICC Color Profile Yes Variable-size To define the color profile for Color management Can also contain a path to an external file containing the Color Profile. When loaded in memory as "non-packed DIB", it is located between the Color Table and Gap1.[3]

DIBs in memory

A Bitmap Image File loaded into memory becomes a DIB data structure - an important component of the Windows GDI API. The in-memory DIB data structure is almost the same as the BMP file format, but it does not contain the 14-byte Bitmap File Header and begins with the DIB Header. For DIBs loaded in memory, the Color Table can also consist of 16 bit entries, that constitute indexes to the currently realized palette[4] (an additional level of indirection), instead of explicit RGB color definitions. In all cases, the Pixel Array must begin at a memory address that is a multiple of 4 bytes. In non-packed DIBs loaded in memory, the optional Color Profile data should be located immediately after the Color Table and before the Gap1 and Pixel Array[3] (unlike in Diag.1).

When the size of Gap1 and Gap2 is zero, the in-memory DIB data structure is customarily referred to as "packed DIB" and can be referred to by a single pointer pointing to the beginning of the DIB Header. In all cases, the Pixel Array must begin at a memory address that is a multiple of 4 bytes. In some cases it may be necessary to adjust the number of entries in the Color Table in order to force the memory address of the Pixel Array to a multiple of 4 bytes.[4] For "packed DIBs" loaded in memory, the optional Color Profile data should immediately follow the Pixel Array, as depicted in Diag.1 (with Gap1=0 and Gap2=0).[3]
"Packed DIBs" are required by Windows clipboard API functions as well as by some Windows patterned brush and resource functions.[5]

Bitmap file header

This block of bytes is at the start of the file and is used to identify the file. A typical application reads this block first to ensure that the file is actually a BMP file and that it is not damaged. The first two bytes of the BMP file format are the character 'B' then the character 'M' in 1-byte ASCII encoding. All of the integer values are stored in little-endian format (i.e. least-significant byte first).

Offset# Size Purpose
0000h 2 bytes the Header field used to identify the BMP & DIB file is 0x42 0x4D in Hexadecimal, same as BM in ASCII. The following entries are possible:
  • BM – Windows 3.1x, 95, NT, ... etc.; and it is not mandatory unless file size is greater or equal to SIGNATURE
  • BA – OS/2 struct Bitmap Array
  • CI – OS/2 struct Color Icon
  • &CP – OS/2 const Color Pointer
  • IC – OS/2 struct Icon
  • *PT – OS/2 Pointer
0002h 4 bytes the size of the BMP file in bytes
0006h 2 bytes reserved; actual value depends on the application that creates the image
0008h 2 bytes reserved; actual value depends on the application that creates the image
000Ah 4 bytes the offset, i.e. starting address, of the byte where the bitmap image data (Pixel Array) can be found.

Equivalent header in C language

#include <stdint.h>
 
/* Note: the magic number has been removed from the bmpfile_header structure
   since it causes alignment problems
     struct bmpfile_magic should be written/read first
   followed by the
     struct bmpfile_header
   [this avoids compiler-specific alignment pragmas etc.]
*/
 
struct bmpfile_magic {
  unsigned char magic[2];
};
 
struct bmpfile_header {
  uint32_t filesz;
  uint16_t creator1;
  uint16_t creator2;
  uint32_t bmp_offset;
};

DIB Header (Bitmap Information Header)

This block of bytes tells the application detailed information about the image, which will be used to display the image on the screen. The block also matches the header used internally by Windows and OS/2 and has several different variants. All of them contain a dword (32 bit) field, specifying their size, so that an application can easily determine which header is used in the image. The reason that there are different headers is that Microsoft extended the DIB format several times. The new extended headers can be used with some GDI functions instead of the older ones, providing more functionality. Since the GDI supports a function for loading bitmap files, typical Windows applications use that functionality. One consequence of this is that for such applications, the BMP formats that they support match the formats supported by the Windows version being run. See the table below for more information.

