ICO (File Format)

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1	handwiki	Vivi Li	--	1386	2022-11-04 01:35:11

The content is sourced from: https://handwiki.org/wiki/ICO_(file_format)

The ICO file format is an image file format for computer icons in Microsoft Windows. ICO files contain one or more small images at multiple sizes and color depths, such that they may be scaled appropriately. In Windows, all executables that display an icon to the user, on the desktop, in the Start Menu, or in Windows Explorer, must carry the icon in ICO format. The CUR file format is an almost identical image file format for non-animated cursors in Microsoft Windows. The only differences between these two file formats are the bytes used to identify them and the addition of a hotspot in the CUR format header; the hotspot is defined as the pixel offset (in x,y coordinates) from the top-left corner of the cursor image where the user is actually pointing the mouse.

pixel offset cursor hotspot

References

Fekete, Gyorgy (March 11, 2009). "Operating System Interface Design Between 1981-2009". Webdesigner Depot. http://www.webdesignerdepot.com/2009/03/operating-system-interface-design-between-1981-2009/. Retrieved June 6, 2011.
Hornick, John (September 29, 1995). "Icons". Windows User Interface Technical Articles (MSDN). Microsoft Corporation. http://msdn.microsoft.com/en-us/library/ms997538.aspx. Retrieved June 5, 2011.
"Windows 95 Architecture Components". Microsoft TechNet. Microsoft Corporation. https://technet.microsoft.com/en-us/library/cc751120.aspx. Retrieved June 6, 2011.
"Shell Icon BPP". Windows 2000 Registry Reference (Microsoft TechNet). Microsoft Corporation. https://technet.microsoft.com/en-us/library/cc938238.aspx. Retrieved June 6, 2011.
"Creating Windows XP Icons". Windows XP Technical Articles (MSDN). Microsoft Corporation. July 2001. http://msdn.microsoft.com/en-us/library/ms997636.aspx. Retrieved June 5, 2011.
"Icons". The evolution of the ICO file format, part 4: PNG images. Raymond Chen - MSFT. https://devblogs.microsoft.com/oldnewthing/20101022-00/?p=12473. Retrieved October 22, 2010.
Butcher, Simon (3 Sep 2003). "image/vnd.microsoft.icon". http://www.iana.org/assignments/media-types/image/vnd.microsoft.icon. Retrieved 3 Jan 2014.
Lawrence, Eric (11 Feb 2011). "IE9 RC Minor Changes List". MSDN Blogs. https://blogs.msdn.microsoft.com/ieinternals/2011/02/11/ie9-rc-minor-changes-list/#comment-1108. Retrieved 20 Aug 2016. See the second comment.
The classic BITMAPINFOHEADER bitmap format supports storing images with 32 bits per pixel. When saved as a standalone .BMP file, "the high byte in each [pixel] is not used". However, when this same data is stored inside a ICO or CUR file, Windows XP (the first Windows version to support ICO/CUR files with more than 1 bit of transparency) and above interpret this byte as an alpha value.
Although Microsoft's technical documentation states that this value must be zero, the icon encoder built into .NET (System.Drawing.Icon.Save) sets this value to 255. It appears that the operating system ignores this value altogether.
Setting the color planes to 0 or 1 is treated equivalently by the operating system, but if the color planes are set higher than 1, this value should be multiplied by the bits per pixel to determine the final color depth of the image. It is unknown if the various Windows operating system versions are resilient to different color plane values.
The bits per pixel might be set to zero, but can be inferred from the other data; specifically, if the bitmap is not PNG compressed, then the bits per pixel can be calculated based on the length of the bitmap data relative to the size of the image. If the bitmap is PNG compressed, the bits per pixel are stored within the PNG data. It is unknown if the various Windows operating system versions contain logic to infer the bit depth for all possibilities if this value is set to zero.
Chen, Raymond (May 2008). "Windows Confidential - 16-Bit Icons Are So Passé". TechNet Magazine. https://technet.microsoft.com/en-us/magazine/2008.05.windowsconfidential.aspx. Retrieved July 1, 2011.

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