1. History

Icons introduced in Windows 1.0 were 32×32 pixels in size and were monochrome.^[1] Support for 16 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.^[2] Windows 95 also introduced a new Device Independent Bitmap (DIB) engine.^[3] 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^[2][4] 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.^[2] Thus, a single icon file could store images of any size from 1×1 pixel up to 256×256 pixels (including non-square sizes) with 2 (rarely used),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 added 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 effects 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 icons throughout the shell to be upscaled.^[2] Microsoft only recommended icon sizes up to 48×48 pixels for Windows XP.^[5] Windows XP can downscale 256×256 and larger icons if no closer image size is available.^[2]

Windows Vista added a 256×256-pixel icon view to Windows Explorer, as well as support for the compressed PNG format. Although compression is not required, Microsoft recommends that all 256×256 icons in ICO files should be stored in PNG format to reduce the overall size of the file.^[6] 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.^[6]

2. MIME Type

While the IANA-registered MIME type for ICO files is image/vnd.microsoft.icon,^[7] it was submitted to IANA in 2003 by a third party and is not recognised by Microsoft software, which uses image/x-icon instead.^[8] Erroneous types image/ico, image/icon, text/ico and application/ico have also been seen in use.^[7]

3. Icon Resource Structure

An ICO or CUR file is made up of an ICONDIR ("Icon directory") structure, containing an ICONDIRENTRY structure for each image in the file, followed by a contiguous block of all image bitmap data (which may be in either Windows BMP format, excluding the BITMAPFILEHEADER structure, or in PNG format, stored in its entirety).^[2]

Images with less than 32 bits of color depth follow a particular format: the image is encoded as a single image consisting of a color mask (the "XOR mask") together with an opacity mask (the "AND mask").^[5] The XOR mask must precede the AND mask inside the bitmap data; if the image is stored in bottom-up order (which it most likely is), the XOR mask would be drawn below the AND mask. The AND mask is 1 bit per pixel, regardless of the color depth specified by the BMP header, and specifies which pixels are fully transparent and which are fully opaque. The XOR mask conforms to the bit depth specified in the BMP header and specifies the numerical color or palette value for each pixel. Together, the AND mask and XOR mask make for a non-transparent image representing an image with 1-bit transparency; they also allow for inversion of the background. The height for the image in the ICONDIRENTRY structure of the ICO/CUR file takes on that of the intended image dimensions (after the masks are composited), whereas the height in the BMP header takes on that of the two mask images combined (before they are composited). Therefore, the masks must each be of the same dimensions, and the height specified in the BMP header must be exactly twice the height specified in the ICONDIRENTRY structure.

32-bit images (including 32-bit BITMAPINFOHEADER-format BMP images^[9]) are specifically a 24-bit image with the addition of an 8-bit channel for alpha compositing. Thus, in 32-bit images, the AND mask is not required, but recommended for consideration. Windows XP and higher will use a 32-bit image in less than True color mode by constructing an AND mask based on the alpha channel (if one does not reside with the image already) if no 24-bit version of the image is supplied in the ICO/CUR file. However, earlier versions of Windows interpret all pixels with 100% opacity unless an AND mask is supplied with the image. Supplying a custom AND mask will also allow for tweaking and hinting by the icon author. Even if the AND mask is not supplied, if the image is in Windows BMP format, the BMP header must still specify a doubled height.

3.1. Outline

All values in ICO/CUR files are represented in little-endian byte order.

Header

Structure of image directory

Referenced image data

All image data referenced by entries in the image directory proceed directly after the image directory. It is customary practice to store them in the same order as defined in the image directory.

Recall that if an image is stored in BMP format, it must exclude the opening BITMAPFILEHEADER structure, whereas if it is stored in PNG format, it must be stored in its entirety.

3.2. PNG Format

The ability to read PNG images from ICO and CUR format images was introduced in Windows Vista.^[6] A PNG image can be stored in the image in the same way as done for a standard Windows BMP format image, with the exception that the PNG image must be stored in its entirety, with its file header.

4. Icon Library

An icon library is a way to package Windows icons. It is typically a 16-bit New Executable or a 32-bit Portable Executable binary file having .ICL extension with icon resources being the packaged icons. Windows Vista and later versions do not support viewing icons from 16-bit (New Executable) files.^[13]