G3-Fax

Group 3 Fax Image

image/g3fax Released 1980 · by CCITT (now ITU-T) / IETF

The Group 3 Fax format (ITU-T T.4), often represented by the MIME type image/g3fax, is a legacy 1-bit monochrome raster image format designed for telefacsimile transmission over analog telephone lines. It utilizes Modified Huffman run-length encoding to compress black-and-white documents efficiently.

Legacy & Document Production Ready

Technical Specifications

MIME Type

image/g3fax

Extensions

.g3

Container

Raw Bitstream

Compression

Lossless (CCITT Group 3)

Algorithms

CCITT Group 3 (Modified Huffman / READ)

Color Depth

1-bit (Monochrome)

Metadata

None

Capabilities

Transparency

Animation

Progressive Loading

Alpha Channel

HDR Support

Official Specification

Aliases: image/fax-g3

Magic Bytes & HTTP Headers

File Signature (Magic Bytes)

None (Raw Bitstream)

HTTP Headers

Content-Type: image/g3fax

HTML Accept Attribute

accept="image/g3fax, .g3"

Browser Support Matrix

Browser	Status	Min Version
Chrome	Not Supported	—
Firefox	Not Supported	—
Safari	Not Supported	—
Edge	Not Supported	—
Internet Explorer	Not Supported	Legacy

Advantages & Limitations

Advantages

Extremely efficient compression for 1-bit black-and-white text and line art
Standardized via RFC 1494 for email and internet fax transmission
Low bandwidth requirements

Limitations

Raw format lacks metadata (such as image width and resolution), requiring out-of-band information to decode accurately
Strictly limited to 1-bit monochrome imagery
Zero native web browser support

Developer Code Snippets

import magic
print(magic.from_file('document.g3', mime=True)) # Will likely return 'application/octet-stream' due to lack of header

const FileType = require('file-type');
const type = await FileType.fromFile('document.g3');
console.log(type?.mime); // Standard parsers will fail to identify raw G3

// Standard HTTP content type detection will fall back to application/octet-stream

$mime = mime_content_type('document.g3');
echo $mime; // application/octet-stream

File Detection Steps

How to programmatically detect a image/g3fax file by reading its raw bytes:

1 Extremely difficult to detect via magic bytes due to the lack of a standardized header.
2 Check file extension (.g3) and attempt to decode the CCITT bitstream looking for valid EOL markers.

Software Support

ImageMagick

I

IrfanView

X

XnView

GIMP

G

Ghostscript

Conversion Tools

Share & Embed

Markdown Badge

[![G3-Fax Format](https://freewebptojpg.com/mime/image/g3-fax/badge.svg)](https://freewebptojpg.com/mime/image/g3-fax)

Interactive iframe Widget

<iframe src="https://freewebptojpg.com/mime/image/g3-fax/embed" width="100%" height="280" style="border:1px solid #cbd5e1;border-radius:8px;" title="G3-Fax Specs"></iframe>

JSON API

All data on this page is available as a free, open JSON API. Use it in your project, documentation, or tooling.

GET https://freewebptojpg.com/mime/image/g3-fax.json Open JSON

Free to use

No auth required

CORS enabled

Use from any domain

Cached 24h

Cache-Control headers set

Frequently Asked Questions

What is the image/g3fax MIME type?

Defined in RFC 1494, image/g3fax is the standard MIME type for transmitting CCITT Group 3 fax images over the internet or email, representing raw bitstream data of 1-bit monochrome document scans.

What is the magic byte for image/g3fax?

Raw G3 fax files lack a standard file header or magic bytes. The bitstream typically begins with a unique End of Line (EOL) synchronization code consisting of eleven zeros followed by a one (000000000001).

How can I view an image/g3fax file?

Web browsers do not natively support image/g3fax. To view these files, they must be converted to modern formats like PNG or wrapped in a TIFF container (TIFF-FX), or opened using specialized image software like ImageMagick or Ghostscript.