Log C and Rec 709 Video

Greyscale Characteristics

ALEXA can output images using Log C, Rec 709 or P3 encoding.

Rec 709 and P3

Material recorded in Rec 709 or P3 has a display specific encoding or in other words "what you see is what you get" characteristics. These characteristics map the actual contrast range of the scene to the total contrast range the display device is able to reproduce. The purpose of these encodings is to directly provide a visually correct representation of the captured material, when it is screened on a HDTV monitors (Rec 709) or digital cinema projectors (DCI P3). Because of this requirement, these display specific encodings provide somewhat reduced choices in color grading.

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 to a code value of 400/1023 (i.e. code value 400 in a total code value range of 0 - 1023 i.e. a 10bit signal). The maximum value of the Log C curve depends on the 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. Two sets of primaries are important in our case. One is defined in ITU-R Recommendation BT.709, more commonly known as Rec 709, which is the international standard for HDTV displays. The other set is defined in the Digital Cinema System Specification and in SMPTE 431-2. We refer to it as DCI P3.

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 can’t 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, which is available at http://www.arri.de/camera/arridigital/tools/lut_generator.html 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.