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Color FAQ

Common question regarding ARRI's Color Management

  • 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.

  • 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.

    x/y graph depicting Rec 2020, Rec 709 and P3 colorspace

  • 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 (LogC3)/308 (LogC4) 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.

    six gray gradients illustrating exposure range of ALEV3 sensor; distribution of stops
    Copyright © ARRI

  • 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.

    A graph depicting the differences between ARRI Wide Gamut 3 and 4

  • The values of the primaries in an image from the sensor of a digital camera 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 ARRI Wide Gamut color space.

  • ARRI always strives to deliver the best overall image quality. As part of that we’re continuously improving our color science.

    With the new ALEV4 sensor we've taken our color science through its biggest rewrite since we introduced the ALEV3 sensor in the first ALEXA in 2010. We've made various improvements over the years, but our color science never really came to a rest. In these last years LogC3 became famous throughout the industry. With ALEV4 LogC3 had to change to hold all the dynamic range the new sensor if capable of. So all is new: ARRI Color Engine, ARRI Wide Gamut 4, LogC4. To sum it up we call the new color science REVEAL.

    The good news: REVEAL is compatible with ALEV3 cameras. One can de-bayer ALEV3 ARRIRAW to ARRI Wide Gamut 4/LogC4 and use all the benefits of REVEAL. If shot in ProRes – no worries LogC3 is and will be supported in all post tools through our SDK.

  • You use our ARRI Reference Tool (ART) to do so. Download ART here.
     
    1.    Open the ARRI Reverence Tool. No need to import a clip. Change to the LOOK room.
    2.    Use the file browser in “My Looks” to find your look file *.alf4 on your storage. Select it. ART automatically applies your look to our Reference Image of Helen and John for reference.
    3.    Choose “Export 3D LUT” from the tool bar.
    4.    A pop-up windows opens. Set your LUT size, file name and location to save the file.


    GUI of the ARRI Reference Tool on macOS

  • With the update to ACES Version 1.3 there was also an update for the ACES IDT for REVEAL (AWG4/LogC4). The ACES Version 1.3 can be implemented by 3rd party vendors e.g. Pomfort already supports ACES 1.3. ACES can be used on the SDI output and in post-production of course. Currently we do not offer an ACES in-camera workflow.

  • All 3D-LUTs you want to use in ALEXA 35 need to be converted to an ARRI Look File 4. You can use our ARRI Reference Tool to convert the LUT for ALEXA 35.

    Keep in mind that we have changed our look workflow and introduced a new color management ("REVEAL") with our ALEXA 35. That means you must use a Log-to-Log 3D-LUT (=without output transform/DRT) inside the look file. Custom DRTs are currently not supported and would end in a double-LUTted look.

    We suggest redesigning and visually matching your older Looks/3D-LUTs (designed for LogC3) for ALEXA 35 as it has a completely new color science and dynamic range. There’s also a small guideline “ALEXA 35 – ARRI Look File ALF4”, which explains this task.


  • ALF2 look files are designed to work with ALEXA Mini, ALEXA SXT, ALEXA 65, ALEXA LF, ALEXA Mini LF, as well as AMIRA (all using our ALEV3 sensor). They combine a creative grade (CDL or custom LUT) with a color-space transform.

    ALF4 is the new generation look file for ALEXA 35 (ALEV4 sensor). This look file separates the creative grade from the decision which output color space you intend to use. The creative grade is a Log-to-Log 3D LUT that makes changes “in log” to the LogC4 image. The following output color space transformation is currently determined by the respective setting in the camera, you can choose between:

    •    REC 709 (SDR)
    •    REC 2020 (SDR)
    •    REC 2100 / PQ (HDR)
    •    REC 2100 / HLG (HDR)

    All these DRTs are also included in our official ARRI LogC4 LUT Package, which can also be used in post-production.The functionality for loading custom DRTs into the ALEXA 35 will be released in one of our upcoming camera SUPs.

    In the following table you will find the naming of the in-camera DRT and the corresponding 3D-LUT. 

