Ico windows icon format

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CUR

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 file 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 to where the user is actually pointing the mouse.^{[citation needed]}

The ANI file format is used for animated Windows cursors.

Icons introduced in Windows 1.0 were 32×32 pixels in size and were monochrome.^[2] Support for 16 colors was introduced in Windows 3.0.^{[citation needed]}

Win32 introduced support for storing icon images of up to 16.7 million colors (TrueColor) and up to 256×256 pixels in dimensions.^[3] Windows 95 also introduced a new Device Independent Bitmap (DIB) engine.^[4] 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^[3][5] 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.^[3] 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 8-bit 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.^[3] Microsoft only recommended icon sizes up to 48×48 pixels for Windows XP.^[6] Windows XP can downscale larger icons if no closer image size is available.^[3]

Windows Vista added full support for 256×256-pixel 32-bit color icons,^{[Notes 1]} as well as support for the compressed PNG format. Although compression is not required, Microsoft recommends that all 32-bit color 256×256 icons in 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.^[7][8]

While the IANA-registered MIME type for ICO files is image/vnd.microsoft.icon,^[9] it was submitted to IANA in 2003 by a third party and is not recognised by Microsoft software, which uses image/x-icon or image/ico instead.^[10][11] Erroneous types image/ico, image/icon, text/ico and application/ico have also been seen in use.^[9]

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).^[3]

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»).^[6][3] 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(1) and which are fully opaque(0). 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.^[12]

32-bit images (including 32-bit BITMAPINFOHEADER-format BMP images^{[Notes 2]}) 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.

It’s important to note that in the AND mask, as for the pixel array of the bitmap, padding bytes must be appended to the end of each row in order to bring up its length to a multiple of four bytes since it’s basically a (monochrome) bitmap.^[3] The AND mask of an 8×8 pixels bitmap would have 1 byte of data and 3 bytes of padding(8*8*1bpp = 64 bits/8 = 8 bytes of total rows, so each row is 1 byte and 3 bytes of padding are needed), a 16×16 bitmap’s AND mask would have 2 bytes of data and 2 bytes of padding, a 32×32 bitmap’s AND mask would have 4 bytes of data and no padding. Note that the quantity of padding bytes needed depend on the dimensions of the bitmap and not its color depth since the AND mask is 1 bit per pixel regardless.

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

ICONDIR structure

Image entry

ICONDIRENTRY structure

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.

If an image is stored in BMP format, it must exclude the opening BITMAPFILEHEADER structure.

The height of the BMP image must be twice the height declared in the image directory. This is because the actual image data will contain two parts: the actual image immediately followed by a 1 bit mask of the same size as the image used to determine which pixels will be drawn.

The mask has to align to a DWORD (32 bits) and should be packed with 0s. A 0 pixel means ‘the corresponding pixel in the image will be drawn’ and a 1 means ‘ignore this pixel’. The pixel colour is either explicit for 24 and 32 bit versions (which do not have colour tables), indexed for the other depths (1,2,4,8,16) in table of a four byte (BGRA) colours that follows the BITMAPINFOHEADER.

For 1 bit, typically the two colours are #00000000 and #00FFFFFF and the A channel is ignored.

The pixel data for 1,2,4,8 and 16 bits is packed by byte and DWORD aligned.

24 bit images are stored as B G R triples but are not DWORD aligned.

32 bit images are stored as B G R A quads.

Originally, ICOs and CURs were intended to be used on monochrome displays and used the formula Output = (Existing AND Mask) XOR Image but on colour screens, the cursor is composed using A channel blending and the mask is used to determine which pixels are included or excluded.

If an image is stored in PNG format, then there is no BITMAPINFOHEADER and the contents of the PNG image in full are used instead.

The ability to read PNG images from ICO format images was introduced in Windows Vista.^[7] 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 in 32bpp ARGB format.^[7]

Note: CUR files cannot use PNG image data and must use BMP image data as per ICOs.^[7]

Icons and cursors in Portable Executable (EXE or DLL) files are organised in resources of type RT_GROUP_ICON, RT_GROUP_CURSOR, RT_ICON and RT_CURSOR.^[13]

RT_GROUP_ICON and RT_GROUP_CURSOR resources contain NEWHEADER structure and one or more RESDIR structures which have almost the same format as corresponding ICONDIR and ICONDIRENTRY structures in ICO/CUR files. Main difference is in RESDIR that the last member of the structure contains two-byte resource identifier of the RT_ICON/RT_CURSOR instead of image offset in the file.

