Convert ICO to PPM / PNM

The ICO format is an image container specifically designed for Microsoft Windows to store icons for executable files, folders, and applications. Introduced with Windows 1.0, it can encapsulate multiple raster images of varying dimensions and color depths (specifically BMP or PNG frames) within a single file to support dynamic scaling across different display resolutions. The structure comprises an ICONDIR header referencing one or more ICONDIRENTRY directory entries, allowing the operating system to select the appropriate 16x16, 32x32, or 48x48 pixel resource based on the current UI scaling settings.

ICO files are natively supported by the Microsoft Windows operating system, which automatically renders them in the File Explorer and taskbar. On macOS, Apple Preview handles the format natively. For editing or detailed inspection of the contained bitmap layers, raster graphics editors such as GIMP, Adobe Photoshop (via plugin), or Paint.NET are required. Web browsers also natively render ICO files when they are designated as favicons.

The Portable Pixel Map (PPM) format is a lowest-common-denominator color image format developed by Jef Poskanzer as part of the Netpbm project. It stores pixels as uncompressed RGB triplets (Red, Green, Blue) in either a plain ASCII text format (P3 header) or a more efficient binary format (P6 header). The file structure is extremely minimal, consisting of a magic number identifier, dimensions (width and height), maximum color value, and the pixel data itself, making it easily parsable by simple scripts but inefficient for storage compared to compressed formats like JPEG or PNG.

Native support for PPM files is absent in standard Windows or macOS photo viewers. Users typically require specialized image manipulation software to view or convert these files. GIMP (GNU Image Manipulation Program) and IrfanView are the primary tools for opening PPM files on desktop environments. On Linux systems, the Netpbm toolkit and ImageMagick suite provide command-line utilities for processing and converting PPM data.