Image MIME Reference
JXL
JPEG XL
image/jxl
VS
Portable Graymap
image/x-portable-graymap
PGM
A complete technical comparison of JPEG XL and Portable Graymap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
JXL wins on browser support
Feature Support
| Feature | JXL | PGM |
|---|---|---|
| Transparency (Alpha) | Yes | ✕ No |
| Animation Support | Yes | ✕ No |
| Progressive Loading | Yes | ✕ No |
| HDR Support | Yes | Yes |
| EXIF Metadata | Yes | ✕ No |
| ICC Color Profile | Yes | ✕ No |
JPEG XL
image/jxl
Extension
.jxl
Container
ISOBMFF / JPEG XL Codestream
Compression
Lossy (VarDCT) / Lossless (Modular)
Algorithm
VarDCT (Variable block-size DCT)Modular (Predictive coding for lossless)
Color Depth
8-bit, 10-bit, 12-bit, 16-bit, 32-bit (Float)
Developed by
Joint Photographic Experts Group (JPEG)
Released
2021
Magic Bytes
FF 0A (Codestream) / 00 00 00 0C 4A 58 4C 20 0D 0A 87 0A (Container)
Portable Graymap
image/x-portable-graymap
Extension
.pgm
Container
Netpbm
Compression
Uncompressed
Algorithm
None
Color Depth
8-bit (Grayscale), 16-bit (Grayscale)
Developed by
Jef Poskanzer (Netpbm Project)
Released
1988
Magic Bytes
50 32 (ASCII) or 50 35 (Binary)
Browser Support Comparison
| Browser | JXL | PGM |
|---|---|---|
Chrome
|
✕ | ✕ |
Firefox
|
✕ | ✕ |
Safari
|
✓ 17+
|
✕ |
Edge
|
✕ | ✕ |
IE (Legacy)
|
✕ | ✕ |
JXL
JXL Strengths
- Capable of losslessly recompressing legacy JPEG files, saving ~20% space while retaining perfect byte-for-byte restorability
- Highly superior progressive decoding (Saliency-driven), allowing recognizable images to load extremely fast on slow connections
- Outperforms WebP and competes favorably with AVIF, especially at high-fidelity settings and for complex textures/photography
- Royalty-free with an open-source reference implementation (libjxl)
Limitations
- Google's removal of JXL support from Chrome severely hindered its adoption as a universal web standard
- Lack of hardware decoding chips compared to video-derived codecs like AVIF (AV1)
- File parsing logic is complex due to the split between raw codestreams and ISOBMFF containers
PGM
PGM Strengths
- Extremely simple format allows custom parsers to be written from scratch in minimal time
- Supports 16-bit precision, making it highly valuable for scientific, medical, and height-map data where 8-bit precision is insufficient
- ASCII encoding ('P2') allows manual editing and visual inspection of pixel values directly within a text editor
Limitations
- Strictly limited to grayscale; cannot display color
- Lack of compression results in massive file sizes, making it entirely impractical for web delivery or consumer storage
- Does not support alpha transparency or modern metadata structures
When to choose which format
JXL
Use JXL when…
- High-End Web Delivery (via Safari/Polyfills)
- HDR Photography
- Legacy JPEG Archiving
- Digital Art & Image Storage
PGM
Use PGM when…
- Computer Vision and Machine Learning Pre-processing
- Scientific and Medical Image Processing
- Unix/Linux Command-Line Image Pipelines