Working with HD Video
To enable the colorist to get the most out of ALEXA material, the images need to be delivered using Log C encoding. Material recorded in ARRIRAW and ProRes 4444 delivers superior 12 bit color precision, whereas shooting Log C as 10 bit 4:2:2 material has become increasingly popular due to its manageability.
ALEXA can output images with different color processing methods (commonly called 'gamma') applied.
- Log C is a so-called scene based encoding. The signal level increases by a fixed amount with each increase of exposure measured in stops. This encoding, which uses an ALEXA-specific wide gamut color space, is similar to files from a film scan and ideal to carry image information. When viewed directly, Log C images look flat with desaturated colors. To correctly display Log C material on an HD monitor (Rec 709) 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 (LUT).
- Rec 709 is a display-based encoding using the RGB primaries specified in ITU Recommendation BT.709, and can be directly output to standard HDTV displays without any conversion. The material can be processed by most HD video production gear in realtime, which enables extremely short production times. Rec 709, however, provides somewhat 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. To enable productions to shoot in Rec 709 color space without the sacrifice of too much highlight information, ARRI provides a special Low Contrast Characteristic (LCC) ARRI Look File that can be applied to change the standard Rec 709 output.
- DCI P3 also is a display based encoding. It is primarily used in digital cinema projectors and an increasing number of LCD displays. DCI P3 has a similar tone mapping to ALEXA Rec 709, but a wider color gamut, which is designed to approximate that of print film.