BMP file format

Applies to:
BG1, BG1: TotS, BG2, BG2: ToB, PST, IWD, IWD:HoW, IWD:TotL, IWD2

General Description
http://www.daubnet.com/formats/BMP.html

Table of Contents
Last updated: Jan 14. 1998



Basic Description
This file format is the MS-Windows standard format. It holds black&white-, 16-color, 256-color and Truecolor images. The palletized 16-color and 256-color images may be compressed via run length encoding. Notice there is also a OS/2-BMP format.



Basic File Format

Name Size Description
Header 14 bytes Windows Structure: BITMAPFILEHEADER
0x00 Signature 2 bytes 'BM'
0x02 FileSize 4 bytes File size in bytes
0x06 Reserved 4 bytes unused (=0)
0x0a DataOffset 4 bytes File offset to Raster Data
InfoHeader 40 bytes Windows Structure: BITMAPINFOHEADER
0x0e Size 4 bytes Size of InfoHeader =40 
0x12 Width 4 bytes Bitmap Width
0x16 Height 4 bytes Bitmap Height
0x1a Planes 2 bytes Number of Planes (=1)
0x1c BitCount 2 bytes Bits per Pixel  
1 = monochrome palette. NumColors = 1  
4 = 4bit palletized. NumColors = 16  
8 = 8bit palletized. NumColors = 256 
16 = 16bit RGB. NumColors = 65536 (?) 
24 = 24bit RGB. NumColors = 16M
0x1e Compression 4 bytes Type of Compression  
0 = BI_RGB   no compression  
1 = BI_RLE8 8bit RLE encoding  
2 = BI_RLE4 4bit RLE encoding
0x22 ImageSize 4 bytes (compressed) Size of Image 
It is valid to set this =0 if Compression = 0
0x26 XpixelsPerM 4 bytes horizontal resolution: Pixels/meter
0x2a YpixelsPerM 4 bytes vertical resolution: Pixels/meter
0x2e ColorsUsed 4 bytes Number of actually used colors
0x32 ColorsImportant 4 bytes Number of important colors 
0 = all
0x36 ColorTable 4 * NumColors bytes present only if Info.BitsPerPixel <= 8  
colors should be ordered by importance
    Red 1 byte Red intensity
Green 1 byte Green intensity
Blue 1 byte Blue intensity
reserved 1 byte unused (=0)
repeated NumColors times
Raster Data Info.ImageSize bytes The pixel data

Raster Data encoding

Depending on the image's BitCount and on the Compression flag there are 6 different encoding schemes. All of them share the following: 

Pixels are stored bottom-up, left-to-right. Pixel lines are padded with zeros to end on a 32bit (4byte) boundary. For uncompressed formats every line will have the same number of bytes. Color indices are zero based, meaning a pixel color of 0 represents the first color table entry, a pixel color of 255 (if there are that many) represents the 256th entry. For images with more than 256 colors there is no color table. 

 

Raster Data encoding for 1bit / black & white images

BitCount = 1 Compression = 0 
Every byte holds 8 pixels, its highest order bit representing the leftmost pixel of those. There are 2 color table entries. Some readers will ignore them though, and assume that 0 is black and 1 is white. If you are storing black and white pictures you should stick to this, with any other 2 colors this is not an issue. Remember padding with zeros up to a 32bit boundary (This can be up to 31 zeros/pixels!) 
 

Raster Data encoding for 4bit / 16 color images

BitCount = 4 Compression = 0 
Every byte holds 2 pixels, its high order 4 bits representing the left of those. There are 16 color table entries. These colors do not have to be the 16 MS-Windows standard colors. Padding each line with zeros up to a 32bit boundary will result in up to 28 zeros = 7 'wasted pixels'.
 

Raster Data encoding for 8bit / 256 color images

BitCount = 8 Compression = 0 
Every byte holds 1 pixel. There are 256 color table entries. Padding each line with zeros up to a 32bit boundary will result in up to 3 bytes of zeros = 3 'wasted pixels'.
 

