ALEXA VFX Frequently Asked Questions
The ALEXA camera is widely used in feature film and TV work today. This includes a lot of VFX-heavy films and TV series. This guideline is intended to help VFX professionals who are new to the ALEXA camera to answer common questions regarding its workflow, color processing and image quality.
We cover the three most commonly used compositing software packages on the market. If the software you are using is not dealt with in this document or if you have any further questions regarding VFX and the ALEXA camera, please feel free to contact us at email@example.com.
1. Camera settings
- What is the right format for shooting green/blue screen shots?
The ALEXA camera offers a multitude on formats for onboard and in-camera recording. ProRes and DNxHD use lossy compressions. While ProRes offers resolutions up to 2K, DNxHD offers HD resolution only. All these are compromises which will have an effect on the keyability and quality of a green and blue screen shot.
While many VFX-heavy productions have successfully used ProRes 4444 and ProRes 4444 XQ for their VFX work, only ARRIRAW provides the full resolution uncompressed sensor data and therefore the best choice for VFX work.
- What are the recommended codecs when shooting green/blue screen shots?
A lot of productions cannot afford ARRIRAW recording for VFX work and therefore want to use ProRes or DNxHD recording. If ARRIRAW is not an option, ARRI recommends to only use the highest quality codecs (ProRes 4444 and ProRes 4444 XQ as well as DNxHD 444) for shooting green and blue screen.
- What is the recommended sensor mode when shooting VFX shots?
ALEXA´ s super clean color separation is particularly important for green screen and other VFX work. Nevertheless sometimes it is important to work with more pixels than used later in the final product (for gaining more resolution for repositioning, resizing, rotating or stabilizing etc).
In addition to the 16:9 and the 4:3 sensor mode, ALEXA XT cameras are capable of Open Gate sensor mode (only ALEXA XT cameras are capable of Open Gate mode). In Open Gate sensor mode the whole area of the sensor (3141 x 2198) is captured; it is only available in ARRIRAW.
Please note that also the 4:3 sensor mode allows to position tracker marks etc in the areas which are not exposed for 1.85:1 or Cinemascope or to stabilize or repostion vertically without any loss of quality.
- Is there a difference between green and blue screens because the ALEXA camera uses a CMOS sensor?
The ALEXA 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 light. Please also see the next topic.
- Is tungsten or daylight better for lighting green/blue screen shots?
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, and therefore a blue screen captured under tungsten will be more noisy than a green screen. Daylight light is preferred.
- What is a good aim for brightness of my green/blue screen?
The brightness of the screen should correspond to the foreground, which needs to be separated. A good aim is, when the foreground is properly exposed for skin tones 1/3 of a stop above 18% grey, the screen should read about 1/3 to 2/3 of stop over 18% gray. Please take care that the background is evenly lit and not underexposed.
- What is the best ASA rating for green/blue screen shots?
The short answer is: 200-400 ASA.
Here is the long answer: The "base" rating for the ALEXA in general is 800 ASA. When setting the camera to 800 ASA you get 7.4 stops of latitude above middle grey and 6.6 stops below. This gives you a very good range similar to that of film.
When shooting green or blue screens in a controlled studio environment, often the 7.4 stops above middle grey are not needed. In a situation like this it could be preferable to set the ALEXA 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 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.
3. Compositing Software: Adobe After Effects
- How do I import my ARRIRAW files into After Effects ?
ARRIRAW files can be imported into After Effects as of version CS 6. From Adobes website the following are known issues:
- The importer works in 16 bit, so set your project to 16 or 32 bpc.
- Footage is always decoded at full resolution, even if a lower frame size is needed.
- There is no exposure or color space control in the importer, and no importer options at all.
- Metadata is not exposed as XMP, so is not available in After Effects.
- Collect Files does not work with ARRIRAW footage.
- How do I import my ProRes files into After Effects?
ProRes files can be imported into Adobe After Effects without any limitation. Just import your ProRes files like you would import any other footage.
- How do view my Log C files in After Effects?
To view Log C footage correctly inside Adobe After Effects, generate a Log C to Video photometric LUT using ARRI's online LUT Generator and use an "Apply Color LUT" effect inside AfterEffects. This only works, when your project is not set to "Linearize Working Space".
4. Compositing Software: Blackmagic Design Fusion
- How do I import my ARRIRAW files into Fusion?
Fusion has native ARRIRAW support. A standard loader can be used to import ARRIRAW files. For standard Log C deliverables set the color space to "ARRI Wide Gamut (Log C)".
- How do I import my ProRes files into Fusion?
Fusion has native ProRes support. Use a standard loader to load ALEXA ProRes files into Fusion.
- How do I view my Log C files in Fusion?
To view Log C footage correctly inside Fusion, generate a Log C to Video Rec 709 photometric, extended to extended range LUT using ARRI's online LUT Generator and use a this LUT to view your images.
5. Compositing Software: The Foundry Nuke
- How do I import my ARRIRAW files into Nuke?
The Foundry Nuke 7.0 or later has native ARRIRAW support. For older versions your ARRIRAW files need to be converted beforehand to either DPX or OpenEXR files and import them using the ALEXAV3 importer color space.
Depending on the desired workflow and deliverables ARRIRAW files can be imported using these two methods using a standard read node inside Nuke.
