Convert ICO to PNG

What is inside an .ico

An ICO is a tiny directory followed by several images, each a different size and colour depth: historically 16×16 and 32×32 at 16 or 256 colours, today usually 16, 32, 48 and 256 pixels with full 32-bit colour. The entries are stored either as Windows bitmaps with a separate transparency mask or — for anything 256 pixels and above, since Vista — as embedded PNGs. The decoder picks the largest entry and hands over its pixels with the mask applied, so the PNG you get is the best picture the file holds.

The alpha channel in an old icon is one bit — a pixel is either there or not — and the edges of a 16-colour icon from the nineties look exactly as jagged in the PNG as they did on screen. A modern icon carries a full 8-bit alpha and its edges come across soft. Nothing is smoothed or enlarged on the way; the PNG is the icon's own pixels.

A favicon downloaded from a website is sometimes not an ICO at all despite the name — many sites serve a PNG or an SVG as favicon.ico. The file is read by its contents, so those decode too; what comes out is simply the picture at its one size.

When you want something else

For every size in the icon rather than the largest, ImageMagick's magick in.ico out.png writes out-0.png, out-1.png and so on, one per entry, and icotool -x from icoutils does the same on Linux and lists what the file contains first. For icons inside a program, Resource Hacker on Windows or wrestool from icoutils extracts them.

If the icon was made from artwork you still have, go back to it: a 256-pixel PNG out of an icon is the end of the line for resolution, where the SVG or the original PNG can be rendered at any size.