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Image MIME Reference
T.38

T.38 Fax over IP (FoIP)

image/t38
VS

Valve Texture Format

image/vnd.valve.source.texture
VTF

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

Equal browser support

Feature Support

Feature T.38 VTF
Transparency (Alpha) ✕ No Yes
Animation Support ✕ No Yes
Progressive Loading ✕ No ✕ No
HDR Support ✕ No Yes
EXIF Metadata ✕ No ✕ No
ICC Color Profile ✕ No ✕ No
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 →
Valve Texture Format
image/vnd.valve.source.texture
Extension
.vtf
Container Valve Texture Format Header
Compression DXTC (DXT1, DXT3, DXT5) / Uncompressed (BGRA8888, RGB888, etc.)
Algorithm
DXT1-DXT5None
Color Depth 8-bit, 24-bit (RGB), 32-bit (RGBA), 16-bit Floating Point (HDR)
Developed by Valve Corporation
Released 2004
Magic Bytes
56 54 46 00
Full VTF reference →

Browser Support Comparison

Browser T.38 VTF
Chrome Chrome
✕ ✕
Firefox Firefox
✕ ✕
Safari Safari
✕ ✕
Edge Edge
✕ ✕
IE IE (Legacy)
✕ ✕
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
VTF

VTF Strengths

  • Extremely fast loading within the Source Engine because the block-compressed data goes directly into the GPU's VRAM without CPU decompression overhead
  • Bundles mipmaps, volumetric slices, cubemap faces, and animation frames entirely within a single file
  • Openly documented by the Valve Developer Community, leading to a massive ecosystem of third-party modding tools
Limitations
  • Strictly tied to the Source Engine ecosystem; entirely useless in standard web or print contexts
  • No native support in standard image editors without hunting down and installing specialized plugins
  • Block compression algorithms (DXT) are lossy and can cause visible artifacting, especially on normal maps or smooth gradients

When to choose which format

T.38
Use T.38 when…
  • Fax over IP (FoIP) Transmissions
  • SIP/SDP Media Negotiation
  • VoIP Gateways and PBX Systems
  • Enterprise Fax Servers
VTF
Use VTF when…
  • Source Engine Game Textures
  • Custom Maps and Modding (Garry's Mod, CS:GO)
  • Animated In-Game Decals and Sprays
  • Environment Cubemaps (Skyboxes)

More comparisons

T.38 vs ACES → VTF vs ACES → T.38 vs APNG → VTF vs APNG → T.38 vs AVCI → VTF vs AVCI → T.38 vs AVCS → VTF vs AVCS → T.38 vs AVIF → VTF vs AVIF → All comparisons →
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