Visual Effects FAQ

Our cameras offer three different encodings of the recorded image:

• Linear encoding 
• Logarithmic encoding 
• Display-ready video encoding for Rec 709 monitors.

While ARRIRAW is encoded as linear data, it can be converted to the other two encodings using the ARRI Reference Tool, ARRIRAW Converter (legacy) or 3rd party applications which support ARRIRAW. For ProRes clips, the encoding can be logarithmic or video (not available on ALEXA Mini LF or later).

ARRI also offers transformation-LUTs to convert from the different encodings back and forth (Cameras with ALEV3 sensor use the LUT generator; for ALEXA 35 we have a LUT package). This can not be done without loss of data. For instance when recording a clip in Rec 709 encoding to ProRes files, these files can be converted Log C, but will have lost detail in blacks and whites.

Linear Encoded Data
Linear data is mostly used in VFX processing because it is the natural encoding for computer generated elements. The well-known ALEV3 sensor for example has a dynamic range of 14+ stops which corresponds to a linear range of more than 15,000:1. In most cases floating point numbers are used to store this range (the OpenEXR image format, for example, is based on 16 bit floating point numbers).

Linear from ARRIRAW files
The most direct way to linear files is to record ARRIRAW and to process the data with the ARRI Reference Tool (ART)/ARRIRAW Converter (ARC – legacy). Third party software vendors also support ARRIRAW in their systems.

Linear from Log C files
Another way to obtain linear data is to undo the Log C curve of images recorded in QuickTime ProRes 4444, ProRes 4444XQ files.

ARRI's online LUT Generator can create LUTs to convert LogC3 images to linear sensor data. When the LogC3 data is converted to linear sensor data, black (corresponding to zero exposure) will be represented by the value 256/65535. This sensor black level is the mean of all pixels. Because of read-out noise, single pixels may be above or below this value. The standard deviation of the read out noise is approximately 2.5 meaning that the offset of 256 is more than high enough to encode the full noise amplitude (usually one assumes a range of three times the standard deviation or ± 8 code values). Using the parameters for linear scene exposure will map the black value, as expected, to 0.0. With the noise, however, single pixels will come out as negative values. When those values cannot be preserved and one does not want to clip them, a small offset of 8/65535 should be added to the relative scene exposure factor. This is equivalent of adding flare to the image data. The amount of flare expressed relative to the scene white will vary with the exposure index. It ranges from 0.1% (for EI 200) to 0.8% (for EI 3200). The flare should be subtracted before the images are converted back to LogC3.

Logarithmic Encoded Data 
The Log C curve was first introduced with the ARRIFLEX D-20 camera. It's an encoding with a transfer characteristic similar to that of a scan from negative film. Because of the fundamental differences between digital cameras and negatives, however, the color characteristics remain different.

Display-Ready Video Encoding for Rec 2020/709 Monitors 
The Rec 2020 or Rec 709 output of the camera is either used for on-set preview or for when the program is edited for television without extensive color correction. Those images are displayed without any further transformation. While this simplifies the post production workflow it reduces the possibilities in color correction. The images have been tone-mapped and transformed into the target color space.

The tone-map curve is applied to the Log C data. This transform is also available as a LUT for post-processing of Log C footage. While this provides a nice image on a video monitor, it also means that some information has been "squeezed out" of the transformed image. The matrix transform applied immediately following the tone-map curve results in an image transcoded into the target color space, but if the original image contained extremely saturated colors, this may also bring a loss of color. If a transformed color is outside the gamut of the display, it will be mapped or clipped to an in-gamut color.

For a green screen shot neither white point setting has an effect on the green channel gain. Daylight and tungsten light deliver the same quality.

For a blue screen shot the blue gain used with a 3200K tungsten light is substantially higher than the blue gain used with a 5600K daylight, therefore a blue screen captured under tungsten will be more noisy than a green screen. Hence daylight is preferred. 

Look files that are active in the camera while recording, get captured as metadata in the file header of ARRIRAW files, ProRes files (and DNxHD files).

