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Image MIME Reference
APNG

Animated Portable Network Graphics

image/vnd.mozilla.apng
VS

X Window Dump (Legacy MIME Alias)

image/x-xwindowdump
XWD

A complete technical comparison of Animated Portable Network Graphics and X Window Dump (Legacy MIME Alias) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

APNG wins on browser support

Feature Support

Feature APNG XWD
Transparency (Alpha) Yes Yes
Animation Support Yes ✕ No
Progressive Loading Yes ✕ No
HDR Support ✕ No ✕ No
EXIF Metadata Yes ✕ No
ICC Color Profile Yes ✕ No
Animated Portable Network Graphics
image/vnd.mozilla.apng
Extension
.apng.png
Container PNG Chunk Stream
Compression Deflate (Lossless)
Algorithm
Deflate (zlib)
Color Depth 8-bit (Indexed), 24-bit (TrueColor), 32-bit (RGBA)
Developed by Mozilla Corporation (Stuart Parmenter & Vladimir Vukićević)
Released 2004
Magic Bytes
89 50 4E 47 0D 0A 1A 0A
Full APNG 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 APNG XWD
Chrome Chrome
✓ 59
 ✕
Firefox Firefox
✓ 3
 ✕
Safari Safari
✓ 8
 ✕
Edge Edge
✓ 79
 ✕
IE IE (Legacy)
✕ ✕
APNG

APNG Strengths

  • Supports 8-bit alpha transparency, completely solving the ugly 'white halo' problem inherent to GIFs
  • Gracefully degrades; viewers that do not support APNG will render a perfectly valid, static PNG of the first frame
  • Provides lossless compression that retains absolute pixel-perfect quality for UI elements and line art
Limitations
  • Because it relies on lossless Deflate compression, file sizes for complex, video-like animations can be massive compared to lossy formats like WebP or AVIF
  • Was initially rejected by the official PNG group, delaying its adoption for nearly a decade
  • Lacks native hardware decoding acceleration, unlike video-based animated formats (like MP4 or WebM)
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

APNG
Use APNG when…
  • High-Quality Web Animations
  • Stickers (Apple iMessage, LINE, Discord)
  • UI Loaders with Alpha Transparency
  • GIF Replacement
XWD
Use XWD when…
  • X Window System (X11) Display Debugging and Screenshots
  • Legacy Unix/Linux System Administration

More comparisons

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