Editing BMP file format (section)

==== Pixel array (bitmap data) ====
The pixel array is a block of 32-bit DWORDs, that describes the image pixel by pixel. Usually pixels are stored "bottom-up", 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.<ref name="DIBhelp" /> Unless {{mono|BITMAPCOREHEADER}} is used, 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).<ref name=DIBhelp/> 
Contemporary DIB Headers allow pixel formats with 1, 2, 4, 8, 16, 24 and 32 bits per pixel (bpp).<ref>MSDN - {{mono|BITMAPINFOHEADER}}: [http://msdn.microsoft.com/en-us/library/dd183376%28VS.85%29.aspx The member biBitCount]</ref> [[Graphics Device Interface|GDI+]] also permits 64 bits per pixel.<ref>{{cite web|url=http://msdn.microsoft.com/en-us/library/ms536393(v=vs.85).aspx|publisher=[[Microsoft Developer Network|MSDN]]|title=Types of Bitmaps|date=2012-06-03|access-date=2014-03-16}}</ref>

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 Arrays 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.<ref name="DIBhelp" /> 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 6 bytes of data and 2 bytes of padding, Width=3 would have 9 bytes of data and 3 bytes of padding, and Width=4 would have 12 bytes of data and no padding.