Raster Data encoding for 16bit / hicolor images

BitCount = 16 Compression = 0 
Every 2bytes / 16bit holds 1 pixel.  
<information missing: the 16 bit was introduced together with Video For Windows? Is it a memory-only-format?> 
The pixels are no color table pointers. There are no color table entries. Padding each line with zeros up to a 16bit boundary will result in up to 2 zero bytes.
 

Raster Data encoding for 24bit / truecolor images

BitCount = 24 Compression = 0 
Every 4bytes / 32bit holds 1 pixel. The first holds its red, the second its green, and the third its blue intensity. The fourth byte is reserved and should be zero. There are no color table entries. The pixels are no color table pointers. No zero padding necessary.
 

Raster Data compression for 4bit / 16 color images

BitCount = 4 Compression = 2 
The pixel data is stored in 2bytes / 16bit chunks.  The first of these specifies the number of consecutive pixels with the same pair of color. The second byte defines two color indices. The resulting pixel pattern will be interleaved high-order 4bits and low order 4 bits (ABABA...). If the first byte is zero, the second defines an escape code. The End-of-Bitmap is zero padded to end on a 32bit boundary. Due to the 16bit-ness of this structure this will always be either two zero bytes or none.  
 
n (byte 1) c (Byte 2) Description
>0 any n pixels are to be drawn. The 1st, 3rd, 5th, ... pixels' color is in c's high-order 4 bits, the even pixels' color is in c's low-order 4 bits. If both color indices are the same, it results in just n pixels of color c
0 0 End-of-line
0 1 End-of-Bitmap
0 2 Delta. The following 2 bytes define an unsigned offset in x and y direction (y being up) The skipped pixels should get a color zero.
0 >=3 The following c bytes will be read as single pixel colors just as in uncompressed files. up to 12 bits of zeros follow, to put the file/memory pointer on a 16bit boundary again.
 
Example for 4bit RLE
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 up. (Windows docs say down, which is wrong)
00 00 End-of-line
09 1E 1 E 1 E 1 E 1 E 1
00 01 EndofBitmap
00 00 Zero padding for 32bit boundary
 
 

Raster Data compression for 8bit / 256 color images

BitCount = 8 Compression = 1 
The pixel data is stored in 2bytes / 16bit chunks.  The first of these specifies the number of consecutive pixels with the same color. The second byte defines their color index. If the first byte is zero, the second defines an escape code. The End-of-Bitmap is zero padded to end on a 32bit boundary. Due to the 16bit-ness of this structure this will always be either two zero bytes or none.  
 
n (byte 1) c (Byte 2) Description
>0 any n pixels of color number c
0 0 End-of-line
0 1 EndOfBitmap
0 2 Delta. The following 2 bytes define an unsigned offset in x and y direction (y being up) The skipped pixels should get a color zero.
0 >=3 The following c bytes will be read as single pixel colors just as in uncompressed files. A zero follows, if c is odd, putting the file/memory pointer on a 16bit boundary again.
 
Example for 8bit RLE
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 up. (Windows docs say down, which is wrong)
00 00 End-of-line
09 1E 1E 1E 1E 1E 1E 1E 1E 1E 1E
00 01 End-of-bitmap
00 00 Zero padding for 32bit boundary
 



Portability
Although BMPs were invented by Microsoft for its Windows platform, a lot of programs on other platforms are capable of reading and writing them. Notice the Intel order in 2byte and 4-byte integer values (Least significant byte first). The 16bit BMPs have been introduced to Windows after the others, still puzzling many applications.



Trademarks, Patents and Royalties
To my knowledge: None.



Cross-Checking Software
This section is for programmers, who wish to cross-check their implementation with others. This is an incomplete list of programs, which are available as freeware / shareware / try-before-buy etc.

The following software is able to decode BMP files

The following software is able to encode BMP files



Online Resources
http://www.dcs.ed.ac.uk/~mxr/gfx/ 



Paper Resources
MS-Windows SDK
[German:] Referenzhandbuch Dateiformate by G√ľnter Born.

The following suggestions are from the Graphics Fileformats FAQ: