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Image MIME Reference
BMP

Windows Bitmap (Legacy Windows MIME Alias)

image/x-windows-bmp
VS

Portable Anymap

image/x-portable-anymap
PNM

A complete technical comparison of Windows Bitmap (Legacy Windows MIME Alias) and Portable Anymap — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

BMP wins on browser support

Feature Support

Feature BMP PNM
Transparency (Alpha) Yes ✕ No
Animation Support ✕ No ✕ No
Progressive Loading ✕ No ✕ No
HDR Support ✕ No ✕ No
EXIF Metadata ✕ No ✕ No
ICC Color Profile Yes ✕ No
Windows Bitmap (Legacy Windows MIME Alias)
image/x-windows-bmp
Extension
.bmp.dib
Container DIB (Device-Independent Bitmap)
Compression Uncompressed / Run-Length Encoding (RLE)
Algorithm
NoneRLE4RLE8
Color Depth 1-bit (Monochrome), 4-bit (16 colors), 8-bit (256 colors), 16-bit (High Color), 24-bit (True Color), 32-bit (True Color with Alpha)
Developed by Microsoft Corporation
Released 1985
Magic Bytes
42 4D
Full BMP reference →
Portable Anymap
image/x-portable-anymap
Extension
.pnm
Container Netpbm
Compression Uncompressed
Algorithm
None
Color Depth 1-bit (PBM), 8-bit to 16-bit (PGM), 24-bit to 48-bit (PPM)
Developed by Jef Poskanzer (Netpbm Project)
Released 1988
Magic Bytes
50 31, 50 32, 50 33, 50 34, 50 35, or 50 36
Full PNM reference →

Browser Support Comparison

Browser BMP PNM
Chrome Chrome
✓ 1
 ✕
Firefox Firefox
✓ 1
 ✕
Safari Safari
✓ 1
 ✕
Edge Edge
✓ 12
 ✕
IE IE (Legacy)
✓
 ✕
BMP

BMP Strengths

  • Universally supported by all major operating systems and modern web browsers
  • Maintains perfect, lossless fidelity because pixel data is stored without lossy compression artifacts
Limitations
  • Massive file size overhead due to a lack of modern compression algorithms
  • Legacy MIME strings like 'image/x-windows-bmp' can trigger validation warnings in strict security or API environments if unhandled
PNM

PNM Strengths

  • Unparalleled simplicity: parsers and writers for PNM files can be written from scratch in C or Python in just a few minutes without relying on external libraries
  • Serves as an excellent foundational teaching tool for computer science students learning about image arrays and raster graphics
  • Flawlessly pipes through Unix command-line utilities for robust automated image processing (via the Netpbm suite)
Limitations
  • Zero compression natively results in astronomical file sizes, making it entirely impractical for web delivery or consumer storage
  • No support for standard metadata formats like EXIF, XMP, or ICC color profiles
  • No native alpha channel support in the classic PNM specification (though the newer PAM format 'P7' addresses this)

When to choose which format

BMP
Use BMP when…
  • Legacy Windows Desktop Registry Associations
  • Older Enterprise Intranet Applications
  • Windows Graphics Archival
PNM
Use PNM when…
  • Unix/Linux Command-Line Image Processing (Netpbm suite)
  • Academic Computer Vision Research
  • Intermediary Format for Image Format Conversion

More comparisons

BMP vs ACES → PNM vs ACES → BMP vs APNG → PNM vs APNG → BMP vs AVCI → PNM vs AVCI → BMP vs AVCS → PNM vs AVCS → BMP vs AVIF → PNM vs AVIF → All comparisons →
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