Looks, Log C, Rec 709 or 2020 and LUT. What is all of that?
Start digging into ARRI’s color science right here in our frequently asked questions on the topic of color where we explain the fundamental terms!
- Rec 709
Material recorded in Rec 709 (short for ITU-Recommendation BT.709) has a display specific encoding or a, in other words, "what you see is what you get" characteristic. The purpose of a display specific encoding is to immediately provide a visually correct representation of the camera material, when it is screened on a certain display device. This is achieved by mapping the actual contrast range of the scene into the contrast range that a display device can reproduce. Examples of display specific encodings are Rec 709 for HDTV screens or DCI P3 for Digital Cinema Projectors. On the downside, a display specific encoding puts some limits on the options for a colorist.
- What is Rec 2020?
Rec 2020 is a wider color space than Rec 709, which is the current industry standard for HD. Rec 2020 promises more brilliant images, though only new display technology is able to show these. Traditional Rec 709 displays (like TVs or ordinary computer displays) can’t display Rec 2020.
While Rec 709 (short for Recommendation ITU-R BT.709) is the encoding color space for HDTV, Rec 2020 is the encoding color space for UHD. The primary colors lie on the spectral locus and it is thus possible to have more saturated colors in images. (It does not mean, however, that all colors look more saturated. When properly converted, a Rec 709 image will look exactly the same when displayed on a Rec 2020 display.)
Rec 2020 is an encoding standard. A TV or display may not support the full gamut. Nevertheless, it will correctly display the colors within its physical gamut. To get the “Premium” logo of the UHD Alliance, for example, a TV needs to support a minimum of 90% of the P3 gamut.
- ARRI Cameras offer the option to record in „Log C“. What is „Log C“?
The Log C curve is a logarithmic encoding of the scene, meaning that the relation between exposure measured in stops and the signal is constant over a wide range. Each stop of exposure increases the signal by the same amount. The overall shape of the Log C curve is similar to the exposure curves of film negatives. Because of fundamental differences between a sensor and negative film, however, the color characteristics remain different.
Log C actually is a set of curves for different EI values/ASA ratings. Each curve maps the sensor signal, corresponding to 18% gray scene luminance, to a code value of 400 in a 10 bit signal. A 10 bit signal offers a total code value range of 0 to 1023. The maximum value of the Log C curve depends on the set EI value. The reason is quite simple: When the lens is stopped down, by one stop for example and the EI setting is increased from, say, 800 to 1600, the sensor will capture one stop more highlight information. Since the Log C output represents scene exposure values, the maximum value increases.
- What is “Color Gamut”?
A color gamut is the range of reproducible colors available on a certain device such as a display for example. A display may have a color gamut that exceeds a certain color space which itself is a subset of the visible spectrum of the human eye.
The color gamut is described by its three primary color values (red, green and blue), which define all possible colors for a color gamut inside of a triangle spanned between these primary colors. The primary colors therefore also define the outer boundaries of the gamut of the specific device. A display cannot produce colors that are outside of its gamut.
- Does a camera have its own gamut?
In an image that is delivered from the sensor of a digital camera, the values of the
primaries relate to the amount of light seen through red, green and blue color filters. There are no colors a camera can’t see, so it does not really make sense to talk about the gamut of a camera. It is, however, necessary to describe the color space used to encode the colors, which in case of the ALEXA camera is called the ALEXA wide gamut color space.
- Are there differences within the ARRI camera line-up as far as color science is concerned?
ARRI always strives to deliver the best overall image quality. As part of that we’re continuously improving our color science. Besides minor changes and improvements within the ALEXA series the introduction of AMIRA came with a new so called “Color Core” enabling a broad feature set for users. Not only is it possible to import a 3D Look-Up-Table (3D LUTs) using the ARRI Look File 2, but furthermore there are “Video Look Parameters” which allow to work with controls known from ENG/video cameras.
ARRI Look File 2 is available in ALEXA SXT, ALEXA Mini and AMIRA.
Following with SUP 5.0 and higher we will introduce the Camera Access Protocol (CAP) to ALEXA Mini, which allows for live-grade functionality as it is implemented in ALEXA SXT already.
ALEXA Classic and ALEXA XT cameras on the other hand are not able to process a 3D-LUT in its Color Core: the ARRI Look File 1 offers look creation “similar to CDL”. If a 3D LUT-color-preview is demanded an external LUT-box has to be utilized to alter the image on the fly.
- My images seem to be lacking contrast/look washed out – what’s the issue?
This is not an error! You have chosen to record in “Log C” – ARRI’s native color encoding. Opposed to a straight encoding to a “video” image (in REC 709 color space) Log C maintains more details in the lowlight as well as in the highlight areas. Log C is also referred to as the camera negative as it is unprocessed footage.
That being said, Log C does not help to accelerate your production. If you’re on a tight schedule keep in mind that an additional rendering step has to take place before the footage can be delivered to a customer. In that case we recommend to shoot in REC 709. Color correction can be skipped in that case, but is still possible although in a limited way.
- Why is there no support for DCI P3?
DCI-P3 color space has been removed from the ALEXA camera menu for reasons of simplicity. A conversion LUT, Log C to DCI-P3, can be generated via our LUT Generator.
- ALEXA SXT, AMIRA and ALEXA Mini can work with 3D LUTs – how?