Size Header Name OS support Features Added (incremental) in Bitmap Files Written by
12 BITMAPCOREHEADER
OS21XBITMAPHEADER
OS/2 and also all Windows versions since Windows 3.0
64 BITMAPCOREHEADER2
OS22XBITMAPHEADER
OS/2 Adds Halftoning. Adds RLE and Huffman 1D compression.
40 BITMAPINFOHEADER all Windows versions since Windows 3.0 Removes RLE-24 and Huffman 1D compression. Adds 16bpp and 32bpp pixel formats. Adds optional RGB bit masks. Adobe Photoshop
52 BITMAPV2INFOHEADER Undocumented. Removes optional RGB bit masks. Adds mandatory RGB bit masks.
56 BITMAPV3INFOHEADER Undocumented. Adds mandatory Alpha channel bit mask. Adobe Photoshop
108 BITMAPV4HEADER all Windows versions since Windows 95/NT4 Adds Color Space type and Gamma correction
124 BITMAPV5HEADER Windows 98/2000 and newer Adds ICC Color Profiles

Versions after BITMAPCOREHEADER only add fields to the end of the header of the previous version. For example: BITMAPV2INFOHEADER adds fields to BITMAPINFOHEADER and BITMAPV3INFOHEADER adds fields to BITMAPV2INFOHEADER (or, BITMAPINFOHEADER is a stripped version of BITMAPV2INFOHEADER)

For compatibility reasons, most applications use the older DIB headers for saving files. With OS/2 being obsolete, for now the common format is the BITMAPINFOHEADER header. See next table for its description. All values are stored as unsigned integers, unless explicitly noted.

Offset # Size Purpose
0Eh 4 the size of this header (40 bytes)
12h 4 the bitmap width in pixels (signed integer).
16h 4 the bitmap height in pixels (signed integer).
1Ah 2 the number of color planes being used. Must be set to 1.
1Ch 2 the number of bits per pixel, which is the color depth of the image. Typical values are 1, 4, 8, 16, 24 and 32.
1Eh 4 the compression method being used. See the next table for a list of possible values.
22h 4 the image size. This is the size of the raw bitmap data (see below), and should not be confused with the file size.
26h 4 the horizontal resolution of the image. (pixel per meter, signed integer)
2Ah 4 the vertical resolution of the image. (pixel per meter, signed integer)
2Eh 4 the number of colors in the color palette, or 0 to default to 2n.
32h 4 the number of important colors used, or 0 when every color is important; generally ignored.

Note: The image size field can be 0 for BI_RGB bitmaps.

Equivalent header in C language

typedef struct {
  uint32_t header_sz;
  int32_t width;
  int32_t height;
  uint16_t nplanes;
  uint16_t bitspp;
  uint32_t compress_type;
  uint32_t bmp_bytesz;
  int32_t hres;
  int32_t vres;
  uint32_t ncolors;
  uint32_t nimpcolors;
} BITMAPINFOHEADER;

The compression method field (bytes #30-33) can have the following values:

Value Identified by Compression method Comments
0 BI_RGB none Most common
1 BI_RLE8 RLE 8-bit/pixel Can be used only with 8-bit/pixel bitmaps
2 BI_RLE4 RLE 4-bit/pixel Can be used only with 4-bit/pixel bitmaps
3 BI_BITFIELDS Bit field or Huffman 1D compression for BITMAPCOREHEADER2 Pixel format defined by bit masks or Huffman 1D compressed bitmap for BITMAPCOREHEADER2
4 BI_JPEG JPEG or RLE-24 compression for BITMAPCOREHEADER2 The bitmap contains a JPEG image or RLE-24 compressed bitmap for BITMAPCOREHEADER2
5 BI_PNG PNG The bitmap contains a PNG image
6 BI_ALPHABITFIELDS Bit field This value is valid in Windows CE .NET 4.0 and later.

