Free Lossless Image Format

Free Lossless Image Format (FLIF) is a lossless image format claiming to outperform PNG, lossless WebP, lossless BPG and lossless JPEG 2000 in terms of compression ratio on a variety of inputs. FLIF supports a form of progressive interlacing (a generalization of the Adam7 algorithm) with which any partial download (greater than couple hundred bytes) of an image file can be used as a lossy encoding of the entire image.

Source: Wikipedia — Free Lossless Image Format (CC BY-SA 4.0)