ALEXA Classic and ALEXA XT use the ARRI Look File 1 (ALF-1),  the following parameters are part of the ARRI Look File:

• Color saturation value (called Saturation)
• RGB offsets (called PrinterLight)
• A mono free form curve applied to all RGB channels (called ToneMapLut)
• The three primitives of the ASC Color Decision List (CDL), separate for each R, G, and B channel (called SOP Node – Slope Offset Power)

ALEXA SXT, LF, Mini LF,  and 65 as well as ALEXA Mini and AMIRA use ARRI Look File 2 (ALF-2) which supports the following parameters:

• Genuine ASC Color Decision List
• 3D-LUT
  ... Or VLP (Video Look Parameters), containing tone mapping (Gamma, Black Gamma and Knee) and saturation by hue (over all saturation, red, yellow, green cyan, blue and magenta)

ALEXA 35 uses the ARRI Look File 4 which supports parallel SDR and HDR output from the camera as well as:

• Genuine ASC Color Decision List
• Creative Modification Transform (Log-to-Log 3D-LUT)
• Use of a custom DRT

Find out more about the ARRI Look File and have a look at our Color FAQ!

The brightness of the screen should correspond to the foreground, which needs to be separated. A good objective is to have the foreground properly exposed for skin tones 1/3 of a stop above 18% grey, the screen should read about 1/3 to 2/3 of a stop over 18% grey, too. Please ensure that the background is evenly lit and not underexposed.

The ALEXA or AMIRA cameras store per-frame and per-shot metadata. This metadata is present in all recorded formats - onboard and in-camera formats ARRIRAW, MXF/ARRIRAW, ProRes, MXF/ProRes, and DNxHD. Per shot metadata is also written to an FinalCut Pro XML file, the ALE, and an AVID AAF when recording to ProRes or DNxHD respectively (depending on the camera).

To extract that metadata use our ARRI Reference Tool, ARRIRAW Converter (legacy), or ARRI META Extract (legacy) for MacOS and Windows.

Please also refer to our Metadata White Paper.

The short answer is: 200-400 ASA for ALEXA/AMIRA cameras using the ALEV3 sensor and 400-800 ASA for ALEXA35 (ALEV4 sensor).

Here's the long answer: The "base" rating for the ALEV3/ALEV4 is 800 ASA. When setting the camera to 800 ASA you get 7.4/9.3 stops of latitude above middle grey and 6.6/7.7 stops below. This gives you a very good range similar or even better to that of film.

When shooting green or blue screens in a controlled studio environment, the 7.4/9.3 stops above middle grey are often not needed. In a situation like this it could be preferable to set the camera to a lower ASA rating. As long as the highlights don't get clipped, a lower ASA rating will give you less noise in the image. When looking at a ALEXA or AMIRA image, often the noise/grain is not visible to the eye, but it is visible to a compositing keyer. Having less noise in the image means that the footage will be easier to key.

A part of the metadata we record is delivered by LDS, the Lens Data System. Dynamic information, such as Lens Focus, Lens Focal Length, Lens Serial Number and Lens Iris. This additional information makes postporduction and documentation easier as the metadata is stored within the image files so it cannot get lost, which is a great difference to film times, where often the information collected on location got lost before reaching post production.

LDS information can be used for easy camera set up, as the values of lens, iris, focal length and focus point, therefore also depth of field, are available. For recording the LDS data the cameras has to be equipped with an LDS lens mount and of course with LDS 1 or 2 capable lenses. Those are all ARRI Signature Primes, ARRI/Zeiss Master Primes, ARRI/Zeiss LDS Ultra Primes, and ARRI/Fujinon Alura Lightweight Zooms. Other lenses might supply data via /i which is undestood by ARRI cameras as well. Non-LDS lenses can be used with ARRI lens motors and the Lens Data Archive (LDA) activated. The LDA will supply necessary lens information, while the motor supplys information about its position. This is being combined to supply values for focus, iris, zu zoom.

In some VFX software packages the LDS information is displayed. In any case, you can use the ARRI Reference Tool to export metadata to a *.json file or ARRI META Extract (legacy) to extract the data into csv files.

