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Image MIME Reference
FITS

Flexible Image Transport System

image/fits
VS

X Window Dump (Legacy MIME Alias)

image/x-xwindowdump
XWD

A complete technical comparison of Flexible Image Transport System and X Window Dump (Legacy MIME Alias) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

Equal browser support

Feature Support

Feature FITS XWD
Transparency (Alpha) ✕ No Yes
Animation Support ✕ No ✕ No
Progressive Loading ✕ No ✕ No
HDR Support Yes ✕ No
EXIF Metadata ✕ No ✕ No
ICC Color Profile ✕ No ✕ No
Flexible Image Transport System
image/fits
Extension
.fits.fit.fts
Container FITS
Compression Uncompressed / Tile Compressed (Rice, GZIP)
Algorithm
UncompressedRiceGZIPHcompress
Color Depth 8-bit, 16-bit, 32-bit, 64-bit (Float/Double)
Developed by IAU FITS Working Group / NASA
Released 1981
Magic Bytes
53 49 4D 50 4C 45 20 20 3D 20
Full FITS reference →
X Window Dump (Legacy MIME Alias)
image/x-xwindowdump
Extension
.xwd.wd
Container XWDHeader / XWDFileHeader Structure
Compression Uncompressed
Algorithm
None
Color Depth 8-bit (Indexed), 16-bit (High Color), 24-bit (True Color), 32-bit (True Color with Alpha/Padding)
Developed by MIT / X Consortium (X Window System)
Released 1987
Magic Bytes
00 00 00 64 (Typically indicates a 100-byte header size followed by version)
Full XWD reference →

Browser Support Comparison

Browser FITS XWD
Chrome Chrome
✕ ✕
Firefox Firefox
✕ ✕
Safari Safari
✕ ✕
Edge Edge
✕ ✕
IE IE (Legacy)
✕ ✕
FITS

FITS Strengths

  • The undisputed global standard for archiving and sharing astronomical data
  • Self-documenting: Headers are written in plain ASCII, ensuring data is readable decades later regardless of software
  • Supports incredibly high dynamic range, N-dimensional arrays, and complex binary tables
Limitations
  • No support in standard consumer software or web browsers
  • Uncompressed raw FITS files from modern telescopes can be excessively large
  • Does not use standard metadata models like EXIF or XMP, requiring specialized parsers
XWD

XWD Strengths

  • Provides an exact, raw pixel dump directly from the X11 display server memory without compression overhead
  • Extremely fast for Unix environments to generate during low-level graphical debugging
Limitations
  • Completely uncompressed, resulting in large, bloated file sizes
  • Zero support in web browsers or modern desktop environments without specialized graphics translation tools
  • Verbose legacy MIME strings like 'image/x-xwindowdump' require backend normalization in modern applications

When to choose which format

FITS
Use FITS when…
  • Astronomy & Astrophysics
  • Space Telescope Data (Hubble, JWST)
  • Scientific Data Archiving
  • Spectroscopy
XWD
Use XWD when…
  • X Window System (X11) Display Debugging and Screenshots
  • Legacy Unix/Linux System Administration

More comparisons

FITS vs ACES → XWD vs ACES → FITS vs APNG → XWD vs APNG → FITS vs AVCI → XWD vs AVCI → FITS vs AVCS → XWD vs AVCS → FITS vs AVIF → XWD vs AVIF → All comparisons →
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