Image encoding for ARRI Wide Gamut color space
ARRI cameras record and output images in Log C wide gamut color space. Only Log C images can transport all the color information and dynamic range captured by ARRI camera sensors. The latest generation of ARRI color science (ARRI REVEAL) was introduced with the ALEXA 35. Now, ALEXA 265 follows as the second camera running REVEAL color science internally.
REVEAL is backwards-compatible with ARRIRAW shot on all previous ALEXA and AMIRA cameras.
Images encoded with Log C (C is for Cineon; the original Cineon log encoding is based on the density of color film negative) can be identified by their flat and desaturated nature. Whites and blacks are not extended to their maximum values. The reason for this: the Log C curve is a logarithmic encoding for images, the encoding has a grayscale characteristic similar to a scan from negative film.
Because of the fundamental differences between digital cameras and negatives, the color characteristics remain different, though. Logarithmic encoding means that the relation between exposure measured in stops and the encoded signal is linear (straight) over a wide range. Each stop of exposure increases the signal by the same amount.
A LogC4 image looks different from a LogC3 image because of their different origins and requirements: ALEV3 (the original ALEXA sensor) and ALEV4 (the new ALEXA 35 sensor) have a different dynamic range and therefore different demands towards a logarithmic encoding of their images. All the knowledge we gained with ALEV3 over the years led to the advancements in color science which ultimately materialized in REVEAL and facilitated LogC4. We can fit images from ALEV3 into the new pipeline, but not the other way around.
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Log C uses what is known as scene-based encoding. The signal level increases by a fixed amount with each increase of exposure, measured in stops. This encoding, which uses an ARRI-specific wide gamut color space, is similar to files from a film scan and ideally suited to carrying image information. To correctly display Log C material on an SDR or HDR monitor (Rec 709/Rec 2020) or in a digital projection (P3), it needs to be tone-mapped and transformed into the target color space. This image conversion is performed by a 3D-LUT (3D Look-Up Table).
For LogC3 LUTs (AMIRA and ALEXA cameras up to Mini LF) please visit our online LUT Generator.
For LogC4 LUTs (ALEXA 265 and ALEXA 35 only), you can download pre-generated LUTs in a cube format.
Rec 709/2020 is a display-based encoding using RGB primaries specified in ITU Recommendation BT.709/2020 and can be directly output to standard HDTV/UHDTV displays without any conversion.
The material can be processed by HD/UHD video production gear in real time, which enables short production times. But Rec 709/2020 limits choices in color grading and, due to its more contrasty characteristic, cannot hold as much highlight information as the much flatter Log C curve. Therefore, Rec 709/2020 is not available as a recording color space in ALEXA Mini LF, ALEXA 35, and ALEXA 265.
Shooting with a combination of ALEXA 35/ALEXA 265 and other ALEXA or AMIRA cameras raises the question of how to handle both Log C versions in one project. We have anticipated this situation and offer a path for footage captured with ALEXA Mini LF or older ALEXA/AMIRA cameras to be used within REVEAL Color Science alongside ALEXA 35 or ALEXA 265 footage through our ARRIRAW SDK. Software using our SDK can develop LogC3 footage to LogC4. This can only be achieved with ARRIRAW footage and is not possible with Apple ProRes footage. See our guide on mixing LogC3/LogC4 for more information.
Wireless Video Optimized LogC4 (WVO LogC4) is an intermediate encoding of LogC4 that is designed to improve image quality when transmitting LogC4 over wireless video transmitters. The WVO encoding is an option built into the ALEXA 265 and ALEXA 35. A matching WVO decoding LUT must be used after signal reception to reconstitute regular LogC4. Download the WVO LogC4 LUT package here!
If you are planning to employ a wireless video setup in your LogC4 signal chain form camera to DIT, carefully check your signal for image artefacts during prep. Due to a reduced bandwith over the air you might experience banding artefacts visible in gradients or patches of even colors.
In the following situations you should not use WVO LogC4:
When SDI processing is set to “Wireless Video Optimized LogC4”, the indication in the SDI status overlays will show “WVO”, where normally “LOG” or “Look” are shown.
Please see our technical note on LogC4 WVO for further details.
