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Image MIME Reference
PJPEG

Progressive JPEG (Legacy IE Alias)

image/pjpeg
VS

Portable Pixmap

image/x-portable-pixmap
PPM

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

PJPEG wins on browser support

Feature Support

Feature PJPEG PPM
Transparency (Alpha) ✕ No ✕ No
Animation Support ✕ No ✕ No
Progressive Loading Yes ✕ No
HDR Support ✕ No Yes
EXIF Metadata Yes ✕ No
ICC Color Profile Yes ✕ No
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 →
Portable Pixmap
image/x-portable-pixmap
Extension
.ppm
Container Netpbm
Compression Uncompressed
Algorithm
None
Color Depth 24-bit (RGB), 48-bit (RGB High Color)
Developed by Jef Poskanzer (Netpbm Project)
Released 1988
Magic Bytes
50 33 (ASCII) or 50 36 (Binary)
Full PPM reference →

Browser Support Comparison

Browser PJPEG PPM
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
PPM

PPM Strengths

  • Absolute structural simplicity allows parsers and renderers (such as custom raytracers) to write directly to the format with virtually no overhead or external libraries
  • Supports 16-bit per channel (48-bit total) precision, preserving deep color accuracy before converting to final delivery formats
  • ASCII encoding ('P3') allows manual editing and visual inspection of RGB values directly within a text editor
Limitations
  • Zero compression results in astronomical file sizes, making it entirely impractical for web delivery or consumer storage
  • Does not natively support an alpha channel for transparency (this is handled by the newer PAM extension format within the Netpbm suite)
  • Lacks support for modern metadata standards like EXIF, XMP, or embedded ICC color profiles

When to choose which format

PJPEG
Use PJPEG when…
  • Legacy Internet Explorer File Uploads
  • Progressive Image Rendering (Historical context)
PPM
Use PPM when…
  • Intermediary Format for Image Format Conversion
  • Academic Computer Graphics and Raytracing Output
  • Unix/Linux Command-Line Image Pipelines

More comparisons

PJPEG vs ACES → PPM vs ACES → PJPEG vs APNG → PPM vs APNG → PJPEG vs AVCI → PPM vs AVCI → PJPEG vs AVCS → PPM vs AVCS → PJPEG vs AVIF → PPM vs AVIF → All comparisons →
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