Image MIME Reference
PJPEG
Progressive JPEG (Legacy IE Alias)
image/pjpeg
VS
RealNetworks RealPix
image/vnd.rn-realpix
RealPix
A complete technical comparison of Progressive JPEG (Legacy IE Alias) and RealNetworks RealPix — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
PJPEG wins on browser support
Feature Support
| Feature | PJPEG | RealPix |
|---|---|---|
| Transparency (Alpha) | ✕ No | Yes |
| Animation Support | ✕ No | Yes |
| Progressive Loading | Yes | Yes |
| HDR Support | ✕ No | ✕ No |
| 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)
RealNetworks RealPix
image/vnd.rn-realpix
Extension
.rp
Container
IMFL (XML Markup)
Compression
Uncompressed (Plain Text Markup)
Algorithm
None (Plain Text)
Color Depth
Dependent on referenced images
Developed by
RealNetworks
Released
1998
Magic Bytes
3C 69 6D 66 6C
Browser Support Comparison
| Browser | PJPEG | RealPix |
|---|---|---|
Chrome
|
✓ All
|
✕ |
Firefox
|
✓ All
|
✕ |
Safari
|
✓ All
|
✕ |
Edge
|
✓ All
|
✕ |
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
RealPix
RealPix Strengths
- Allowed for 'Ken Burns'-style animated slideshows at a fraction of the bandwidth required for actual video, crucial for 1990s dial-up connections
- Plain text XML architecture made it easy for developers and hobbyists to hand-code complex multimedia presentations
- Perfectly synchronized with RealAudio streams using SMIL-like timing parameters
Limitations
- Completely obsolete and heavily reliant on the proprietary, defunct RealPlayer browser plugin
- Not a true image format, causing confusion when handled by standard image processing libraries which expect binary pixel data
- Lacks modern CSS or HTML5 canvas equivalents without completely rewriting the presentation logic
When to choose which format
PJPEG
Use PJPEG when…
- Legacy Internet Explorer File Uploads
- Progressive Image Rendering (Historical context)
RealPix
Use RealPix when…
- Historical Dial-up Streaming Presentations
- RealPlayer Slideshows
- Bandwidth-efficient 'Video' Alternatives (late 1990s)