Please see our LDS FAQ for more detailed information on LDS 1 or 2.
 

Yes, there is. Because we're using a classic bayer-pattern schema on the sensor, the ALEV3 and ALEV4 CMOS sensor has twice the number of green photosites than blue photosites; therefore the resolution of a green screen is higher compared to a blue screen. Also the green channel is less noisy than the blue channel when using tungsten lighting.

There's a rule of thumb: "The cleaner the image the better the key".

While many VFX-heavy productions have successfully used ProRes 4444 and ProRes 4444 XQ for their VFX work, only ARRIRAW (or bit-identical conversion to HDE) provides full resolution uncompressed sensor data and therefore is the best choice for VFX work.

Beginning with ALEXA Classic cameras SUP 3.0 until SUP 7.0 ARRI offered a film style matrix that could be applied to the Log C output. The same transform was also available as a 3D-LUT for post-processing of Log C footage.

The film style matrix makes the color characteristics of the Log C image similar to negative film scanned on an ARRISCAN. The matrix is most usefully applied when the data is previewed or converted with a print film emulation (PFE) . This is the common workflow in Digital Intermediate where the PFE is applied as a 3D-LUT in the display path.
The film-matrix can in some situations achieve a bigger color separation and therefore make the keying of some shots easier.

Due to new developments in color grading and digital workflow our film style matrix has been discontinued.

The Log C curve is a logarithmic encoding for images that is used in all of ARRI's digital cameras. The encoding has a transfer characteristic similar to that of a scan from negative film. Because of the fundamental differences between digital cameras and negatives, however, the color characteristics remain different.

Logarithmic encoding means that the relation between exposure measured in stops and the encoded signal is constant (straight) over a wide range. Each stop of exposure increases the encoded signal by the same amount. The slope of this part of the curve is called its gamma. At the bottom the curve has a toe. The toe accommodates the fact that the sensor cannot see as many distinctions between low light levels as it can at higher light levels. The resulting overall shape of the curve is similar to the exposure curves of film negatives.

There are two version of Log C currently in use, its type depends on the camera's sensor in use:

• ARRI ALEXA Classic, ALEXA XT, ALEXA SXT, ALEXA Mini, and AMIRA, ALEXA LF, and Mini LF and ALEXA 65 output LogC3. (If you're recording ARRIRAW this can be debayered to LogC4)
• ARRI ALEXA 35 outputs LogC4 natively.

For more detailed information about Log C, please have a look at our Log C website, the Color FAQ, and our whitepaper "ALEXA Log C Curve – Usage in VFX".

Linear Color Gamut
Modern keyers best work when supplied with linear encoded images. Working in a linear color space will most often give better results than working with Log C encoded plates.

Noise/Grain
It is useful to degrain/denoise ALEXA/AMIRA green and blue screen plates. Even when the eye cannot see any noise present in the image, there will still be noise present that will have an effect on the keying of these plates.

ES mode (ALEXA 35)
If you find yourself in a green/bluescreen shooting environment that asks for higher ISO settings and the camera at hand is an ALEXA 35, it is adviseable to shoot a test and see if Enhanced Sensitivity mode is applicable. Combine it with ARRI Textures and use L345 Shadow to reduce noise to a minimum.

Sharpening
In case you are working with ARRIRAW files, you have the chance to set the sharpening value according to your wishes. This means that you can reduce the sharpening after the debayering and might find that you can pull a better key. You can use a second pass with increased sharpening for the fill or do a resharpen in your compositing system. Over-sharpened images create a bad key, less sharpened images a better key.

If ARRIRAW is not an option, ARRI recommends to use the highest quality codecs for VFX work: ProRes 4444 and ProRes 4444 XQ.

The ALEXA 35 stores per-frame and per-shot metadata. This metadata is present in all recorded formats (MXF/ARRIRAW, and MXF/ProRes). Per shot metadata is also written to the ALE. To extract that metadata to a *.json file, use our ARRI Reference Tool for MacOS and Windows.