BMP file format

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.

Device-independent bitmaps and the 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

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 file is composed of structures in the following order:

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).

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.

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.

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:

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:

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 (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 two 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:

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:

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:

,

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:

,

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 WindowsDIB header – the BITMAPINFOHEADER  (some other DIB header versions will be larger or smaller than that as described by the table above) and the expression   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  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 .^[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). 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:

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).

Diag.3 – The pixel format with an alpha channel in a 16 bit pixel (in RGBAX sample Lngth notation) actually generated by Adobe Photoshop^[11]

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

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)

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 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).

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 run-length encoding 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

• Comparison of graphics file formats

References

External links

• For a table view of the bitmap file format at sourceforge
• Bitmap File Structure
• An introduction to DIBs (Device Independent Bitmaps)
• BMP test images
• Simple bitmap loader C++ class
• The BMP File Format, Part 1 By David Charlap, Dr. Dobb's journal of software tools, Volume 20, March 1995
• Another source for BMP entry in GFF (the Encyclopedia of Graphic File Formats)