Note: BI_JPEG and BI_PNG are for printer drivers and are not supported when rendering to the screen.[6]

Equivalent header in C language (compression methods)

typedef enum {
  BI_RGB = 0,
  BI_RLE8,
  BI_RLE4,
  BI_BITFIELDS, //Also Huffman 1D compression for BITMAPCOREHEADER2
  BI_JPEG,      //Also RLE-24 compression for BITMAPCOREHEADER2
  BI_PNG,
} bmp_compression_method_t;

The OS/2 BITMAPCOREHEADER header is also popular:

Offset Size Purpose
Eh 4 the size of this header (12 bytes)
12h 2 the bitmap width in pixels.
14h 2 the bitmap height in pixels.
16h 2 the number of color planes; 1 is the only legal value
18h 2 the number of bits per pixel. Typical values are 1, 4, 8 and 24.

Note: OS/2 BITMAPCOREHEADER bitmaps cannot be compressed and cannot be 16 or 32 bits/pixel. All values in the OS/2 BITMAPCOREHEADER header are unsigned integers.

A 16-bit and 32-bit version of DIB with an integrated alpha channel has been introduced with the undocumented BITMAPV3INFOHEADER and with the documented BITMAPV4HEADER (since Windows 95) and is used within Windows XP logon and theme system as well as Microsoft Office (since v2000); it is supported by some image editing software, such as Adobe Photoshop since version 7 and Adobe Flash since version MX 2004 (then known as Macromedia Flash). It is also supported by GIMP, Google Chrome, MS-PowerPoint and MS-Word.

Color Table

The Color Table (a.k.a. Palette) occurs in the BMP image file directly after the BMP file header, the DIB header (and after optional three Red, Green and Blue bitmasks if the BITMAPINFOHEADER Header with BI_BITFIELDS option is used). Therefore, its offset is the size of the BITMAPFILEHEADER plus the size of the DIB header (plus optional 12 bytes for the three bit masks).
Note: On Windows CE the BITMAPINFOHEADER Header can be used with the BI_ALPHABITFIELDS[2] option in the biCompression member. In such case, four optional bitmasks follow the BITMAPINFOHEADER Header instead of the three bitmask mentioned above, thus the Color Table's offset is the size of the BITMAPFILEHEADER plus the size of the BITMAPINFOHEADER Header plus the 16 bytes of the four bitmasks (Red, Green, Blue and Alpha).

The number of entries in the palette is either 2n or a smaller number specified in the header (in the OS/2 BITMAPCOREHEADER header format, only the full-size palette is supported).[1][7] In most cases, each entry in the color table occupies 4 bytes, in the order Blue, Green, Red, 0x00 (see below for exceptions).

The Color Table is a block of bytes (a table) listing the colors used by the image. Each pixel in an indexed color image is described by a number of bits (1, 4, or 8) which is an index of a single color described by this table. The purpose of the color palette in indexed color bitmaps is to inform the application about the actual color that each of these index values corresponds to. The purpose of the Color Table in non-indexed (non-palettized) bitmaps is to list the colors used by the bitmap for the purposes of optimization on devices with limited color display capability and to facilitate future conversion to different pixel formats and paletization.

The colors in the Color Table are usually specified in the 4-byte per entry 8.8.8.0.8 format (in RGBAX notation). The Color Table used with the OS/2 BITMAPCOREHEADER uses the 3-byte per entry 8.8.8.0.0 format.[1][7] For DIBs loaded in memory, the Color Table can optionally consist of 2-byte entries - these entries constitute indexes to the currently realized palette[4] instead of explicit RGB color definitions.

Microsoft does not disallow the presence of a valid Alpha channel bit mask[8] in BITMAPV4HEADER and BITMAPV5HEADER for 1bpp, 4bpp and 8bpp indexed color images, which indicates that the Color Table entries can also specify an Alpha component using the 8.8.8.[0-8].[0-8] format via the RGBQUAD.rgbReserved[9] member. However, some versions of Microsoft's documentation disallow this feature by stating that the RGBQUAD.rgbReserved member "must be zero".

