Image MIME Reference
DVB-SUB
DVB Subtitle
image/vnd.dvb.subtitle
VS
Portable Pixmap
image/x-portable-pixmap
PPM
A complete technical comparison of DVB Subtitle and Portable Pixmap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
Equal browser support
Feature Support
| Feature | DVB-SUB | PPM |
|---|---|---|
| Transparency (Alpha) | Yes | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | ✕ No |
| HDR Support | ✕ No | Yes |
| EXIF Metadata | ✕ No | ✕ No |
| ICC Color Profile | ✕ No | ✕ No |
DVB Subtitle
image/vnd.dvb.subtitle
Extension
.sub.sup.ts
Container
MPEG-2 Transport Stream (MPEG-TS) PES Packets
Compression
Lossless (Run-Length Encoding)
Algorithm
RLE (Run-Length Encoding)
Color Depth
2-bit (Indexed), 4-bit (Indexed), 8-bit (Indexed CLUT)
Developed by
DVB Project (ETSI)
Released
1997
Magic Bytes
20
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)
Browser Support Comparison
| Browser | DVB-SUB | PPM |
|---|---|---|
Chrome
|
✕ | ✕ |
Firefox
|
✕ | ✕ |
Safari
|
✕ | ✕ |
Edge
|
✕ | ✕ |
IE (Legacy)
|
✕ | ✕ |
DVB-SUB
DVB-SUB Strengths
- Guarantees 100% accurate visual representation (fonts, colors, layouts) exactly as intended by the broadcaster
- Effortlessly supports complex character sets (e.g., Arabic, Asian languages) without relying on the client device possessing the necessary fonts
- Supports region-specific dynamic positioning to avoid obscuring important on-screen graphics or burned-in lower thirds
Limitations
- Requires significantly more bandwidth than text-based subtitles
- Inaccessible to screen readers and cannot be indexed by search engines
- Requires OCR (Optical Character Recognition) to translate back into standard text-based web formats
- Zero native support in HTML5 video elements
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
DVB-SUB
Use DVB-SUB when…
- Digital Television Broadcasts (DVB-T, DVB-S, DVB-C)
- IPTV and OTT Streaming
- Hardware Set-top Boxes
PPM
Use PPM when…
- Intermediary Format for Image Format Conversion
- Academic Computer Graphics and Raytracing Output
- Unix/Linux Command-Line Image Pipelines