- What is the difference between ARRI Look File 1 (ALEXA Classic and ALEXA XT) and ARRI Look File 2 (AMIRA, ALEXA Mini and ALEXA SXT)?
ARRI Look File 1 offers CDL-like controls as well as printer lights, saturation and a user-definable tone mapping curve. The ALF-1 does not apply LIFT/Gamma/Gain before but after the conversion to REC 709. Usually CDL is done in Log C because there’s more image information in Log C (low- and highlights) still there to work with.
ARRI Look File 2 offers full CDL and 3D LUT or Video Look Parameters. Look files in the file header can be utilized in post. The ALF2 enables support for REC 2020 (please see second question: "What is Rec 2020") and provides the option for live grading.
ARRI Look File 2 – Looks in use
- Can I convert an ALF-1 to an ALF-2?
Conversion of ALF-1 looks is easily possible with our online LUT Generator. Choose the LOOK-tab and upload your look file. Create a 3D LUT (*.cube – DaVinci Resolve format) with 33^3 mesh points.
Next, import the 3D LUT into the ARRI Color Tool and save it as an ALF-2 look file.
- How do I live grade with ALEXA SXT or AMIRA and ALEXA Mini (SUP 5.0 or higher)?
- What is the ARRI Color Tool?
Use the ARRI Color Tool (ACT) to create ARRI’s Look File 2 to use in ALEXA SXT, ALEXA Mini and AMIRA. Based on a Log C-clip or a *.dpx still frame you can create looks using Color Decision List (CDL) parameters, “Video Look Parameters” (VLP) or import 3D LUTs created in a grading session.
ACT is a free application for Mac and Windows computers.
ARRI Color Tool
- How do I extract a Look File from a clip?
Import the clip into the ARRI Color Tool by browsing to its location in the file browser. Double-click its name/thumbnail to load the clip into the viewer.
To export a ALF-2 choose FILE > SAVE LOOK (CMD + S)
For a 3D LUT choose FILE > EXPORT > 3D LUT (CMD + L)
- How do I use a look in post?
A look can be used in a destructive and a non-destructive way. Destructive equals “quick” – no further color-postproduction will take place. Whereas non-destructive means the look is used as a tool on set but a grading session is usually scheduled later in production.
For grading and mastering the original camera negatives can be used to extract the look from the file header (please see previous question). That means as long as the originals exist, the look can’t get lost. It can be used as a reference for grading or to create “burnt-in” clips (dailies) for editorial and director’s review.
We’ve created two whitepapers with in-depth guides on the ARRI Look File 2:
ARRI Look File 2 – Looks in Editorial - Workflow Guide Line
ALEXA Dailies Creation in Blackmagic Resolve 12.5
ARRI Look File 2 – Looks in use
- Do ARRI cameras support HDR?
Yes, since the very first ALEXA back in 2010!
Find out more on HDR in our separate HDR FAQ
- I need a conversion LUT for postproduction. Where do I get those?
As most of the postproduction tools use their own notation for look up tables, ARRI offers conversion LUTs through the online LUT Generator which can create over a dozen different LUT formats.
When dailies are going to be color timed, the colorist may prefer using a photometric conversion LUT as he or she will use just this one LUT for all shots and perform adjustments to the black and white levels anyway. An example of LUT generator settings for color-corrected dailies are shown below. For an automated dailies process, the normalized LUTs must be used in order to compensate for the different ASA settings on the camera.
If „ARRI Look File 1“-looks were created and used during the shoot, they can also be converted with the ARRI LUT Generator to create 3D LUTs that include look file adjustments (Choose preset "Look"). This will allow the creation of Dailies including the intended look from the DP.
- Normalized & Photometric conversion – When is it used and which should I use when?
As explained in [Log C] the maximum value of the Log C function depends on the exposure index. In a video image, however, one wants to have the cliping point of the sensor represented by the maximum signal (100%) regardless if this clipping point is 6, 7, or 8 stops above mid gray. Therefore, the LUTs for conversion from LogC to Video normalize this range, e.g. the video signal range will always go from approx 1% to 100%.
This normalization is done always in the camera. For post-production, it’s possible to use a LUT that is computed without this normaization step, and it’s called “photometric” (the name is a bit misleading). When one works with a timeline of shots recorded with different settings of the exposure index, it’s easier to use one conversion LUT for all.
One side effect of this normalization is that you get lower blacks. Users who want to use a single LUT while maintaining the look of the image as output by the camera should use the LUT computed for EI800.
- What is Scene Linear?
This is an encoding where the digital values are proportional to the relative brightness in the scene. In more technical terms it’s said that the digital values are radiometrically linear.
- What is Wide Gamut?
Wide gamut is a generic terms for color gamuts that are larger than Rec. 709, i.e. P3 or Rec. 2020. ALEXA Wide gamut is the name of the color encoding space used in combination with Log C.
- What is a 1D LUT?
A 1D LUT describes a transformation applied to each of the red, green and blue (RGB) channels independently. A 1D LUT contains an input value for each of the three colors and a corresponding output value. So for a given value of R, G, or B input, there is a given value of R, G, and B output.
- What is a 3D LUT?
3D LUTs are more powerful than 1D LUTs as they can cross-convert colors between channels and independently control saturation, brightness and contrast. However, they are significantly larger than 1D LUTs and require some computation, since they only store some key values and any in-between values are interpolated.