Free webp to jpg
EN
Image MIME Reference
ICO

Windows Icon (Legacy MIME)

image/x-icon
VS

Portable Anymap

image/x-portable-anymap
PNM

A complete technical comparison of Windows Icon (Legacy MIME) and Portable Anymap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

ICO wins on browser support

Feature Support

Feature ICO PNM
Transparency (Alpha) Yes ✕ No
Animation Support ✕ No ✕ No
Progressive Loading ✕ No ✕ No
HDR Support ✕ No ✕ No
EXIF Metadata ✕ No ✕ No
ICC Color Profile ✕ No ✕ No
Windows Icon (Legacy MIME)
image/x-icon
Extension
.ico
Container ICO Directory Structure
Compression Uncompressed (DIB/BMP) / Deflate (Embedded PNG)
Algorithm
NoneDeflate (via PNG)
Color Depth 1-bit, 4-bit, 8-bit, 24-bit (True Color), 32-bit (RGBA)
Developed by Microsoft Corporation
Released 1985
Magic Bytes
00 00 01 00
Full ICO reference →
Portable Anymap
image/x-portable-anymap
Extension
.pnm
Container Netpbm
Compression Uncompressed
Algorithm
None
Color Depth 1-bit (PBM), 8-bit to 16-bit (PGM), 24-bit to 48-bit (PPM)
Developed by Jef Poskanzer (Netpbm Project)
Released 1988
Magic Bytes
50 31, 50 32, 50 33, 50 34, 50 35, or 50 36
Full PNM reference →

Browser Support Comparison

Browser ICO PNM
Chrome Chrome
✓ 1
 ✕
Firefox Firefox
✓ 1
 ✕
Safari Safari
✓ 1
 ✕
Edge Edge
✓ 12
 ✕
IE IE (Legacy)
✓
 ✕
ICO

ICO Strengths

  • Universally supported by every web browser in existence for website favicons
  • Packs multiple resolutions into a single file, eliminating the need to serve different files for the taskbar vs. the desktop
  • Modern support for embedded PNGs (since Windows Vista) allows for excellent 32-bit transparency (alpha channels) and drastically reduced file sizes
Limitations
  • Internal file structure is convoluted due to decades of legacy cruft, such as the requirement for BMP data to lack a standard file header but double the height property for an archaic XOR/AND mask
  • Larger file size compared to a single optimized PNG or SVG, as it literally stores the same image multiple times at different scales
  • The MIME type landscape is highly fragmented (x-icon vs. vnd.microsoft.icon), causing occasional caching or strict-MIME validation errors on restrictive servers
PNM

PNM Strengths

  • Unparalleled simplicity: parsers and writers for PNM files can be written from scratch in C or Python in just a few minutes without relying on external libraries
  • Serves as an excellent foundational teaching tool for computer science students learning about image arrays and raster graphics
  • Flawlessly pipes through Unix command-line utilities for robust automated image processing (via the Netpbm suite)
Limitations
  • Zero compression natively results in astronomical file sizes, making it entirely impractical for web delivery or consumer storage
  • No support for standard metadata formats like EXIF, XMP, or ICC color profiles
  • No native alpha channel support in the classic PNM specification (though the newer PAM format 'P7' addresses this)

When to choose which format

ICO
Use ICO when…
  • Website Favicons (favicon.ico)
  • Windows Application Executable Icons (.exe)
  • Desktop Shortcut Icons
PNM
Use PNM when…
  • Unix/Linux Command-Line Image Processing (Netpbm suite)
  • Academic Computer Vision Research
  • Intermediary Format for Image Format Conversion

More comparisons

ICO vs ACES → PNM vs ACES → ICO vs APNG → PNM vs APNG → ICO vs AVCI → PNM vs AVCI → ICO vs AVCS → PNM vs AVCS → ICO vs AVIF → PNM vs AVIF → All comparisons →
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