Image encoding for ARRI Wide Gamut color space
ARRI cameras record and output images in Log C wide gamut color space. Log C images can transport all the color information and high dynamic range captured by ARRI’s camera sensors. As our cameras evolve, so does Log C - with the introduction of ALEXA 35 we launched its newest update: LogC4.
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: the original ALEXA sensor (for LogC3) and new ALEXA 35 sensor (for LogC4) have a different dynamic range and therefore different demands towards a logarithmic encoding of their images. All knowledge we've gained with ALEV3 lead to the advancements in color science ultimately facilitated ARRI LogC4.
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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 can be performed using a 3D Look-Up Table (3D LUT).
For LogC3 LUTs (all ALEXA and AMIRA cameras – not for ALEXA 35) please visit our online LUT Generator.
For LogC4 (ALEXA 35 only), you can download pre-generated LUTs in a cube format.
Rec 709/2020 is 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 most HD/UHD video production gear in real time, which enables short production times. Rec 709/2020 provides reduced choices in color grading and, due to a 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 and ALEXA 35 cameras.
Shooting with a combination of ALEXA 35 and other ALEXA or AMIRA cameras raises the question of how to handle both Log C versions on one project. We have anticipated this situation and offer a path for footage captured with the previous ALEXA sensor (LogC3) to be used within REVEAL Color Science alongside ALEXA 35 footage (LogC4) 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 35 from Software Update Package SUP 1.1 on. 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.
