ICO → PNG 변환기

An ICO starts with a directory listing each image it holds: its width, height, colour depth and where its bytes begin. A favicon.ico commonly carries 16×16, 32×32 and 48×48 versions, and Windows application icons add a 256×256 one. Smaller entries are usually stored as BMP bitmaps with a separate 1-bit transparency mask; the 256×256 entry is usually a complete PNG file embedded as is, because PNG is far smaller at that size.

The arithmetic shows why. A single 32×32 entry at 32 bits per pixel needs 4,096 bytes of colour data, a 40-byte bitmap header and a 128-byte mask, so an icon holding only that entry weighs 4,286 bytes with its directory. Add 16 and 48 px bitmaps and the three entries come to 15,102 bytes; a 256 px bitmap on its own would need over 260,000, while the same icon stored as PNG usually takes a few KB to a few tens of KB.

When a browser shows an .ico in an image tag, it chooses which entry to display, and that choice is not guaranteed to be the largest. Converting through that path can leave you with a 32×32 PNG from an icon that also contained a 256×256 version. So this page parses the ICO directory directly and selects the entry with the most pixels, using colour depth to break ties.

If that entry is a PNG, its bytes are copied out unchanged: an embedded PNG of 9,000 bytes comes out as a 9,000-byte PNG, with no re-encoding. If it is a bitmap, the tool decodes the 32, 24, 8, 4 or 1-bit pixel data, applies the transparency mask and writes a PNG from those pixels. The directory’s size field cannot express anything above 256, so dimensions are read from the embedded image’s own header, which means a 500×500 PNG inside an ICO is exported at its real 500×500.

Recovering a usable logo from an old site’s favicon.ico when the original artwork is gone; pulling an application icon into a slide, document or README; turning a Windows .ico into a PNG for a web app manifest, which expects PNG icons such as 192×192 and 512×512; or checking what a favicon actually contains before replacing it. If the largest entry is small, say 32×32, the PNG will be small too. Enlarging it in an editor adds blur rather than detail, so for a sharp large logo you need the original artwork.

The ICO is read into your browser’s memory, parsed and turned into a PNG there; no server receives the file. Each icon converts on its own, up to 30 files per drop at 50 MB each, and the results download one by one or together as a ZIP. A file with an .ico name that actually holds a PNG or JPEG (it happens with favicons saved from websites) is still converted, through the browser’s own decoder.