Log C and Rec 709 Video
Greyscale Characteristics
ALEXA can output images using Rec 709 or Log C encoding.
Rec 709
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.
Log C
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.
Color
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.