    Naming in ARRI LogC4 LUT Package  

    Naming in ALEXA 35

    ARRI_LogC4-to-Gamma24_Rec709-D65_v1

    REC 709 (SDR)

    ARRI_LogC4-to-Gamma24_Rec2020-D65_v1

    REC 2020 (SDR)

    ARRI_LogC4-to-St2084_1K_Rec2100-D65_DW100_v1

    REC 2100 / PQ (HDR)

    ARRI_LogC4-to-HLG_1K_Rec2100-D65_DW100_v1

    REC 2100 / HLG (HDR)

    ALF2 and ALF4 look files are not compatible and cannot be converted.



  • ALEXA 35 uses our new REVEAL Color Science. REVEAL introduces a new Log C curve (LogC4) for the new ARRI Wide Gamut 4 color space to make use of the advancements made in sensor technology for ALEV4.

    Because the new LogC4 curve is too different from the well-known LogC3 characteristic a conversion of Look files (or 3D-LUTs) is not possible to convert existing ALF2 look files (this applies to 3D-LUTs created for LogC3 as well). Looks/LUTs must be recreated visually in LogC4.

    Please note: We highly recommend not to create an inversion of an ALF2 look to create an ALF4 look. This would result in a technically wrong image.

  • To use the Look Library in post production, download the Log-to-Log version for ALEXA 35 here.Please keep in mind that those need to be need to be combined with the ARRI official Display Render Transform LUT e.g. for Rec709: 

    Look Library Look for LogC4 (log-2-log) + ARRI_LogC4-to-Gamma24_Rec709-D65_v1

    This example would result in a correct image incl. the look for Rec. 709. Or you combine the Look Library Look with the official ARRI DRT for HDR e.g.

    Look Library Look for LogC4 (log-2-log) + ARRI_LogC4-to-St2084_1K_Rec2100-D65_DW100_v1

  • Currently you don’t have the possibility to use custom DRTs in-camera. In ALEXA 35 you can choose between different SDR and HDR transforms:

    • REC 709 (SDR)
    • REC 2020 (SDR)
    • REC 2100 / PQ (HDR)
    • REC 2100 / HLG (HDR)

    All these DRTs are also included in our official ARRI LogC4 LUT Package, which can also be used in post-production.The functionality for loading custom DRTs into the ALEXA 35 will be released in one of our upcoming camera SUPs.

    In the following table you will find the naming of the in-camera DRT and the corresponding 3D-LUT. 

    Naming in ARRI LogC4 LUT Package  

    Naming in ALEXA 35

    ARRI_LogC4-to-Gamma24_Rec709-D65_v1

    REC 709 (SDR)

    ARRI_LogC4-to-Gamma24_Rec2020-D65_v1

    REC 2020 (SDR)

    ARRI_LogC4-to-St2084_1K_Rec2100-D65_DW100_v1

    REC 2100 / PQ (HDR)

    ARRI_LogC4-to-HLG_1K_Rec2100-D65_DW100_v1

    REC 2100 / HLG (HDR)


  • There’s an easy fix: setting the SDI 1 OVERLAY BRIGHTNESS to 1 (or 2) brings back the green/red bullet (and other colored icons).

    We will address the problem in a future SUP for ALEXA 35. Until then, the workaround is needed.

    Screenshot of the camera menu

  • This is not an error! You have chosen to record in “Log C” – ARRI’s native color encoding. Opposed 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.

  • DCI-P3 color space has been removed from the ALEXA camera menu for reasons of simplicity. A conversion LUT, LogC3 to DCI-P3, can be generated via our LUT Generator.

  • The ARRI Look File 2 enables different ways to influence the image on ARRI cameras. Here’s an overview on the workflow:

    flow graph describing the ALF-2 workflow
    Copyright © ARRI

  • 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 – Using the ALF2
    ARRI Look File 2 - Looks in Editorial

  • Conversion of ALF-1 looks is easily possible with our ARRI Color Tool. Open your ALF-1 (*.xml) look file and export it as an ALF-2 look file (File > Save Look as).

