Free webp to jpg
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Image MIME Reference
ICO

Windows Icon (Legacy MIME)

image/x-icon
VS

PTI Image (Personal/Vanity Tree)

image/prs.pti
PTI

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

ICO wins on browser support

Feature Support

Feature ICO PTI
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 →
PTI Image (Personal/Vanity Tree)
image/prs.pti
Extension
.pti
Container Proprietary
Compression Unknown / Proprietary
Algorithm
Unknown
Color Depth Unknown
Developed by Jürn Laun
Released 1999
Magic Bytes
Unknown
Full PTI reference →

Browser Support Comparison

Browser ICO PTI
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
PTI

PTI Strengths

  • Provided a registered MIME type namespace for a specific application's proprietary image format without conflicting with standard web types
Limitations
  • Completely undocumented and unsupported in modern ecosystems
  • Zero web browser compatibility
  • Falls into the 'vanity' tree, making it irrelevant for standard cross-platform data exchange

When to choose which format

ICO
Use ICO when…
  • Website Favicons (favicon.ico)
  • Windows Application Executable Icons (.exe)
  • Desktop Shortcut Icons
PTI
Use PTI when…
  • Legacy Application Specific Workflows
  • Proprietary Image Storage

More comparisons

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