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Windows Icons(1) General Commands Manual Windows Icons(1)
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 logi-
cally 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 shad-
ows.
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:
o Windows 1 used monochrome 32x32 icons only.
o Windows 3.0 added color icons with bpp values up to 8.
o Windows 4.0 (a.k.a. Windows95) added support for 32-bpp images and resolutions up
to 256 by 256.
o NT 5.1 (a.k.a. Windows XP) added support for the 8-bit transparency channel in the
unused bits of 32-bpp images.
o NT 6.0 (a.k.a. Windows Vista) added support 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
<TD rowspan=3>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)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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