- for Log C deliverables:
- Set the ARRIRAW color space to "Log C ( wide gamut )".
- et Nukes color space to "ALEXAV3LogC".
- for ACES deliverables:
- Set the ARRIRAW color space to "ACES".
- Set Nukes color space to "Linear".
- Use OCIO for setting your workflow to ACES.
- for Log C deliverables:
- How do I import my ProRes files into Nuke?
Nuke has built-in native ProRes support, but as of Nuke 7.0v4, decoding ProRes this way exhibits a saturation shift when compared to the results of ARRI's reference color processing. ARRI recommends first converting your ALEXA ProRes files to Log C DPX files outside of Nuke, and then reading those Log C DPX files into Nuke (using the AlexaV3LogC color space setting at the bottom of the Read node's properties panel).
- How do I view my Log C files in Nuke?
For viewing ALEXA files imported using ALEXAV3LogC import color space it is best to build your own viewer process using a "vector field node". Download a "K1S1" photometric 3D LUT from ARRI's online LUT Generator and activate the LUT in the "vector field node" using the "ALEXAV3LogC" for color space in and "Linear" for color space out.
When using this viewer process you will get the same preview picture then what the ALEXA renders as an Rec 709 image.
- I have problems keying ALEXA footage. What can I do to improve the key quality?
Linear Color Gamut
Modern keyers best work when supplied with linearly-encoded images. Working in a linear color space will most often give better results than working with Log C encoded plates.
Most often it is useful to degrain/denoise ALEXA 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.
The film matrix can in some situations achieve a bigger color separation and therefore make the keying of some shots easier. In this case the film matrix should only be rendered as a pre-grade step before the keyer and should not be rendered into the final VFX shot.
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. Oversharpened images create a bad key, less sharpened images a better key.
- How does the ALEXA color processing work?
The ALEXA camera offers 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. ARRIRAW can be easily converted to the two other encodings using the 'ARRIRAWConverter" debayer software, or by using other applications which support ARRIRAW.
For ProRes and DNxHD clips the encoding can be logarithmic or video.
ARRI also offers transformation LUTs to convert from the different encodings back and forth. This can not be done without loss of data. For instance when recording a video Rec709 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 ele- ments. The ALEXA camera 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 ARRIRAWConverter (ARC). Several 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 on tape or in QuickTime ProRes 4444 and DNxHD 444 files.
ARRI's online lut generator can create LUTs to convert Log C images to linear sensor data. When the Log C 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 Log C.
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. See also the next topic: "16. What is the Log C format?"
Display-Ready Video Encoding Rec709 Monitors
The 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 brings 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.
- What is the film-matrix and how do I use it?
From SUP 3.0 onwards ARRI offers a film style matrix that can be applied to the Log C output. The same transform is also available as a 3DLUT 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 3DLUT 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.
- What is the Log C format?
The Log C curve is a logarithmic encoding for images that is used in the ARRI ALEXA. 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.
For more detailed information about the Log C format used inside the ALEXA camera you can download "ALEXA Log C Curve – Usage in VFX" from our downloads page: www.arri.com/alexa/downloads
- The production used ARRI Look Files on set. Where are these looks stored and how can I use them in a VFX pipeline?
Look files that are active in the camera while recording, get recorded as metadata in the file header of ARRIRAW files, ProRes files and DNxHD files.
An ARRI Look File contains a set of parameters specific to the ALEXA camera. The look files can be converted into 3D LUT files using the ARRI Online LUT Generator at www.arri.com/camera/digital_cameras/tools/lut_generator.html A conversion of 3D LUT files into ARRI Look Files, however, is not possible.
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)
ARRI's metadata extract tool can extract the look file inside the file headers. Please also see section 6. Where can I get metadata of the ALEXA from?
- Where can I get metadata of the ALEXA from?
The ALEXA camera stores per-frame and per-shot metadata. This metadata is present in all recorded formats - onboard and in-camera formats ARRIRAW, ProRes, DNxHD. Per shot metadata is also written to an FinalCut Pro XML file and an AVID AAF when recording to SxS cards.
Please refer to our metadata workflow paper on our download website for more detailed information (ALEXA Metadata White Paper).
On the download page you can also find ARRI META Extract, an extraction tool for metadata for OS A and Windows.
- What is LDS metadata?
A part of these metadata is LDS (Lens Data System) information, such as Lens Focus, Lens Focal Length, Lens Serial Number, Lens Iris. This additional information makes documentation easier as the metadata is stored within the image files so it cannot get lost, which is a differences to film times, where often the information collected on location got lost before reaching post production.
LDS information can then be used for easy camera set up, as the values of lens, iris, focal length and focus point, therefore also depth of field, is available. For recording the LDS data the cameras has to be equipped with an LDS lens mount and of course with LDS capable lenses. Those are all ARRI/Zeiss Master primes, all ARRI/Zeiss LDS Ultra Primes, all ARRI/Fujinon Alura Lightweight Zooms. Other lenses can be used when the Lens Data Archive LDA is activated.
In some vfx software packages the LDS information is displayed. In any case, you can use ARRI META Extract to extract the data into a csv files (Please see 8.Metadata).
10. More Information
- More information
Log C and Rec 709