PNG

PNG (Portable Network Graphics) is a lossless image format established as a W3C Recommendation in 1996. Developed as a patent-free GIF alternative, it combines lossless compression with transparency support.

PNG uses Deflate (LZ77 + Huffman coding). Row-level filtering (Sub, Up, Average, Paeth) increases redundancy before compression. Being lossless, images can be re-saved indefinitely without degradation.

• Alpha channel: colour types with alpha allow sample depths of 8 or 16 bits (the spec as a whole covers 1 to 16 bits), so the common 8-bit case gives 256 semi-transparency levels and 16-bit reaches 65,536, unlike GIF's binary approach
• Color modes: True color (24/48-bit), grayscale (1-16 bit), indexed (up to 256 colors)
• Interlacing: Adam7 displays progressively across 7 passes
• Gamma correction: gAMA chunk compensates for cross-display color differences

The specification itself keeps evolving. The third edition, published as a W3C Recommendation in June 2025, folds animated PNG (APNG) into the official spec and defines chunks that carry HDR color information (cICP, mDCV, cLLI) along with EXIF storage. These are backward-compatible extensions of the 1996 original, so existing files keep working, but as of August 2026 the idea that PNG is still-image-only and SDR-only is out of date.

PNG's weakness is file size - photographs can be several times larger than JPEG. It excels with logos, icons, UI assets, and screenshots with sharp edges. It remains essential as a lossless intermediate format in editing workflows.