NEWHEADER/RESDIR structures also referred as GRPICONDIR/GRPICONDIRENTRY in many sources.^[14]

NEWHEADER structure

RESDIR structure

RT_ICON and RT_CURSOR resources have the same image data format as in ICO files and can store PNG images as well. Additionally, first four bytes of RT_CURSOR resource data contain the cursor hotspot data, as two WORD (16 bit) values (in contrast to CUR files, in which hotspot data is contained in the ICONDIRENTRY structure).^[15]

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 an .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.^[16]

• Apple Icon Image format
• BMP file format
• Computer icon
• Favicon
• List of icon software

• Icon design guidelines for Windows Vista icons
• Icon design guidelines for Windows 10 app icons
• Icons (1995 Microsoft Technical Article)
• The evolution of the ICO file format (Part 1, Part 2, Part 3, Part 4)
• The format of icon resources
• The format of icon resources, revisited
• A Look Inside Windows Icons, Part 1 — PC Mag Jan 26, 1993 Vol.12 No. 2
• A Look Inside Windows Icons, Part 2 — PC Mag Feb 9, 1993 Vol.12 No. 3
• Enable High Color Icons in Windows 95 and later
• Enhance Java GUIs with Windows Icons

Relevant source files

• ico/d3.ico
• ico/program_exe.ico
• ico/program_install.ico

Purpose and Scope

This document provides detailed technical specifications of the ICO file format as used in the Folder-Ico repository. It outlines the binary structure of ICO files, their headers, directory entries, and image data formats. This information is essential for developers who need to work with, validate, or generate compliant ICO files. For information about how PNG image data is embedded within ICO files, see PNG Data in ICO Files.

Overview of ICO Format

The ICO format is Microsoft’s standard format for icon files in Windows operating systems. Each ICO file serves as a container that can hold multiple icon images in different sizes, color depths, and resolutions. This allows applications to select the most appropriate icon representation based on display settings.

Sources: ico/program_install.ico1-10

The ICO file starts with a fixed-size header that contains basic information about the file type and the number of images contained within.

In the repository’s ICO files, the header typically indicates the file is an ICO file (type=1) and contains multiple images in different sizes.

Sources: ico/program_install.ico1-2

Icon Directory

Following the header is the icon directory, which consists of a series of entries, one for each image in the file. Each entry provides metadata about the corresponding image and indicates where its data is located within the file.

Directory Entry Structure

Each directory entry is 16 bytes in size and structured as follows:

The directory entries in Folder-Ico’s icons typically include variants with different bit depths, with 32-bit (RGBA) being common for high-quality icons.

Sources: ico/program_install.ico1-10

Image Data Formats

The ICO format supports two main types of image data: BMP (legacy format) and PNG (modern format). The structure differs based on the format used.

BMP Format (Legacy)

For BMP format, the image data consists of:

1. DIB (Device-Independent Bitmap) header:

   • Usually the BITMAPINFOHEADER structure (40 bytes)
   • Specifies dimensions, color depth, and compression

2. Pixel data:

   • XOR mask (color data)
   • AND mask (transparency mask for older formats)

PNG Format (Modern)

For PNG format, the image data is a standard PNG file with:

1. PNG signature: The bytes 89 50 4E 47 0D 0A 1A 0A
2. PNG chunks (IHDR, IDAT, IEND, etc.)

The PNG format is preferred in modern icons because it provides:

• Better compression
• True alpha channel transparency
• Wider color support
• Better cross-platform compatibility

In the Folder-Ico repository, many icons use the PNG format embedded within the ICO container to provide high-quality icons with transparency.

Sources: ico/program_install.ico2-246

Multi-Size Icon Storage

One of the key features of the ICO format is its ability to store multiple images of different sizes within a single file. This allows applications to choose the most appropriate size based on the display context.

Common Icon Sizes in Repository

The Folder-Ico repository includes ICO files with multiple image sizes to ensure compatibility with different display contexts. Common sizes include:

Each size version is stored as a separate image entry within the ICO file.

