Log C and Rec 709 Video
ALEXA can output images using Rec 709 or Log C encoding.
Material recorded in Rec 709 has a display specific encoding or, in other words, "what you see is what you get" characteristics. 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.
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, the sensor will capture one stop more highlight information. Since the Log C output represents scene exposure values, the maximum value increases.
There are three values for each pixel, which relate to the amounts of three primary colors that are mixed to create a certain color. The primary colors define the outer boundaries of the so-called gamut of the display. A display cannot produce colors that are outside of this gamut. The primaries for an HDTV screen are defined in the international standard ITU-R Recommendation BT.709, more commonly known as Rec 709.
In an image that is delivered from the sensor of a digital camera, the three values relate to the amount of light seen through three color filters. There are no colors a camera cant 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 is called wide gamut RGB color space.
Log C to Video Conversion
When a Log C image is displayed on a standard HDTV monitor, it will look flat with desaturated colors as shown here. This is because:
- the logarithmic scene encoding is different from the display specific image encoding and
- the colors cannot be reproduced by the gamut of the display.
In order to show a grayscale characteristic and color reproduction that is visually correct, the Log C material needs to be tone-mapped for the right encoding and transformed into the target color space. Depending on the possibilities of your equipment, you want to apply a LUT or a 3D LUT to the images. A one-dimensional LUT can perform the tone-mapping so the resulting image will at least have a grayscale characteristic suitable for display. The transform into the target color space, however, needs to be done with a 3D LUT. This type of LUT contains both, the grayscale and the color transformation.
LUTs can be applied in most tools for dailies creation or mastering or directly in a monitor such as the Cine-tal Cinemage.
ARRI LUT Generator
ARRI provides 3D LUTs for most tools through the ARRI LUT Generator. The web application can generate one-dimensional and three-dimensional lookup tables in a variety of formats.
The example here shows the settings needed to receive a Log C to video tone-map conversion and transform to Rec 709 color space for The Foundry Nuke. To find out more about all settings or Log C and Rec 709 encoding, please have a look at the documentation in the download section.
ARRI TECH TALK >>
Working with ALEXA Log C ProRes and DNxHD material in AVID Media Composer 8.1. How to AMA link and transcode clips and apply external 3D LUT’s combined with CDL grading.