NAME

       Microsoft Windows icon files

DESCRIPTION

       A Microsoft Windows icon file contains one or more images, at resolutions up to 256 by 256
       pixels and various bpp values.  The images are encoded either as Portable Network Graphics
       file (PNG), or in a format similar to Microsoft's BMP format.

       If  encoded  as  BMP,  the image includes an "AND mask", which contains 1-bit transparency
       data.  It may also contain additional 8-bit transparency  data  together  with  the  color
       information.

   Color Depth
       Except  for  the 16 bits per pixel images and images with bit fields, which both are rare,
       the colors in all BMP encoded images are RGB with 8 bits per  channel.   Images  with  bpp
       values lower than 16 use a palette.

       I.e. the bpp value gives the number of distinct colors, not the color depth.

   XOR Mask and AND Mask
       BMP  encoded  images  contain  two  pixel  maps: The so-called "XOR mask" stores the color
       information for each pixel, and the "AND mask" stores the transparency belonging to it.

       The names and the function of these maps are most easily understood by looking  at  how  a
       1-bpp  icon  image  is  rendered  on  a  monochrome  screen:  The pixels on the screen are
       logically AND-ed with the bits on the AND mask, then the result is logically  XOR-ed  with
       the bits on the XOR mask.

       The  result  is  that  if a bit on the AND mask is reset, the corresponding bit on the XOR
       mask determines the color of the pixel on the screen.  If a bit in the AND mask is set and
       the  corresponding  bit  in  the  XOR  mask  is  black  (reset), the image is transparent.
       Finally, if the bits are set in both the AND and XOR mask (the pixel on the  XOR  mask  is
       white), the background of the screen is inverted.

       In  color  environments,  a  pixel on the XOR mask outside the opaque area of the image is
       usually black and sometimes white, but a color other than black and white will hardly give
       predictable results.

       Since  Windows  XP,  there may also be an 8-bit transparency channel in 32-bpp BMP encoded
       icon images. The AND mask, however,  is  still  required  and  used  e.g.  for  generating
       shadows.

       PNG  encoded images don't contain AND masks.  While rendering a PNG encoded image, Windows
       constructs an AND mask on the fly from the transparency channel, if present.

   Evolution of Windows Icons
       The Windows icon file format has undergone some extensions since it was  invented  in  the
       mid-eighties for Windows 1:

       •      Windows 1 used monochrome 32x32 icons only.

       •      Windows 3.0 added color icons with bpp values up to 8.

       •      Windows 4.0 (a.k.a. Windows95) added option for 32-bpp images and resolutions up to
              256 by 256.

       •      NT 5.1 (a.k.a. Windows XP) added option for the 8-bit transparency channel  in  the
              unused bits of 32-bpp images.

       •      NT 6.0 (a.k.a. Windows Vista) added option for PNG encoded images

   Common Resolutions and BPP Values
       Typical resolutions and bpp values of the Windows shell icons include:

       ────────────────────────────────────────────────
       OS             resolutions           bpp values
       Windows 3      32x32                 1, 4

       Windows 4      16x16, 32x32, 48x48   4, 8
       NT 5           16x16, 32x32, 48x48   4, 8, 32
       NT 6           16x16, 32x32, 48x48   4, 8, 32
       24x24, 96x96   8, 32

       256x256        32 (PNG encoded)

       Within  the  icon file, the images with low bpp values are usually stored first.  With the
       same bpp value, the images are sorted by resolution, large images first.

   MIME Type and File Name Extension
       The MIME type of Windows icon files is registered by IANA as image/vnd.microsoft.icon, but
       the unofficial name image/x-icon is still widely used.

       The  file  name extension (used by Microsoft operating systems as file type identifier) is
       .ico.

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/winicon.html