History
Icons introduced in Windows 1.0 were 32×32 pixels in size and were monochrome. Support for 256 colors was introduced in Windows 3.0.
Win32 introduced support for storing icon images of up to 16.7 million colors (Truecolor) and up to 256x256 pixels in dimensions. Windows 95 also introduced a new Device Independent Bitmap (DIB) engine. However, 256 color was the default icon color depth in Windows 95. It was possible to enable 65535 color (Highcolor) icons by either modifying the Shell Icon BPP value in the registry or by purchasing Microsoft Plus! for Windows 95. The Shell Icon Size value allows using larger icons in place of 32×32 icons and the Shell Small Icon Size value allows using custom sizes in place of 16×16 icons. Thus, even prior to Windows XP, a single icon file could store images of any size from 1×1 pixel up to 256×256 pixels (including non-square sizes) with 16, 256, 65535, or 16.7 million colors; but the shell could not display very large sized icons. The notification area of the Windows taskbar was limited to 16 color icons by default until Windows Me when it was updated to support high color icons.
Windows XP adds support for 32-bit color (16.7 million colors plus alpha channel transparency) icon images, thus allowing semitransparent areas like shadows, anti-aliasing, and glass-like shapes to be drawn in an icon.
Windows XP, by default, employs 48×48 pixel icons in Windows Explorer. Windows XP can be forced to use icons as large as 256×256 by modifying the Shell icon size value but this would cause all 32×32 sized icons throughout the shell to be upsized. Microsoft only recommended icon sizes up to 48×48 pixels for Windows XP. Windows XP can read 256×256 pixel icon images and larger and it can resize them to use if no closer image size is available.
Windows Vista adds support for displaying 256×256 pixel icon images as a separate view in Windows Explorer, and supports (but does not require) the compressed PNG format. Microsoft recommends that all 256×256 icons in the ICO files should be stored in PNG format to reduce the overall size of the file. The Windows Vista Explorer supports smoothly scaling icons to non-standard sizes which are rendered on the fly even if an image is not present for that size in the icon file. The Windows Vista shell adds a slider for "zooming" the icon sizes in and out. With users using higher resolutions and high DPI modes, larger icon formats (such as 256×256) are recommended.
Read more about this topic: ICO (file Format)
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—Ralph Waldo Emerson (1803–1882)
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—Anton Pavlovich Chekhov (1860–1904)