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

netpbm documentation                              12 April 2013                                 Windows Icons(1)