As mentioned above, the Color Table is normally not used when the pixels are in the 16-bit per pixel (16bpp) format (and higher); there are normally no Color Table entries in those bitmap image files. However, the Microsoft documentation (on the MSDN web site as of Nov. 16, 2010) specifies that for 16bpp (and higher), the Color Table can be present to store a list of colors intended for optimization on devices with limited color display capability, while it also specifies, that in such cases, no indexed palette entries are present in this Color Table. This may seem like a contradiction if no distinction is made between the mandatory palette entries and the optional color list.

Pixel storage

The bits representing the bitmap pixels are packed in rows. The size of each row is rounded up to a multiple of 4 bytes (a 32-bit DWORD) by padding.
For images with Height > 1, multiple padded rows are stored consecutively, forming a Pixel Array.

The total number of bytes necessary to store one row of pixels can be calculated as:

\mbox{RowSize} = \left\lceil\frac { \mbox{BitsPerPixel} \cdot \mbox{ImageWidth} }{32}\right\rceil \cdot 4,
ImageWidth is expressed in pixels.

The total amount of bytes necessary to store an array of pixels in an n Bits per Pixel (bpp) image, with 2n colors, can be calculated by accounting for the effect of rounding up the size of each row to a multiple of a 4 bytes, as follows:

\mbox{PixelArraySize} = \mbox{RowSize} \cdot \left | \mbox{ImageHeight} \right  \vert
ImageHeight is expressed in pixels. The absolute value is necessary because ImageHeight can be negative

The total Bitmap Image File size can be approximated as:

\mbox{FileSize} \approx 54 + 4 \cdot 2^\mbox{bpp} + \mbox{PixelArraySize},
for BPP ≤ 8 ( because for pixels larger than 8 bits, the palette is not mandatory )

Only images with 8 or fewer Bits per Pixel must account for the palette. 16bpp images (or higher), may omit the palette part from the size calculation, as follows:

\mbox{FileSize} \approx 54 + \mbox{PixelArraySize},
for Bits per Pixel > 8.

In the formulas above, the number 54 is the combined size of the 14 byte Bitmap File Header and the 40 byte popular Windows DIB Header - the BITMAPINFOHEADER  (some other DIB Header versions will be larger or smaller than that as described by the table above) and the expression 4 \cdot 2^n  is the size of the color palette in bytes.

This total file size formula is only an approximation, since the size of the color palette will be 3 \cdot 2^n bytes for the OS/2 DIB Header version BITMAPCOREHEADER, and some files may define only the number of colors needed by the image, potentially fewer than 2n.[1]
An additional size uncertainty is introduced by the optional presence of the 12 or 16 bytes needed for the extra bit masks stored immediately after the BITMAPINFOHEADER DIB Header and the variable-size GAP depiced in Diag.1

Pixel Array (bitmap data)

The Pixel Array is a block of 32-bit DWORDs, that describes the image pixel by pixel. Normally pixels are stored "upside-down" with respect to normal image raster scan order, starting in the lower left corner, going from left to right, and then row by row from the bottom to the top of the image.[1]
Uncompressed Windows bitmaps also can be stored from the top to bottom, when the Image Height value is negative.

In the original OS/2 DIB, the only four legal values of color depth were 1, 4, 8, and 24 bits per pixel (bpp).[1] Contemporary DIB Headers allow pixel formats with 1, 2, 4, 8, 16, 24 and 32 bits per pixel (bpp).[10]

Padding bytes (not necessarily 0) must be appended to the end of the rows in order to bring up the length of the rows to a multiple of four bytes. When the Pixel Array is loaded into memory, each row must begin at a memory address that is a multiple of 4. This address/offset restriction is mandatory only for Pixel Array's loaded in memory. For file storage purposes, only the size of each row must be a multiple of 4 bytes while the file offset can be arbitrary.[1]
A 24-bit bitmap with Width=1, would have 3 bytes of data per row (blue, green, red) and 1 byte of padding, while Width=2 would have 2 bytes of padding, Width=3 would have 3 bytes of padding, and Width=4 would not have any padding at all.

