Gamma and log coding for D‑cinema
A 30‑minute talk
at Oslo Digital Cinema Conference (ODCC)
by Charles Poynton
We explore the relationship between exposure and image data values, a relationship that is central to digital cinematography. In modern cinema cameras, that relationship – the “OETF” – is roughly logarithmic (hence, “shooting log”). The logarithm arises from D-log E, the classic sensitometry that relates the logarithm of exposure (log E) and the resulting optical density (D) in photochemical camera negative or release print film. Log plots have their origins in 1890 in the work of Hurter & Driffield (H & D), and are still evident in film data sheets today. Video systems have historically used a power function, characterized by “gamma,” instead of logarithmic coding. Various cinema camera manufacturers today use various log codes as the basis for OETFs; we’ll describe how these relate to the classical formulation, and explore what advantage (if any!) there is of tuning an OETF curve to the characteristics of particular sensors. Gamma of 2.2, 2.4, and 2.6 are standardized and widely used today, for EOTFs; we’ll explore how these numbers relate to cameras. Log and gamma functions have implications for noise performance, for image appearance, and for post-production and colour grading. The talk will be relevant to technically minded cinematographers and colourists, DITs, and image engineers.
Charles Poynton is an independent researcher and image/colour scientist based in Toronto. He wrote the book Digital Video and HD Algorithms and Interfaces, now in its second edition. Thirty years ago, he chose 1080 image rows for HD standards, by which “square pixels” were established for HD and digital cinema. He earned his PhD in 2018 from Simon Fraser University with a dissertation entitled Colour Appearance Issues in Digital Video, HD/UHD, and D‑cinema. ∎