Editing BMP file format (section)

==== Pixel format ====

* 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.<ref name="DIBhelp" /> 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:<ref name="PSformats">MSDN: [http://msdn.microsoft.com/en-us/library/ms959648.aspx Windows CE - {{mono|BITMAPINFOHEADER}} Structure]</ref>).  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]].<ref name="DIBhelp" />  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 [[Sample (graphics)|samples]] of the pixel.
* The 24-bit per 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).<ref name="DIBhelp" />
* 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 (digital image)|channel]]. The following diagram defines this mechanism:

{{Plain image with caption|BitfieldsSLN.svg|Diag. 2 – The BITFIELDS mechanism for a 32-bit pixel depicted in RGBAX sample length notation|753px|center}}

The sample fields defined by the BITFIELDS bit masks have to be contiguous and non-overlapping, but the order of the sample fields is arbitrary. The most ubiquitous field order is: Alpha, Blue, Green, Red (MSB to LSB). 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<ref name=AlphaBitFields/> ([[Windows CE]] only).

{{Plain image with caption|SLNotation44440.svg|Diag. 3 – The pixel format with an alpha channel for a 16-bit pixel (in RGBAX sample Length notation) actually generated by Adobe Photoshop<ref name=PSformats2>Adobe Photoshop: [http://livedocs.adobe.com/en_US/Photoshop/10.0/WSfd1234e1c4b69f30ea53e41001031ab64-7751.html BMP Format] {{Webarchive|url=https://web.archive.org/web/20110922225022/http://livedocs.adobe.com/en_US/Photoshop/10.0/WSfd1234e1c4b69f30ea53e41001031ab64-7751.html |date=2011-09-22 }}</ref>|448px|center}}

{{Plain image with caption|AllBMPformats.png|All of the possible pixel formats in a DIB|700px|center}}