The journey to JPEG XL started with guetzli (which started with butteraugli to guide loss in JPEG encoding) and brunsli. Guetzli is a great (but very very slow)

The journey to JPEG XL started with guetzli (which started with butteraugli to guide loss in JPEG encoding) and brunsli. Guetzli is a great (but very very slow) JPEG encoder. Brunsli is a classic JPEG1 recompressor. We mixed those and got the first version of PIK. We added some forced format-level progression, adaptive quantization, filtering, larger DCTs, integerated HUIF as lossless/super-progressive coder, and adopted some WebP lossless features into HUIF (Select, 2d-LZ77, entropy clustering). The final version still had brunsli-like features and we surfaced those as a JPEG recompression system.