  • An Ethernet or WIFI connection to the camera is the key element in a live grading setup. We’ve created an overview to show the big picture of such a setup:

    wkflw_faq-color_ALF2_Livegrading
    Copyright © ARRI

  • 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.

    Please note: The ARRI Color Tool has been discontinued. The functionality will be covered by our ARRI Reference Tool (ART) from now on.
     

  • Yes, since the very first ALEXA back in 2010!

    Find out more on HDR in our separate HDR FAQ

  • As most of the postproduction tools use their own notation for look up tables, ARRI offers conversion LUTs through the online LUT Generator (for LogC3 only! here's the LUT package for LogC4) 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 Color Tool 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.

    screen shot of ARRI's LUT Generator
    Copyright © ARRI

  • 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. Scene Linear files are stored in a 16bit float container and are mostly used for VFX pulls. 

  • Wide gamut is a generic term for color gamuts that are larger than Rec. 709, i.e. P3 or Rec. 2020. ARRI Wide Gamut 3/4  is the name of the color encoding space used in combination with Log C.

  • 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.

  • 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.

  • The Look Library "for post production" is available as Log-to-Log 3D-LUTs (*.cube) for LogC3 and LogC4 on our Look Files Website.

    Some 3rd party applications are able to read the ALF-2 information directly out of the ProRes clip or ARRIRAW file header (e.g. Blackmagic Design’s “DaVinci Resolve Studio”).


  • Here's an overview on ALF-1/ALF-2 support in 3rd party tools:

    a table showing the feature set and ALF-2/1 support of various postproduction software
    Copyright © ARRI

  • Generally speaking you need to export the look as a 3D-LUT and build this into an ARRI Look File.

    For ALEXA 35 this needs to be a Log-to-Log 3D-LUT based on actual Logc4 footage; the transform to Rec 709 is currently handled by the camera. Using custom DRTs will be enabled in a future software update. Use the ARRI Reference Tool to create the look file.

    For all ALEXA and AMIRA cameras using the previous look file format (ALF-2), the look must be based on LogC3 footage. A 3D-LUT to use in an ALF-2 needs to have a transform to Rec 709 baked-in. Use the ARRI Color Tool to create the look file.




    ARRI Look Library LogC3

    Collection of log-to-log 3D LUTs for ALEXA Classic, ALEXA Mini, AMIRA, ALEXA 65, ALEXA LF, and ALEXA Mini LF


  • The in-camera ARRI Look Library files are protected. You have to rename your exported look name before loading it back to the camera. Rename the exported  “1110-1.aml” file to a different filename, such as “1110-1_v2.aml” on the system to which the Look was exported.Then import the renamed look back into the camera.

  • The ARRI Look Library is available free of charge. You can download the whole package in the ARRI Look File format or as 3D-LUTs. Pick ALF-2/LogC3 for all cameras but ALEXA 35) and ALF-4/Logc4 for ALEXA 35.

    To find the downloads please head to our look files website.

  • There are different parameters for sharpness, detail and the ARRI noise redcution and in what way it will affect the final image. A more detailed explanation regarding all settings you will find in our ARRI Texture Control Whitepaper.

    This does not apply to ALEXA 35 cameras, which have a dedicated feature called "ARRI Textures" to control the image.

  • To get a good, correct color representation, you typically want to adjust the white balance of the camera to the lighting of your scene. As a result, the live output and recorded image of a neutral grey target should not exhibit a color cast (at least while no artistic look is applied). Since it may be difficult to see a color cast depending on the viewing environment, the best way is to look for a color shift with an RGB waveform display or a vecorscope display.

    The camera offers two dials that allow you to set the white balance:
    - the color temperature [K] will move the white point between blue and red, or "colder and warmer".
    - the tint or color cast [CC] will move the white point between green and magenta or "plus and minus green".

    You can either set these values manually or have the camera calculate an automated white balance while it is looking at the neutral grey target.