Free webp to jpg
EN
Image MIME Reference
ICO

Windows Icon (Legacy MIME)

image/x-icon
VS

Silicon Graphics Image

image/sgi
SGI

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

ICO wins on browser support

Feature Support

Feature ICO SGI
Transparency (Alpha) Yes Yes
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 →
Silicon Graphics Image
image/sgi
Extension
.sgi.rgb.rgba.bw.int.inta
Container SGI Image Format
Compression Uncompressed / Lossless (Run-Length Encoding)
Algorithm
RLENone
Color Depth 8-bit, 16-bit (Per Channel)
Developed by Paul Haeberli (Silicon Graphics)
Released 1990
Magic Bytes
01 DA
Full SGI reference →

Browser Support Comparison

Browser ICO SGI
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
SGI

SGI Strengths

  • Pioneered multi-channel and 16-bit per channel imaging capabilities required for high-end 1990s visual effects
  • Extremely robust and simple header structure
  • Reliable Run-Length Encoding provided solid lossless compression for flat-shaded 3D renders
Limitations
  • Completely obsolete for modern web and application development
  • Zero native web browser compatibility
  • Largely replaced by TIFF and OpenEXR in modern professional VFX and 3D pipelines

When to choose which format

ICO
Use ICO when…
  • Website Favicons (favicon.ico)
  • Windows Application Executable Icons (.exe)
  • Desktop Shortcut Icons
SGI
Use SGI when…
  • Legacy 3D Animation & VFX
  • Silicon Graphics IRIX Workflows
  • Historical CGI Archiving

More comparisons

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