Any lossless format can be made lossy by simply performing a lossy pre-pass prior to the lossless compression. The real challenge is finding a good way to perf

Any lossless format can be made lossy by simply performing a lossy pre-pass prior to the lossless compression. The real challenge is finding a good way to perform the pre-pass; it's easy to pick a simple naive method, but optimality is likely exponential-time, or if one is lucky, polynomial with an intractable constant (using trellis). The fact that PNG uses a dictionary-based compressor instead of a PPMD-like system likely makes the problem much harder.