TABLE OF CONTENTS
Currently, color management is achieved through careful best practices and collaboration between creatives, post facilities, and the Creative Tech Workflows & Imaging team. While there are open standards for Color Management like ACES, this document provides an overview of what we mean when we say a project is ‘color managed’ - whether a project is using ACES or not. The long term plan is to increase industry understanding of these concepts, and improve existing standards like ACES v2.0, in order to make color management easier and more accessible.
The term ‘color managed’ is used a lot at Netflix and within our industry, but can mean different things to different people. For some content types, ‘color managed’ could mean shooting in Rec. 709, finishing on a calibrated Rec. 709 monitor, and uploading a ProRes to YouTube. In its most advanced form, say for a high-end VFX facility, it could mean creating textures and CG elements in a scene-referred color space, such as ACES, pre-converting assets using OpenColorIO, compositing with live-action camera footage, and exporting DPX or EXR files to a DI facility for final color grading. As you can see, there is a wide range of interpretations. The purpose of this document is to define what ‘color managed’ means for Netflix productions.
For Netflix, there are three main goals of color management. The first is to ensure a predictable and repeatable way to view images and convert between different color spaces, based on a variety of capture devices and display types. This ensures that creative intent is maintained throughout production and post processes. The second is to ensure all color decisions and VFX work are done in a scene-referred color space, in order to maximize creative flexibility in post, and third, to ensure that our picture archival masters are of high quality and future proof.
A tech blog post was written by members of Creative Technologies in order to give some historical and scientific context around what we mean by color management, some tips on how to achieve it, and how it helps enable certain archival assets to be generated.
A color pipeline represents the essential color space definitions, transforms, and deliverables for a project. This usually requires knowledge of the source(s), working, and final delivery color spaces to ensure that all points in a color pipeline are aligned. This must be defined in coordination with all departments responsible for viewing, sending, and receiving images on a production.
Input Color Space(s)
This is based on the initial image capture device or source color space. Some examples are Sony SLog3 / SGamut3.cine, RED Log3G10 / WideGamutRGB, ARRI LogC / WideGamut. For certain workflows, this could include archival footage or graphics already in a display-referred color space such as Rec. 709 / BT.1886 or sRGB.
Working Color Space
This is the color space where manipulations of the image take place. If the source is not already in the working color space, the source should be converted into the working color space prior to any image manipulation done. Examples of working color spaces are Sony SLog3 / SGamut3.cine, RED Log3G10 / WideGamutRGB, ARRI LogC / WideGamut, or ACES. When viewing things in their working color space, without the use of some Output Transform (see below), they may look incorrect to the human eye. This is because these color spaces are designed to store the full dynamic range and color information from the scene, rather than storing the image in the display’s color space.
This is an agreed-upon color transform which converts the image from the Working Color Space to the Display Color Space. Using an Output Transform or “viewing LUT” — such as a creative LUT designed by a colorist or DI facility, or even a default camera LUT like ARRI’s 709 LUT — can not only serve as the base “look” of the show, but it protects and preserves the working color space and the full dynamic range it has to offer for color and VFX.
Display Color Space
This is the color space that a display or monitor is capable of supporting where the images look correct. Examples of display color spaces are Rec. 709/BT.1886, PQ (ST.2084) P3-D65 or Rec. 2020, etc.
In this example, the working color space is Log (logarithmic) and the display color space is Rec. 709 / BT.1886. The Output Transform separates the two, and is only baked in for the Rec. 709 / BT.1886 streaming master. The archival master (NAM) remains in Log color space.
What Is The Value of Color Management?
Confidence and Trust in the Image
Confidence and trust in the color pipeline means that productions spend less time on support and troubleshooting and more time on creative decision making. Fixing or re-doing work due to color issues can be very expensive and time consuming. This is why it's so important for all parties in the production and post-production stages who are responsible for viewing, sending, receiving, and/or manipulating images to align on the color pipeline.
Flexibility in Viewing Images
A color pipeline like the one described in this article enables the possibility of viewing images in different scenarios with more confidence. Whether it is an SDR or HDR display, a monitor or projector, the appearance of the image should be consistent and creative decisions should be maintained. This is only possible with a solid color management system.
Color management, and specifically defining a working color space that contains all of the information captured (or CG generated), helps to protect all of the creative work in a future proof picture archival master. For example, color managed images from past seasons can easily plug into new seasons or projects, VFX or Animation elements can be used across projects, or simply the archival masters from current projects can live on in the archive for historical or remastering purposes.
