Free webp to jpg
EN
Image MIME Reference
PJPEG

Progressive JPEG (Legacy IE Alias)

image/pjpeg
VS

Sigma X3F RAW Image

image/x-sigma-x3f
Sigma-X3F

A complete technical comparison of Progressive JPEG (Legacy IE Alias) and Sigma X3F RAW Image — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

PJPEG wins on browser support

Feature Support

Feature PJPEG Sigma-X3F
Transparency (Alpha) ✕ No ✕ No
Animation Support ✕ No ✕ No
Progressive Loading Yes ✕ No
HDR Support ✕ No Yes
EXIF Metadata Yes Yes
ICC Color Profile Yes Yes
Progressive JPEG (Legacy IE Alias)
image/pjpeg
Extension
.jpeg.jpg.pjpeg.pjp.jfif
Container JPEG (ISO/IEC 10918-1)
Compression Lossy (DCT)
Algorithm
Discrete Cosine Transform (DCT)
Color Depth 8-bit, 12-bit
Developed by Joint Photographic Experts Group / Microsoft (Alias)
Released 1992
Magic Bytes
FF D8 FF E0 (JFIF) / FF D8 FF E1 (Exif)
Full PJPEG reference →
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 →

Browser Support Comparison

Browser PJPEG Sigma-X3F
Chrome Chrome
✓ All
 ✕
Firefox Firefox
✓ All
 ✕
Safari Safari
✓ All
 ✕
Edge Edge
✓ All
 ✕
IE IE (Legacy)
✓
 ✕
PJPEG

PJPEG Strengths

  • Progressive JPEGs (which this MIME type attempted to describe) provide excellent perceived loading speeds on slow connections by rendering a low-resolution preview first
  • Files sent with this MIME type are fully backwards compatible with any standard JPEG decoder
Limitations
  • It is a non-standard MIME type that violates IANA specifications
  • Causes strict file upload validation scripts to fail if not explicitly whitelisted or mapped
  • Creates unnecessary confusion, as the official image/jpeg MIME type already fully encompasses progressive encoding
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

When to choose which format

PJPEG
Use PJPEG when…
  • Legacy Internet Explorer File Uploads
  • Progressive Image Rendering (Historical context)
Sigma-X3F
Use Sigma-X3F when…
  • Professional Digital Photography
  • Sigma / Foveon High-Acuity Imaging
  • Non-destructive RAW Post-Processing

More comparisons

PJPEG vs ACES → Sigma-X3F vs ACES → PJPEG vs APNG → Sigma-X3F vs APNG → PJPEG vs AVCI → Sigma-X3F vs AVCI → PJPEG vs AVCS → Sigma-X3F vs AVCS → PJPEG vs AVIF → Sigma-X3F vs AVIF → All comparisons →
Share this comparison