Methods, systems, and bitstream syntax are described for metadata signaling and conversion for film grain encoding and synthesis. Given a bitstream with MPEG film-grain SEI messaging, for each picture, a processor: detects if the film grain model is suitable for film-grain synthesis using the AV1 autoregressive with additive blending noise model, and then: transcodes the MPEG film grain SEI parameters to corresponding AV1 film grain parameters, synthesizes the film grain, and adds it to the decoded video pictures according to the AV1 specification. An example process for translating AV1 parameters to MPEG film-grain SEI messaging is also provided.