Image MIME Reference
HEIC-Sequence
HEVC Image Sequence (HEIF Container)
image/heic-sequence
VS
Portable Graymap
image/x-portable-graymap
PGM
A complete technical comparison of HEVC Image Sequence (HEIF Container) and Portable Graymap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
HEIC-Sequence wins on browser support
Feature Support
| Feature | HEIC-Sequence | PGM |
|---|---|---|
| Transparency (Alpha) | Yes | ✕ No |
| Animation Support | Yes | ✕ No |
| Progressive Loading | ✕ No | ✕ No |
| HDR Support | Yes | Yes |
| EXIF Metadata | Yes | ✕ No |
| ICC Color Profile | Yes | ✕ No |
HEVC Image Sequence (HEIF Container)
image/heic-sequence
Extension
.heics.heic
Container
HEIF / ISOBMFF (ISO/IEC 14496-12)
Compression
Lossy / Lossless
Algorithm
HEVC (H.265)
Color Depth
8-bit, 10-bit, 12-bit, 16-bit
Developed by
Moving Picture Experts Group (MPEG) / Apple
Released
2017
Magic Bytes
XX XX XX XX 66 74 79 70 68 65 76 63
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 | HEIC-Sequence | PGM |
|---|---|---|
Chrome
|
✕ | ✕ |
Firefox
|
✕ | ✕ |
Safari
|
✓ 11+
|
✕ |
Edge
|
✕ | ✕ |
IE (Legacy)
|
✕ | ✕ |
HEIC-Sequence
HEIC-Sequence Strengths
- Incredibly efficient storage for bursts and short animations compared to GIF or motion JPEG
- Supports 16-bit color, HDR, and alpha channel transparency across all frames
- Shares frame data blocks, meaning the base image and animation data are heavily optimized for space
Limitations
- Zero support in web browsers outside the Apple ecosystem (Chrome, Firefox, Edge do not support it)
- Heavily encumbered by HEVC patent pools, hindering open-source adoption
- Complex container structure makes parsing and decoding difficult without specialized libraries
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
HEIC-Sequence
Use HEIC-Sequence when…
- Apple Live Photos
- Burst Photography
- High-Quality Animations
- Cinemagraphs
PGM
Use PGM when…
- Computer Vision and Machine Learning Pre-processing
- Scientific and Medical Image Processing
- Unix/Linux Command-Line Image Pipelines