Sources: ico/program_install.ico1-274

ICO File Byte Structure Example

The actual byte structure of an ICO file follows this pattern:

Offset    Size    Description
------    ----    -----------
0x0000    2       Reserved (0)
0x0002    2       Type (1 for ICO)
0x0004    2       Image count

// For each image, there's a directory entry:
0x0006    1       Width of first image
0x0007    1       Height of first image
0x0008    1       Color count
0x0009    1       Reserved (0)
0x000A    2       Color planes
0x000C    2       Bits per pixel
0x000E    4       Size of image data
0x0012    4       Offset to image data

// More directory entries follow...

// Image data starts at the specified offsets

The complexity of ICO files lies in their format flexibility, particularly in how they handle the image data (BMP or PNG) and the organization of multiple images of different sizes.

Sources: ico/program_install.ico1-10

ICO Format Limitations and Considerations

Despite its versatility, the ICO format has certain limitations and special considerations that developers should be aware of:

For optimal compatibility, the Folder-Ico repository typically includes multiple size variants in each ICO file, with both high-quality PNG data for modern systems and BMP data for legacy support.

Sources: ico/program_install.ico1-274

Relationship to System Architecture

Within the Folder-Ico repository, ICO files serve as the primary visual assets and are paired with hash files for integrity verification.

Understanding the ICO file format is essential for developers working with the Folder-Ico repository, as it helps ensure the correct generation, modification, and validation of icon files within the system.

Sources: ico/program_install.ico1-274

ICO (Windows Icon) Format

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Материал из РУВИКИ — свободной энциклопедии

У этого термина существуют и другие значения, см. ICO.

ICO (Windows icon) — формат хранения файлов значков в Microsoft Windows.

Формат ICO аналогичен формату CUR (Windows cursors), предназначенному для хранения курсоров. Отличие состоит в численном значении одного поля в заголовочной структуре и интерпретации значений двух других полей этой же структуры.

Один ICO-файл содержит один или несколько значков разных размеров и разрешений. Размер значка может быть любым, но наиболее употребимы квадратные значки со стороной 16, 32 и 48 пикселей. Также используются значки с размером 24, 40, 60, 72, 92, 108, 128 пикселей.

Значки бывают в естественном цвете (True Color, глубина цвета 24 бит), High Color (глубина цвета 16 бит) или с фиксированной палитрой (из двухсот пятидесяти шести, шестнадцати или всего из двух цветов). В этом случае число, соответствующее каждому пикселю, указывает не на цвет, а на номер цвета в палитре.

По своей структуре изображения в файле ICO наиболее близки к BMP, но принципиально отличаются от них наличием дополнительного изображения — маски, накладываемой на задний план при помощи операции побитового «И», что позволяет реализовать (полную) прозрачность рисунка. Последующее наложение основного изображения с помощью «исключающего ИЛИ» может даже дать «инверсные» пиксели в тех местах, где задний план не был замаскирован.

Кроме того, начиная с Windows XP поддерживаются 32-битные значки — каждому пикселу соответствует 24-бита цвета и 8-битный альфа-канал, позволяющий реализовать 256 уровней частичной прозрачности. С помощью альфа-канала можно отображать значки со сглаженными (размытыми) краями и тенью, сочетающимися с любым фоном; маска в этом случае игнорируется.

Начиная с Windows Vista, формат поддерживает размеры значков более 256 пикселей на сторону и сжатие с помощью формата PNG. В этом случае маска не содержится в файле отдельно, а генерируется автоматически из альфа-канала PNG изображения.^[2]

Файл состоит из заголовка фиксированной длины, каталога информации о изображениях и непосредственно изображений.

Заголовок[править | править код]

Заголовок имеет размер 6 байт:

Каталог информации об изображениях[править | править код]

Представляет собой последовательные записи фиксированного размера (16 байт), следующие одна за другой. Количество записей определяется полем count заголовка.

Часть полей может содержать 0 и легко получается из других данных; например, если изображение не хранится в формате PNG, тогда количество битов на пиксель рассчитывается на основе информации о размере растра, а также его ширине и высоте.

• Значок
• favicon
• Глубина цвета