Demystifying Colour Spaces, Transfer Functions, and Colour Standards

July 31, 2024

Filmlight engineer Daniele Siragusano unpacks the foundations of colour science and how these concepts define essential industry workflows.


Series

Filmlight’s Daniele Siragusano Part 2: Untangling industry standards and white papers

As with Part One of this mini-series, my aim for this Insight is to demystify the confusing and slightly intimidating rabbit hole that is colour science. Fortunately, Filmlight engineer Daniele Siragusano is joining me on this endeavour once again, and every time we talk, I learn at least five new nerdy things.

This Insight covers the fundamental building blocks of what we call ‘color spaces’.

It covers the myriad of interconnected standards that colourists rely on daily to deliver accurate deliverables to their clients.

Clearing the jargon hurdle

Breaking down colour spaces into their individual components can quickly become an intimidating exercise. Although significant technical detail is available in standards and reports, this information is not easily digestible or understood by the everyday, only vaguely technical, colourist (e.g., me).

This is where Daniele will step in to help!

This Insight will highlight industry standards and use practical examples in Baselight to help digest this information in a slightly more palatable way. Although I’ll warn you, even with an image engineer literally there to hold my hand, some of the concepts found in standards went over my head.

Even if you don’t understand every detail of Daniele’s explanation, the overarching topics and summaries that we cover are valuable.

Key takeaways from this Insight

By the end of this Insight you should understand:

  • The foundational building blocks of colour space: a transfer function and colour primaries
  • How camera stops relate to brightness levels
  • How to read industry White Papers and why you should focus on the footnotes
  • The major ITU standards that define Standard Definition, High Definition, and PQ / Hybrid Log-Gamma (HLG) workflows

Our Eyes, Cameras & Displays

What is a ‘colour space’?

“A colour space is a transformation to and from an observer.”

Daniele Siragusano

When we step outside and view the world, our eyes are pretty phenomenal cameras. There is complex machinery in our retina and in our brains that allows us to translate this light data into something meaningful and useful.

When we attempt to capture and display this raw, infinite supply of photons using camera and display technologies, there are significant barriers that modern post-production workflows have tried to alleviate.

Different camera manufacturers will use different capture colour spaces to attempt to record a pleasing image, but that’s only half the battle.

Different display technologies have different display and viewing standards that also attempt to reproduce a pleasing image. For this handover to work effectively, there needs to be clear industry standards that define the correlation between the media that is played on our displays and the displays themselves.

A colour space can be thought of simply as how data is encoded, either referring to the capture of information through a camera or the reproduction of information on any given display.

How To Build A Colour Space: Transfer Functions and Color Primaries

A typical RGB colour space can be built using two foundational concepts, joined together:

  • Transfer Function
  • Colour Primary

These two concepts have many different names, and they are nearly all interchangeable (with some minor differences).

Transfer Function

Transfer functions are often referred to as power functions, tone curves, or, depending on whether it’s a camera or display transfer function:

  • Optical-electrical transfer function (OETF) – A camera transfer function describing when photons (visual information) are converted to electrons (to be captured by a recording device).
  • Electro-optical transfer function (EOTF) – A display transfer function describing when electronic information is converted to photons (so the human eye can see it).

Colour Primary

Colour primaries are often called primary vectors, a colour gamut or a colour matrix. These primary colours cannot be created by mixing other colours together. Different colour models may have different colour primaries (think: sRGB vs. Rec. 709 vs. P3-D65).

Colour primaries can be proprietary, defined by the camera manufacturer, separately from the display primary, although many cameras can optionally record in one—or several—display colour primaries. Proprietary display colour primaries are not common in our industry.

How Transfer Functions and Colour Primaries work together

On the camera end, transfer functions are useful on-set because capturing light values in a linear fashion using a camera is inefficient. A camera can encode only a limited number of digital samples (directly related to its bit depth).

To encode this information more efficiently, logarithmic transfer functions reproduce a tone scale (or a grayscale from pure black to pure white) that better aligns with how human eyes perceive the world.

On the display end, transfer functions are used to ensure consistent image reproduction on various display formats and viewing environments.

Colour primaries can be considered the range of possible colours that can be captured or displayed. Camera manufacturers create unique colour gamuts that capture a technologically limited selection of colours. Display technologies reproduce a technologically limited selection of colours within the current limitations of screen technology.

Common OETFs and colour primaries

Camera ManufacturerCommon Camera Transfer Functions (OETF)Common Camera Color Primaries
ARRIARRI LogC3ARRI Wide Gamut 3
CanonCanon LogCanon Cinema Gamut
DJIDJI D-LogDJI D-Gamut
REDRED Log3G10REDWideGamutRGB
SonySony S-Log3Sony S-Gamut3.Cine
This is a list of common camera transfer functions and colour primaries designed to store linear light values efficiently.

Common EOTFs and colour primaries

DisplaysCommon Display Transfer Functions (EOTF)Common Display Color Primaries
Apple Displays2.2 GammaP3 D65
Broadcast Television2.4 GammaRec. 709
DCI Theatre2.6 GammaP3 D60
HDR DisplaysST 2084 PQRec. 2020
Consumer Displays2.2 GammaRec. 709
A list of common display transfer functions and colour primaries designed to reproduce colour standards in a pleasing manner.

Questions or Comments? Join the discussion!

This was the final part of my chat with Daniele, but if I’m being honest, I want another excuse to learn more from him! So, if you could convince an image engineer to have a coffee with you, what would you ask? Maybe we’ll come up with some questions that’ll get Daniele back on Mixing Light 🙂

I’d also like to organize a deep dive to discuss the benefits of logarithmic encoding versus encoding linear light, as that’s a topic I’ve never been able to explain elegantly! Do you want to see more of this kind of conversational Insight format? Let me know in the comments.

– Luke

PS – Logged-in Mixing Light members, scroll down below the video for technical definitions and the PDF white papers of the ITU colour space standards discussed in this Insight.


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