General Requirements for Proper Color Management
Consistent Color Transforms
Standard color pipelines like ACES are an attempt to have consistent color transforms between different software and hardware, and know ‘how do I view this working image’ without having the image baked into a display color space. While ACES defines the transforms themselves, software like OpenColorIO (OCIO) make implementation of a color pipeline more consistent across software, as well as opening up the ability to define custom color pipelines, and making sharing them between facilities and software packages easier. OCIO has been well adopted in the VFX industry, but needs work in on-set, editorial, and final color grading software.
High Bit-Depth Precision for Formats and Processing
A high-quality color management system needs high bit-depth processing in order to transform images between color spaces non-destructively. We recognize that certain non-linear color spaces (such as Rec. 709 and Log flavors) exist so that high-quality work can be done at lower bit-depths, but increasingly there are linear representations and complex operations that require 16-bit floating point or higher precision to be done non-destructively. We would consider 10-bit a bare minimum for any professional imaging system handling SDR images, and 12-bit for HDR images. Ideally, all systems would be capable of floating point (16-bit half or 32-bit full float) in order to reduce the chances of clipping or other destructive operations.
Stages of Color Management
This is a crucial point of communication which increases the likelihood of a smooth color experience through a production. Once a primary camera is chosen, or even before, conversations should be had with all parties responsible for the image. This includes the DoP, DIT, colorist, VFX facilities, and Netflix representatives responsible for the project. While certain decisions may not be able to be made at this stage (for example the exact final colorist may be TBD), it is important to have these conversations as early as possible. What camera and recording format will be used? What is the working color space? Will they use ACES? If not, is there an agreed upon show LUT?
Calibration of all displays to industry color standards is essential. This ensures that the color transforms used are relevant, image appearance is consistent, and color decisions are meaningful. Exact methods of display calibration are outside of the scope of this document, but the important standards and target color spaces can be found in our Color Critical Calibration Guidelines.
On-set is usually the first place where color and images are judged, so it is arguably the most important to get right. Hopefully, the pre-production conversations have informed an agreed-upon and widely communicated setup of on-set video signal path, CDL + LUT boxes, and calibrated monitors. The goal of on-set monitoring is usually to have a non-destructive and repeatable color pipeline, in order to give flexibility and consistency to the downstream dailies process.
Dailies & Editorial
Dailies is where the color pipeline has the biggest effect on post, since the same camera RAW files which will be used in finishing are used to generate color-baked dailies and editorial media. The on-set color decisions should be communicated via ASC CDLs and be passed along into ALEs which are ingested into the Avid. Those CDLs and the same show LUT / Output Transform from on-set should be applied in the same working color space so that editorial sees the same color that was seen on-set.
VFX vendors should receive documentation outlining the color pipeline for every project they are working on. Ideally, a VFX pull should include: a description of the VFX plate color encoding and format (i.e. 16-bit EXR, Linear ARRI Wide Gamut), a color ‘recipe’ to achieve dailies color (i.e. CDL + LUT, working color space), and a reference frame for checking color against existing dailies. The VFX plate color encoding is essential in order for VFX to be done in a scene-referred color space for high-quality compositing and using physics-based rendering methods. The color ‘recipe’ and reference frames are essential in order to send completed VFX shots back into the editorial cut with matching color to the surrounding shots.
Final Color Grading
The color pipeline culminates at the point of the online conform of original camera files (OCF) and final color grading. Ideally, the colorist should have access to the color decisions (CDL + LUT) that was used for dailies, in order to have a starting point similar in appearance to what has been seen throughout the process. Color grading should be done in the working color space, ahead of the Output Transform / show LUT, in order to protect those decisions for the archival deliverables explained below. This can be achieved by grading in ACES, or by setting up the grading system with careful color management settings to ensure that all creative decisions are applied in the working color space (order of operations is key).
When this type of color management is employed, it enables the delivery of the NAM (Non-graded Archival Master) archival deliverable in a single unified color space. For more information about this deliverable, please see the NAM specification.
Example Color Pipeline
As explained above, it is important to ensure that images are stored in the widest possible color space until the very final stages of the color pipeline. This is easily achieved by using ACES, which keeps images in a high dynamic range, wide color gamut space, and allows the user to simply select the Output Transform for the target viewing display. This can also be achieved while working in the dominant camera color space with careful, pro-active color management and the use of conversion and viewing LUTs. As an example, the diagram below shows a color pipeline at the on-set, editorial, and finishing stages.