PNGJPEG

PNGJPEG is a proposed image format that seeks to combine the strengths of PNG and JPEG within a single container. It targets scenarios where both high-fidelity edges and efficient photographic compression are desirable, such as web imagery with transparency or archival storage of mixed-content photographs.

In its conceptual design, PNGJPEG stores image data as a hybrid, two-layer data stream. A lossless PNG-like layer preserves sharp edges, text, and transparency, while a lossy JPEG-like layer encodes broader photographic regions with chroma subsampling and quantization. A quality map or segmentation data guides how the final image is reconstructed, allowing editors to prioritize lossless regions or apply different compression settings to different areas. The format is envisioned to support common color spaces, alpha channels, gamma, and color management metadata.

Encoding typically involves separating the image into regions, applying PNG compression to the lossless layer and JPEG-like compression to the lossy layer, then packaging the streams in a container with synchronized metadata. Decoding reconstructs the image by compositing the two streams according to the quality map or using a fallback to a single stream if only one mode is available.

Status and usage: PNGJPEG has appeared in academic discussions and experimental implementations but is not standardized by major bodies and has limited adoption. It requires custom tooling and is not widely supported by browsers, editors, or image viewers. Potential advantages include flexible trade-offs between image quality and file size, preservation of transparency with sharp details, and suitability for mixed-content archives.