File extension
.png
Portable Network Graphics (PNG)
.png is primarily used for Portable Network Graphics images: lossless raster graphics with strong support for web use, transparency, metadata, and preservation.
Known meanings
Formats and reference material for .png
Portable Network Graphics (PNG)
Lossless raster image format; current W3C PNG Third Edition Recommendation, 2025
Portable Network Graphics Specification (historical)
Historical Portable Network Graphics specification
Portable Network Graphics Specification (historical)
Historical Portable Network Graphics specification
PNG-16 File Format Proposal
Historical nonstandard PNG-related 16-bit-pixel proposal
Portable Network Graphics Information
Portable Network Graphics Information
Details
Format notes
Portable Network Graphics (PNG)
PNG is a lossless raster image format designed for portable network graphics, especially on the web. It replaced many GIF use cases by supporting better compression, truecolor images, grayscale images, indexed-color images, and alpha transparency without relying on patented compression. It remains important for screenshots, diagrams, icons, transparency-heavy graphics, and preservation of pixel-exact imagery.
Datastream Structure
A PNG file starts with an 8-byte signature:
89 50 4E 47 0D 0A 1A 0A. The rest of the file is a sequence of chunks. Each
chunk stores a length, a four-byte chunk type, chunk data, and a CRC. Critical
chunks define the image itself: IHDR gives image dimensions and color type,
PLTE stores palette data when needed, IDAT stores compressed image data, and
IEND marks the end of the stream.
Ancillary chunks carry optional information such as gamma, chromaticity, ICC profiles, text, timestamps, physical pixel dimensions, Exif metadata, and animation control data in newer specifications. Unknown ancillary chunks can generally be skipped; unknown critical chunks must not be ignored. This extension model is a major reason PNG has aged well.
Compression And Filtering
PNG uses scanline filtering before compression. Each scanline may use a filter that predicts pixel values from neighboring pixels, making the remaining data more compressible. The filtered byte stream is then compressed using zlib/Deflate. Because filtering is lossless, a decoded PNG should reproduce the original sample values exactly, subject to color-management interpretation.
Compression level affects file size and encoding time, not image quality. If a PNG changes visually after recompression, the tool probably changed color metadata, bit depth, palette choices, transparency, or gamma handling rather than the compression itself.
Color And Transparency
PNG supports palette images, grayscale, truecolor, and optional alpha channels.
For web use, alpha transparency is one of its defining strengths. For archival
or prepress workflows, color chunks such as sRGB, gAMA, cHRM, and iCCP
matter because they influence how sample values should be displayed.
The 2025 W3C PNG Third Edition is the stable W3C Recommendation and incorporates
APNG and HDR-related updates. W3C also maintains a newer Fourth Edition editor’s
draft, but that draft is not the stable Recommendation. Older PNG files remain
valid, but validators and preservation workflows should record which chunks are
present rather than assuming every .png is a simple static image.
Preservation And Security Notes
PNG is comparatively simple, but decoders still need limits for huge dimensions,
large compressed streams, malformed chunk lengths, CRC failures, and excessive
ancillary data. Preservation checks should verify the signature, chunk ordering,
CRC values, final IEND, dimensions, color type, bit depth, and decompressible
IDAT stream. Keep original color metadata when converting or optimizing files.
Further Reading
- W3C PNG Specification, Third Edition:
https://www.w3.org/TR/png-3/ - PNG home site and libpng resources:
http://www.libpng.org/pub/png/ - Related formats: APNG, MNG, JNG, GIF, WebP