Study Mission on Film Grain Synthesis

Executive summary

Film grain can be an important aspect of artistic intent in content creation, particularly for movies. However, the process of video compression tends to reduce film grain, often in an uncontrolled manner. Film grain synthesis (FGS) is a technique supported by recent video codecs whereby the film grain in the source video is analysed and parameterised prior to encoding. The FGS parameters are conveyed within the encoded bitstream by metadata and the decoding device can use these parameters to recreate equivalent grain (in terms of level, texture, colour, variation, etc.) in the decoded video. With this in mind, DVB initiated a study mission to investigate whether there was evidence of a complete value chain for the use of Film Grain Synthesis techniques in DVB environments.

The study mission organised an online workshop in March 2024 and sent out a fact-finding survey in August 2024. After analysing the responses to the survey, the study mission did not find evidence that a complete value chain would be in place for the use of Film Grain Synthesis techniques in DVB environments. No further work on FGS is recommended in DVB at this stage, but the ongoing work of other SDOs in this area should be kept under review.

Introduction

DVB specifications for the use of video codecs in DVB applications[1] define many aspects of the encoding and decoding processes, generally in the form of constraints on the encoded bitstream and the minimum level of support required in the decoding device. However, one aspect of recent video coding standards that has yet to be addressed by DVB is Film Grain Synthesis (FGS).

Natural film grain is the random optical texture of processed photographic film and is present in digitized film content. Film grain effects may be added by content creators in modern post-production for aesthetic purposes. Film grain can be part of the artistic intent and an integral aspect of the story telling.
Since film grain is noise-like, the process of video compression can tend to reduce film grain, often in an uncontrolled manner, or even remove it. Accurately conveying the original film grain through a conventional encoding and decoding process would require a much higher bitrate, which may be unacceptable for many applications.

Film grain synthesis (FGS) is a technique where the film grain in the source video is analysed and parameterised, prior to any subsequent grain reduction or removal for encoding. The FGS parameters are conveyed within the encoded bitstream by metadata. The decoding device then uses the FGS parameters within the metadata to recreate equivalent grain to add to the decoded video. The goal is to maintain the original artistic intent at a lower bitrate than would be possible without using FGS techniques.

The DVB Commercial Module (CM) set up the Film Grain Synthesis Study Mission in order to explore the commercial drivers and obstacles to the potential use of FGS in DVB systems. The Study Mission held an online workshop on 6 March 2024, with the participation of both DVB members and non-members. A fact-finding survey was sent out on 14 August 2024 to all DVB members and also the non-members who had participated in the workshop. The survey included questions covering considerations relating to artistic, commercial and legacy issues.