The Ace of Colour Management
ACES, the colour management system hailing from the Oscars has spread across the VFX industry with great success, Greg Barta from scivfx describes its basic principles with 3D visualisations
Inside the latest industry standard and tool that’s rocking Hollywood
The nature of the tasks that 3D artists usually do is kind of a mixture of many art forms such as photography and painting. Painters start with a blank white canvas and compile the wide range of colour values of reality into their much narrower palette to achieve a compelling image in its viewing environment and lighting. They can achieve brighter colours than the white of the canvas using special pigments, but it does almost nothing if we hang that painting on the wall of a movie theatre with the screening light settings. However if we put it in front of the screen and light it with the pure white of the projector, this can make a good painting a quite impressive experience. Besides having a multi-thousand watt light source in the projector that defines the dynamic range in this case, the wide range of compounds, origins and different manufacturing processes of the paint pigments provide the rich colour palette for the painter to create scenic paintings. Filmmakers have just three colours, but the features of photochemical processes, like cross-talking, give them a unique, cinematic look. In contrast, today’s digital imaging tech don’t just use only three colours, but these are just data most of the time and converted to an image signal when displayed.
Every 3D artist probably remembers a time when they achieved a good-looking render then changed some basic parameters, like exposure, and the new result ended up looking a bit odd. There could be many reasons for this but the most obvious problem is the lack of proper colour management. In this case, artists tend to use physically-inaccurate values to compensate for the inappropriate rendition of the colours on the display, which is the worst idea when working with physically-based renderers.
The 3D/CG community woke up a long time ago and realised that directly displaying the raw scene colour values on the display is a technical glitch, thus it is normally compensated with the well-known gamma correction.
However the presence of a proper colour management system was still the privilege of
VFX houses until recently and is still not a widely adopted practice.
Our culturally-affected collective colour memories strongly rely on photos and paintings we’ve previously seen, it can make the colours of a raw – and even gamma corrected – 3D render unfriendly. There are also more basic principles, which affect the perception of the image on a display. Today’s display technologies, even HDR monitors/tvs, have a maximum achievable brightness, which are orders of magnitudes lower than in reality, and the saturation of each colour component limits the gamut. Thus, we should reinterpret all or almost all the colour values of the scene properly into the dynamic and gamut range of the display to get a compelling image. Firstly we set the exposure of the render – or worse the lights – however this is just a multiplication of the RGB values. With such simple maths, a daylight scene tends to be dim, flat and washed out on our relatively dark display. Luckily the nature of photographic processes usually compensate for these automatically and we get a compelling image at the end. If we use digital cameras, the camera’s raw developer softwares and colour-grading suites offer similar opportunities.
Most renderers have advanced colourtweaking capabilities like highlight compression and LUT import but these are still far away from the capabilities and benefits of a standardised colour-management system like ACES.
Today’s display technologies have a maximum achievable brightness, which are orders of magnitudes lower than in reality, and the saturation of each colour component limits the gamut
Guardians Of The Galaxy Vol 2 used ACES colour management