A new PNG spec

Original Article ↗ Hacker News Discussion ↗

Backwards Compatibility & Fragmentation

  • Major concern: if PNG adds new compression methods or filters, old decoders will see a valid PNG they can’t actually decode, echoing the “USB‑C but for images” problem where capability isn’t visible from the extension.
  • Some argue PNG was explicitly designed for extensibility: unknown chunks are to be skipped, and files using unsupported compression should simply fail to decode.
  • Others counter that in practice software often assumes rarely‑used fields never change, so new compression could break real‑world code and user expectations (“it used to work, now it doesn’t”).
  • Several call for a distinct extension/media type (PNG2/PNGX) to make incompatibility explicit; others say that would kill adoption and note many browsers already support the new spec.
  • Thread participants involved in the work state that all new features are optional, old PNGs decode exactly as before, and new PNGs should remain “recognizably correct” (e.g., a red apple) on old software, even if not optimally rendered.

HDR, Color Spaces, and cICP

  • The main spec change many focus on is HDR and wide‑gamut support via a new cICP chunk.
  • Debate over whether existing ICC v2/v4 profiles could already express HDR; proponents say ICC’s relative luminance model, gamma assumptions, and LUT size/performance issues make it a poor fit for PQ/HLG workflows.
  • cICP is presented as a compact way to describe common HDR color spaces; criticism that it omits widely used RGB spaces (e.g., Adobe RGB, ProPhoto) and so still requires ICC profiles for those.
  • Users report that HDR PNGs often appear “washed out” in non‑HDR‑aware viewers, meaning backward compatibility can degrade from “limited sRGB” to “wrongly mapped wide gamut,” which some see as a serious regression.

Animation and Competing Formats

  • Officializing APNG (animated PNG) is welcomed by those who prefer lossless, alpha‑capable animations for UI, logos, and “GIF‑like” loops.
  • Others point out APNG is poor for real video compared with WebP/AV1 and that APNG support in upload workflows is still sparse.
  • Broader discussion compares PNG to JPEG XL, WebP, AVIF, TIFF, and OpenEXR. Many feel JPEG XL is technically superior but hamstrung by browser politics; others stress PNG’s ubiquity and archival stability as its core value.

Metadata and Tooling

  • Official EXIF-in-PNG gets praise (dates, camera data) but also concern: rotation flags have historically caused inconsistent rendering and privacy leaks, so many services strip EXIF.
  • Sidecar vs embedded metadata and approximate date standards (e.g., EDTF) are discussed; lack of consistent handling across major photo platforms is seen as a long‑standing pain point.
  • Some celebrate PNG’s chunk system for storing arbitrary app data (e.g., diagram JSON, editor state) while warning about interoperability and hidden‑data risks.