Image MIME Reference
ICO
Windows Icon (Legacy MIME)
image/x-icon
VS
PWG Raster Format
image/pwg-raster
PWG-Raster
A complete technical comparison of Windows Icon (Legacy MIME) and PWG Raster Format — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
ICO wins on browser support
Feature Support
| Feature | ICO | PWG-Raster |
|---|---|---|
| Transparency (Alpha) | Yes | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | ✕ No |
| HDR Support | ✕ No | ✕ No |
| EXIF Metadata | ✕ No | ✕ No |
| ICC Color Profile | ✕ No | ✕ No |
Windows Icon (Legacy MIME)
image/x-icon
Extension
.ico
Container
ICO Directory Structure
Compression
Uncompressed (DIB/BMP) / Deflate (Embedded PNG)
Algorithm
NoneDeflate (via PNG)
Color Depth
1-bit, 4-bit, 8-bit, 24-bit (True Color), 32-bit (RGBA)
Developed by
Microsoft Corporation
Released
1985
Magic Bytes
00 00 01 00
PWG Raster Format
image/pwg-raster
Extension
.pwg.ras
Container
CUPS Raster v2 Container
Compression
PackBits (Run-Length Encoding)
Algorithm
PackBits (Apple RLE variant)
Color Depth
1-bit (Monochrome/K), 8-bit (Grayscale), 24-bit (sRGB / Adobe RGB), 32-bit (CMYK)
Developed by
IEEE-ISTO Printer Working Group (PWG)
Released
2011
Magic Bytes
52 61 53 32
Browser Support Comparison
| Browser | ICO | PWG-Raster |
|---|---|---|
Chrome
|
✓ 1
|
✕ |
Firefox
|
✓ 1
|
✕ |
Safari
|
✓ 1
|
✕ |
Edge
|
✓ 12
|
✕ |
IE (Legacy)
|
✓
|
✕ |
ICO
ICO Strengths
- Universally supported by every web browser in existence for website favicons
- Packs multiple resolutions into a single file, eliminating the need to serve different files for the taskbar vs. the desktop
- Modern support for embedded PNGs (since Windows Vista) allows for excellent 32-bit transparency (alpha channels) and drastically reduced file sizes
Limitations
- Internal file structure is convoluted due to decades of legacy cruft, such as the requirement for BMP data to lack a standard file header but double the height property for an archaic XOR/AND mask
- Larger file size compared to a single optimized PNG or SVG, as it literally stores the same image multiple times at different scales
- The MIME type landscape is highly fragmented (x-icon vs. vnd.microsoft.icon), causing occasional caching or strict-MIME validation errors on restrictive servers
PWG-Raster
PWG-Raster Strengths
- The undisputed global standard for modern, driverless printing environments, eliminating the nightmare of installing manufacturer-specific print drivers
- Extremely lightweight to decode on embedded hardware, requiring minimal RAM buffers on the printer side
- Strict color space and resolution rules guarantee that the printed output will successfully process without syntax errors
Limitations
- Generates massive file sizes and network payloads compared to vector-based formats like PDF or PostScript, as every page is fully rasterized at 300+ DPI before transmission
- No support for native rendering in web browsers, image viewers, or graphic design software without specialized CUPS developer tools
- Cannot store complex metadata, vector text, or layers
When to choose which format
ICO
Use ICO when…
- Website Favicons (favicon.ico)
- Windows Application Executable Icons (.exe)
- Desktop Shortcut Icons
PWG-Raster
Use PWG-Raster when…
- Driverless Network Printing (IPP Everywhere)
- Apple AirPrint and Mopria Print Service Pipelines
- CUPS (Common UNIX Printing System) Spooling