Webinar –
Math and Colour Science for Colourists (ACES)

Presenter: Charles Poynton
Duration: Three, two-hour sessions
Date/time: Tue./Wed./Thu. Dec. 3‑4‑5, 11:00 EST–13:00 EST
Registration: GoToTraining, fee CAD 240

(A 1-page flyer in PDF format is available.)


In principle, it should be possible for a cinema or HD colourist to work completely by eye, using perhaps half a dozen controls having no calibration markings. In practice, this is impossible. Colourists do not only do art, they also do craft and science. They are expected to import image data which, in the worst case, is of uncertain origin; and at best, is accompanied by data (CDLs, LUTs, or look files) that have exact numerical interpretations. Colourists are expected to export image data that is referenced to particular data values. Successful colourists deal with numbers, and with the logarithmic or power function equations that relate them.

It is not just numbers that are required, it is the relationships between numbers and signal code values, and between numbers and colours. The task of tracking these relationships is made difficult by widespread misuse of terms like brightness, luminance, and intensity that, properly used, have precise technical meanings, but unfortunately are interpreted differently by different people in the industry. Even the term linear is confused.

Colours are plotted in two dimensions in circles, hexagons, triangles, squares, and other geometric figures. Why so many shapes? The key lies in the matrix (affine) and projective transforms applied to colour coordinates. An understanding of these transforms unlocks the mysteries of the vectorscope and the [x, y] chromaticity diagram.

Beyond the numbers and the terminology, an appreciation of cinema and HD digital imagery requires familiarity with logarithms and power functions. Many people in the industry believe that film is inherently logarithmic, but film transmittance is related to a power (about 0.6) of light exposure; the logarithmic behaviour only comes about when density is computed. The behaviour of the logarithm of exposure is central to the emergent ACES (scene-referred) model of cinema image processing.

In this 3‑session webinar, Charles Poynton will discuss the math behind colour science – both classic “1931” colorimetry, and the modern science of colour appearance modelling. He will describe how logarithmic (density) measures were used in Cineon/DPX digital intermediates, and he will explain how and why that scheme is giving way to ACES processing. He will describe the ACES colourspace (and its close relative, OCES), and describe the four key colour transforms in the scheme: IDT, LMT, RRT, and ODT. He will explain how the scene-linear model is applied to the DI pipeline, and how it aids CGI/VFX integration. He will explain the power functions used in video and HD, and explain when and where 2.6, 2.4, 2.2, and even 2.0 powers (“gamma” values) are appropriate.

Who Should Attend: The Webinar will be suitable for highly experienced practicing colourists who are consulting and/or teaching. The attendee should be familiar with digital cinema, HD, and digital video. We’ll start from high-school mathematics (y = m·x + b), and delve into power functions, logarithms, affine transforms, and projective transforms from there – always remaining close to the goal of making pictures. Many graphs and equations will be shown!

Registration: At GoToTraining CAD 240. Detailed handout notes – some of which form portions of the second edition of Mr. Poynton’s book – will be provided. For information, contact Charles Poynton, charles@poynton.com, +1 416 535 7187.

Copyright © 2013-11-25 23:16 EST
Charles Poynton