BMP file structure analysis by PHP

phptuts May 4, 2013

Part 1: overview BMP file structure (Reference: http://en.wikipedia.org/wiki/BMP_file_format#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.

Diag. 1 – The structure of the bitmap image file

(Source of picture: WikiPedia.org)

Referring to the diagram 1, the bitmap 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
(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.

Pixel format

In a bitmap image file on a disk or a bitmap image in memory, the pixels can be defined by a 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.Each bit is an index into a table of 2 colors. 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). Each pixel value is a 2-bit index into a table of up to 4 colors.
  • 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. Each pixel value is a 4-bit index into a table of up to 16 colors.
  • 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.
  • 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).
  • 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.

Read more: PHP function to parse BMP header
Part 2: The 16-bit per pixel (16bpp) BMP

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