Bitmap Image Files are typically much larger than image file formats compressed with other algorithms, for the same image. For example, the 1058×1058 Wikipedia logo, which occupies about 271 KB in the lossless PNG format, takes about 3358 KB as a 24bpp BMP Image File. Uncompressed formats are generally unsuitable for transferring images on the Internet or other slow or capacity-limited media.

Compression

Indexed color images may be compressed with 4-bit or 8-bit RLE or Huffman 1D algorithm.
OS/2 BITMAPCOREHEADER2 24bpp images may be compressed with the 24-bit RLE algorithm.
The 16bpp and 32bpp images are always stored uncompressed.
Note that images in all color depths can be stored without compression if so desired.

Pixel Format

In bitmap image file on a disk or bitmap image in memory, the pixels can be defined by varying number of bits.

  • The 1-bit per pixel (1bpp) format supports 2 distinct colors, (for example: black and white, ...or yellow and pink). The pixel values are stored in each bit, with the first (left-most) pixel in the most-significant bit of the first byte.[1] Each bit is an index into a table of 2 colors. This Color Table is in 32bpp 8.8.8.0.8 RGBAX format. An unset bit will refer to the first color table entry, and a set bit will refer to the last (second) color table entry.
  • The 2-bit per pixel (2bpp) format supports 4 distinct colors and stores 4 pixels per 1 byte, the left-most pixel being in the two most significant bits (Windows CE only[11]). Each pixel value is a 2-bit index into a table of up to 4 colors. This Color Table is in 32bpp 8.8.8.0.8 RGBAX format.
  • The 4-bit per pixel (4bpp) format supports 16 distinct colors and stores 2 pixels per 1 byte, the left-most pixel being in the more significant nibble.[1] Each pixel value is a 4-bit index into a table of up to 16 colors. This Color Table is in 32bpp 8.8.8.0.8 RGBAX format.
  • The 8-bit per pixel (8bpp) format supports 256 distinct colors and stores 1 pixel per 1 byte. Each byte is an index into a table of up to 256 colors. This Color Table is in 32bpp 8.8.8.0.8 RGBAX format.
  • The 16-bit per pixel (16bpp) format supports 65536 distinct colors and stores 1 pixel per 2 byte WORD. Each WORD can define the Alpha, Red, Green and Blue samples of the pixel.
  • The 24-bit pixel (24bpp) format supports 16,777,216 distinct colors and stores 1 pixel value per 3 bytes. Each pixel value defines the Red, Green and Blue samples of the pixel (8.8.8.0.0 in RGBAX notation). Specifically in the order (Blue, Green and Red, 8-bits per each sample ).[1]
  • The 32-bit per pixel (32bpp) format supports 4,294,967,296 distinct colors and stores 1 pixel per 4 byte DWORD. Each DWORD can define the Alpha, Red, Green and Blue samples of the pixel.

In order to resolve the ambiguity of which bits define which samples, the DIB Headers provide certain defaults as well as specific BITFIELDS which are bit masks that define the membership of particular group of bits in a pixel to a particular channel.
The following diagram defines this mechanism:

The BITFIELDS mechanism depicted in RGBAX Sample Length notation

Diag.2 - The BITFIELDS mechanism for a 32 bit pixel depicted in RGBAX Sample Length notation

The sample fields defined by the BITFIELDS bit masks have to be contiguous and non-overlapping.
The Red, Green and Blue bit masks are valid only when the Compression member of the DIB Header is set to BI_BITFIELDS.
The Alpha bit mask is valid whenever it is present in the DIB Header or when the Compression member of the DIB Header is set to BI_ALPHABITFIELDS[2] (Windows CE only).

The BITFIELDS mechanism depicted in RGBAX Sample Length notation

Diag.3 - The pixel format with an Alpha channel in a 16 bit pixel (in RGBAX Sample Length notation) actually generated by Adobe Photohop[11]

The table below lists all of the possible pixel formats of a DIB (in RGBAX notation).
All of the possible pixel formats in a DIB

The BITFIELD mechanism described above allows for the definition of tens of thousands different pixel formats, however only several of them are used in practice,[11] such as:

  • 8.8.8.0.0
  • 8.8.8.0.8
  • 8.8.8.8.0
  • 5.5.5.0.1
  • 5.5.5.1.0
  • 5.6.5.0.0
  • 4.4.4.0.4
  • 4.4.4.4.0
  • All palettized formats (marked in yellow in the table above)

[The list above is in the RGBAX notation]

Example 1

Following is an example of a 2×2 pixel, 24-bit bitmap (Windows DIB Header BITMAPINFOHEADER) with pixel format 8.8.8.0.0 (in RGBAX notation).

Example of a 2×2 Pixel Bitmap, with 24 bits/pixel encoding
Offset Size Hex Value Value Description
BMP Header
0h 2 42 4D "BM" Magic number (unsigned integer 66, 77)
2h 4 46 00 00 00 70 Bytes Size of the BMP file
6h 2 00 00 Unused Application specific
8h 2 00 00 Unused Application specific
Ah 4 36 00 00 00 54 bytes Offset where the Pixel Array (bitmap data) can be found
DIB Header
Eh 4 28 00 00 00 40 bytes Number of bytes in the DIB header (from this point)
12h 4 02 00 00 00 2 pixels Width of the bitmap in pixels
16h 4 02 00 00 00 2 pixels Height of the bitmap in pixels
1Ah 2 01 00 1 plane Number of color planes being used
1Ch 2 18 00 24 bits Number of bits per pixel
1Eh 4 00 00 00 00 0 BI_RGB, no Pixel Array compression used
22h 4 10 00 00 00 16 bytes Size of the raw data in the Pixel Array (including padding)
26h 4 13 0B 00 00 2,835 pixels/meter Horizontal resolution of the image
2Ah 4 13 0B 00 00 2,835 pixels/meter Vertical resolution of the image
2Eh 4 00 00 00 00 0 colors Number of colors in the palette
32h 4 00 00 00 00 0 important colors 0 means all colors are important
Start of Pixel Array (bitmap data)
36h 3 00 00 FF 0 0 255 Red, Pixel (0,1)
39h 3 FF FF FF 255 255 255 White, Pixel (1,1)
3Ch 2 00 00 0 0 Padding for 4 byte alignment (could be a value other than zero)
3Eh 3 FF 00 00 255 0 0 Blue, Pixel (0,0)
41h 3 00 FF 00 0 255 0 Green, Pixel (1,0)
44h 2 00 00 0 0 Padding for 4 byte alignment (could be a value other than zero)

Example 2

Following is an example of a 4×2 pixel, 32-bit bitmap with opacity values in the alpha channel (Windows DIB Header BITMAPV4HEADER) with pixel format 8.8.8.8.0 (in RGBAX notation).

Example of a 4×2 Pixel Bitmap, with 32 bits/pixel encoding
Offset Size Hex Value Value Description
BMP Header
0h 2 42 4D "BM" Magic number (unsigned integer 66, 77)
2h 4 9A 00 00 00 154 Bytes Size of the BMP file
6h 2 00 00 Unused Application specific
8h 2 00 00 Unused Application specific
Ah 4 7A 00 00 00 122 bytes from the start of the file Offset where the Pixel Array (bitmap data) can be found
DIB Header
Eh 4 6C 00 00 00 108 bytes Number of bytes in the DIB header (from this point)
12h 4 04 00 00 00 4 pixels Width of the bitmap in pixels
16h 4 02 00 00 00 2 pixels Height of the bitmap in pixels
1Ah 2 01 00 1 plane Number of color planes being used
1Ch 2 20 00 32 bits Number of bits per pixel
1Eh 4 03 00 00 00 3 BI_BITFIELDS, no Pixel Array compression used
22h 4 20 00 00 00 32 bytes Size of the raw data in the Pixel Array (including padding)
26h 4 13 0B 00 00 2,835 pixels/meter Horizontal physical resolution of the image
2Ah 4 13 0B 00 00 2,835 pixels/meter Vertical physical resolution of the image
2Eh 4 00 00 00 00 0 colors Number of colors in the palette
32h 4 00 00 00 00 0 important colors 0 means all colors are important
36h 4 00 00 FF 00 00FF0000 in big-endian Red channel bit mask (valid because BI_BITFIELDS is specified)
3Ah 4 00 FF 00 00 0000FF00 in big-endian Green channel bit mask (valid because BI_BITFIELDS is specified)
3Eh 4 FF 00 00 00 000000FF in big-endian Blue channel bit mask (valid because BI_BITFIELDS is specified)
42h 4 00 00 00 FF FF000000 in big-endian Alpha channel bit mask
46h 4 20 6E 69 57 LCS_WINDOWS_COLOR_SPACE Type of Color Space
4Ah 24h 24* 00...00 CIEXYZTRIPLE Color Space endpoints Unused when LCS_WINDOWS_COLOR_SPACE is specified
6Eh 4 00 00 00 00 0 Red Gamma Unused when LCS_WINDOWS_COLOR_SPACE is specified
72h 4 00 00 00 00 0 Green Gamma Unused when LCS_WINDOWS_COLOR_SPACE is specified
76h 4 00 00 00 00 0 Blue Gamma Unused when LCS_WINDOWS_COLOR_SPACE is specified
Start of the Pixel Array (the bitmap Data)
7Ah 4 FF 00 00 7F 255 0 0 127 Blue (Alpha: 127), Pixel (0,1)
7Eh 4 00 FF 00 7F 0 255 0 127 Green (Alpha: 127), Pixel (1,1)
82h 4 00 00 FF 7F 0 0 255 127 Red (Alpha: 127), Pixel (2,1)
86h 4 FF FF FF 7F 255 255 255 127 White (Alpha: 127), Pixel (3,1)
8Ah 4 FF 00 00 FF 255 0 0 255 Blue (Alpha: 255), Pixel (0,0)
8Eh 4 00 FF 00 FF 0 255 0 255 Green (Alpha: 255), Pixel (1,0)
92h 4 00 00 FF FF 0 0 255 255 Red (Alpha: 255), Pixel (2,0)
96h 4 FF FF FF FF 255 255 255 255 White (Alpha: 255), Pixel (3,0)

Note that the bitmap data starts with the lower left hand corner of the image.

Usage of BMP format

The simplicity of the BMP file format, and its widespread familiarity in Windows and elsewhere, as well as the fact that this format is relatively well documented and free of patents, makes it a very common format that image processing programs from many operating systems can read and write.

Many older graphical user interfaces used bitmaps in their built-in graphics subsystems;[12] for example, the Microsoft Windows and OS/2 platforms' GDI subsystem, where the specific format used is the Windows and OS/2 bitmap file format, usually named with the file extension of .BMP or .DIB.

While most BMP files have a relatively large file size due to lack of any compression (or generally low-ratio RLE on palletized images), many BMP files can be considerably compressed with lossless data compression algorithms such as ZIP (in extreme cases of non-photographic data, up to 0.1% of original size) because they contain redundant data. Some formats, such as RAR, even include routines specifically targeted at efficient compression of such data.

Related formats

The X Window System uses a similar XBM format for black-and-white images, and XPM (pixelmap) for color images. There are also a variety of "raw" formats, which saves raw data with no other information. The Portable Pixmap (PPM) and Truevision TGA formats also exist, but are less often used – or only for special purposes; for example, TGA can contain transparency information.

Numerous other bitmap file formats are in existence, though most are not widely used.[13]

See also

References

External links


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