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Image MIME Reference
Sigma-X3F

Sigma X3F RAW Image

image/x-sigma-x3f
VS

T.38 Fax over IP (FoIP)

image/t38
T.38

A complete technical comparison of Sigma X3F RAW Image and T.38 Fax over IP (FoIP) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

Equal browser support

Feature Support

Feature Sigma-X3F T.38
Transparency (Alpha) ✕ No ✕ No
Animation Support ✕ No ✕ No
Progressive Loading ✕ No ✕ No
HDR Support Yes ✕ No
EXIF Metadata Yes ✕ No
ICC Color Profile Yes ✕ No
Sigma X3F RAW Image
image/x-sigma-x3f
Extension
.x3f
Container Foveon Proprietary Header
Compression Uncompressed / Proprietary Lossless
Algorithm
NoneFoveon Proprietary Huffman-based
Color Depth 12-bit (Per Channel), 14-bit (Per Channel)
Developed by Foveon, Inc. / Sigma Corporation
Released 2002
Magic Bytes
46 4F 56 62
Full Sigma-X3F reference →
T.38 Fax over IP (FoIP)
image/t38
Extension
.t38
Container UDPTL or TCP (Network Transport)
Compression Lossless (MH, MR, MMR / T.4, T.6)
Algorithm
MH (Modified Huffman)MR (Modified Read)MMR (Modified Modified Read)
Color Depth 1-bit (Monochrome B&W)
Developed by ITU-T (International Telecommunication Union)
Released 1998
Magic Bytes
None (Network Stream)
Full T.38 reference →

Browser Support Comparison

Browser Sigma-X3F T.38
Chrome Chrome
✕ ✕
Firefox Firefox
✕ ✕
Safari Safari
✕ ✕
Edge Edge
✕ ✕
IE IE (Legacy)
✕ ✕
Sigma-X3F

Sigma-X3F Strengths

  • Provides absolute per-pixel color accuracy without the interpolation artifacts or moiré patterns typical of Bayer-sensor RAW files
  • Unique 'FOVb' magic byte signature prevents accidental misidentification by standard file sniffers, unlike many TIFF-based raw formats
  • Incredible sharpness and detail resolution at the raw sensor level
Limitations
  • Decoding is extremely computationally expensive and completely proprietary, meaning third-party software often struggles to match the color science of Sigma's own (famously slow) Photo Pro application
  • Massive file sizes because it records three complete RGB channels for every single spatial pixel coordinate
  • Zero support in web browsers or standard OS image viewers
T.38

T.38 Strengths

  • Designed specifically to overcome the timing and packet-loss issues that plague traditional fax over standard VoIP audio codecs (like G.711)
  • Reduces bandwidth requirements by transmitting fax protocol data rather than uncompressed audio tones
  • Universally supported by enterprise SIP trunks and Session Border Controllers (SBCs) for FoIP
Limitations
  • Not an actual image file format, causing confusion due to its 'image/' MIME classification
  • Extremely complex to implement correctly over lossy UDP networks (relies on UDPTL redundancy mechanisms)
  • Zero support in web browsers or standard image viewing software

When to choose which format

Sigma-X3F
Use Sigma-X3F when…
  • Professional Digital Photography
  • Sigma / Foveon High-Acuity Imaging
  • Non-destructive RAW Post-Processing
T.38
Use T.38 when…
  • Fax over IP (FoIP) Transmissions
  • SIP/SDP Media Negotiation
  • VoIP Gateways and PBX Systems
  • Enterprise Fax Servers

More comparisons

Sigma-X3F vs ACES → T.38 vs ACES → Sigma-X3F vs APNG → T.38 vs APNG → Sigma-X3F vs AVCI → T.38 vs AVCI → Sigma-X3F vs AVCS → T.38 vs AVCS → Sigma-X3F vs AVIF → T.38 vs AVIF → All comparisons →
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