Image MIME Reference
CMU-Raster
CMU Window Manager Raster
image/x-cmu-raster
VS
Portable Bitmap
image/x-portable-bitmap
PBM
A complete technical comparison of CMU Window Manager Raster and Portable Bitmap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
Equal browser support
Feature Support
| Feature | CMU-Raster | PBM |
|---|---|---|
| Transparency (Alpha) | ✕ No | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | ✕ No |
| HDR Support | ✕ No | ✕ No |
| EXIF Metadata | ✕ No | ✕ No |
| ICC Color Profile | ✕ No | ✕ No |
CMU Window Manager Raster
image/x-cmu-raster
Extension
.ras
Container
CMU Window Manager Bitmap
Compression
Uncompressed
Algorithm
None
Color Depth
1-bit (Monochrome), 8-bit (Indexed/Grayscale)
Developed by
Carnegie Mellon University (CMU)
Released
1980s
Magic Bytes
F1 00 40 BB
Portable Bitmap
image/x-portable-bitmap
Extension
.pbm
Container
Netpbm
Compression
Uncompressed
Algorithm
None
Color Depth
1-bit (Monochrome)
Developed by
Jef Poskanzer (Netpbm Project)
Released
1988
Magic Bytes
50 31 (ASCII) or 50 34 (Binary)
Browser Support Comparison
| Browser | CMU-Raster | PBM |
|---|---|---|
Chrome
|
✕ | ✕ |
Firefox
|
✕ | ✕ |
Safari
|
✕ | ✕ |
Edge
|
✕ | ✕ |
IE (Legacy)
|
✕ | ✕ |
CMU-Raster
CMU-Raster Strengths
- Incredibly simple, uncompressed binary structure that was easy for early, memory-constrained UNIX workstations to parse and render
Limitations
- Completely obsolete and unsupported outside of niche historical command-line tools like Netpbm
- The shared '.ras' file extension causes frequent file misidentification and system conflicts
- Lacks all modern imaging features, including metadata, compression, and alpha channel transparency
PBM
PBM Strengths
- Absolute structural simplicity allows parsers to be written from scratch with just a few lines of code
- ASCII encoding allows manual editing and debugging directly within a standard text editor
- Serves as a flawless, lossless intermediary for 1-bit raster data like scanned documents or barcodes
Limitations
- Strictly limited to 1-bit monochrome images; cannot natively display shades of gray or color
- Lack of compression results in extremely large file sizes, particularly when using the ASCII 'P1' encoding
- No support for alpha transparency or modern metadata standards like EXIF
When to choose which format
CMU-Raster
Use CMU-Raster when…
- Historical UNIX / Andrew Project Archival
- Legacy Academic Computing Research
PBM
Use PBM when…
- Optical Character Recognition (OCR) Pre-processing
- Unix/Linux Command-Line Image Pipelines
- Academic Image Processing Teaching