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ARRI ALEXA and Apple ProRes: Navigating Log C, LUTs and Looks Generator in Post

One of hottest cameras in high-end television production this past year has been the ARRI ALEXA. It features a 3392 x 2200 pixel Super 35mm-sized CMOS sensor using Bayer pattern color filtering. The recording mechanism can generate images in either the 2880 x 1620 ARRIRAW format or in any flavor of 1920 x 1080 Apple ProRes. Recorded images at all speeds are downsampled from the full sensor, so you are always using the full dimensions, regardless of the output resolution. Thanks to large photosites (the light-receiving pixels of the sensor), ALEXA boasts a 14-stop dynamic range.

An example of using Magic Bullet Colorista II to grade Log C
shots inside After Effects

ARRI has taken great care to generate a digital image that most cinematographers consider very close to the look of 35mm film. The design criteria in these chosen sizes was to emulate in a video camera a process that is similar to film scanning. An ARRILASER scans at 3K (equivalent to ALEXA’s sensor size) and most digital intermediate post is still done at 2K (close to HD at 1920 x 1080). These are basically the same steps performed by the in-camera circuitry.

I’m going to focus this article on the ProRes workflow. ALEXA is capable of larger-than-HD camera raw recordings using an external recorder connected via ARRI’s proprietary T-Link protocol; however, that option is attractive mainly for feature films and visual effects. Most television, corporate, commercial or web projects will likely stick with the ProRes workflow because of its ease in post. To date, I’ve cut more than 50 commercials and short-form web videos that originated with an ALEXA recording to some form of Apple ProRes.

Tools as simple as Magic Bullet Colorista Free can be used
to create a nice dailies grade within Final Cut.

Camera Recording Options
When set to record HD video, ARRI ALEXA is capable of recording one video signal to the onboard SxS cards while sending a second signal out through SDI ports. Both of these are 1920 x 1080 progressive signals at base speeds of 23.98, 25 or 29.97 fps. ALEXA can also record higher rates for in-camera slow motion at up to 120 fps on updated cameras.

Another unique feature is that the camera can work in two color profiles: Rec. 709 and Log C. The latter is a flat image profile that applies a gamma curve to squeeze a wide dynamic range into recordable video space. Both or either of these profiles can be assigned to the SxS cards and the ports. It’s possible to record to the cards in Log C plus externally in Rec. 709—or the other way around—or use a single profile on all outputs.

In this example, the log to video correction is made using the
standard FCP color corrector.

With the recent updates, ARRI has introduced a software Look Generator, which allows cinematographers and digital imaging techs to create custom profiles. These can be loaded into the camera from an SD card and applied to the Rec. 709 profile. One of these is a Low Contrast Curve (LCC) Look, which is similar to Log C. It’s not quite as flat and stays within the Rec. 709 color space. The LCC Look is designed for folks who want a flat profile to preserve dynamic range, but who want to deal only with simple color-correction filters to restore the image in post. In that case, the Rec. 709 (with the LCC Look applied) profile is recorded instead of Log C.

Most of the spots I have worked on were recorded using the Apple ProRes 4444 or ProRes HQ codecs and the Log C profile. The Log C video profile was designed to approximate the uncorrected image of a 35mm film negative when it is digitally scanned. This combination of codec and gamma curve gives you the highest quality video image with the most flexibility for grading in post. The camera’s electronics have already taken care of demosaicing the Bayer-pattern CMOS image (turning linear sensor data into RGB video), so these are QuickTime movies ready to drop into the NLE of your choice. An FCP 7-compatible XML file is also recorded with each card. The file contains additional camera metadata; however, you can work directly with the QuickTime movies straight from the card. Unlike P2, there’s no need to maintain the folder structure of the card, since each file includes embedded audio, video, reel ID (the card number) and timecode. The camera records high-quality audio but will also output timecode for external sound recorders in double-system sound situations.

The Antler Post FCP filter may be used to correct the flatter Log C
image into an enhanced picture that is more pleasing for initial
editing.

Dealing with Log C in Post
The biggest concern most editors have is dealing with the Log C color profile. You commonly hear this on forums: “Help! Why is the video so flat?” There are several ways to deal with Log C in post, and these vary with each NLE or grading application. In all cases, they involve some method of reversing the gamma curve in order to get back to the gorgeous image seen on the monitors during production. Generally the image viewed during the edit won’t be the final one, since it’s going to be altered in a color-correction session anyway. Applying a corrected look during editing is mainly for the purpose of seeing a pleasing image while making editorial decisions. To see a corrected image, you need to apply a proper LUT (lookup table) or apply a color correction that approximates the same results.

The projects I’ve cut so far have all been in Final Cut Pro 7. The two easiest ways of working with Log C ALEXA footage in FCP are to use either the built-in color correctors or apply one of the available third-party Log-C-to-video filters. The earliest of these is the Antler Post LUT filter developed by image consultant Nick Shaw. It’s designed for dailies (8-bit only) and adds optional burn-ins for text and timecode. This is an unofficial LUT that’s been designed by Shaw to approximate the application of ARRI’s internal Log-C-to-709 conversion. You can get similar results in the standard FCP color corrector or three-way filter by expanding the contrast (lower black level, raise highlights) and increasing saturation. Both of these will run in real time as unrendered effects on a fast Mac Pro. Because FCP permits filters to be added to master clips, you can quickly go through a preparation step of applying filters to all clips before you start to edit. Then the files will appear corrected every time you load a clip.

The uncorrected Log C color profile from an ARRI ALEXA file
yields an initial image that appears quite flat.

Two other solutions have recently come to the market. One of these is the Glue Tools ARRI Camera Toolkit, which is based on ARRI’s own color science. They have broken out the Log C filter as a separate plug-in for Final Cut and Motion as a lower-cost solution for editors who don’t need to work with ARRIRAW. Using their batch importer, Log C ProRes files are imported with the filter applied to the footage. Another new option is the Look2Video FCP 7 and FCP X plug-in from Pomfort. In addition to standard correction, custom looks can be designed in Pomfort’s Silverstack SET media management tool. These looks can be applied to the imported ALEXA files.

If you are editing in Premiere Pro or Media Composer, the best solution is to apply a correction in the timeline. Use the standard correction tools and the clips will run in real time. Typically a slight S-curve luma correction is a closer approximation to what the LUT is doing than simply increasing contrast. This is an easy task in the Media Composer color-correction mode.

Pomfort’s ALEXA Look2Video filter offers Log C and other
correction inside FCP X.

Other solutions include using Red Giant Magic Bullet LUT Buddy to apply LUTs from other sources or use other converters, like the After Effects Cineon conversion utility. If you plan on generating your own adjustments using standard color-correction tools, it’s a good idea to have the crew shoot some charts for you, like a Macbeth chart, color bar chart or even just a slate with color bars stripes on it. This will help take some of the guesswork out of your correction.

Offline to Online
Since ALEXA permits dual-recording to the cards and an external source, you can also employ a proxy workflow for offline-online editing. For example, record Log C profile ProRes 4444 movies to the SxS cards and a simultaneous Rec. 709 image (with the DP’s custom “look” applied) to an external recorder, like a Sound Devices PIX 240 or an AJA Ki Pro Mini. Record the external signal in ProRes Proxy or ProRes LT for lightweight offline editing. When the cut is locked, relink to the high-quality Log C ProRes 4444 movies for online finishing and color grading.

ARRI has also provided a web-based LUT generator that can create application-specific 1D and 3D LUTs. For example, I’ve used it to generate one for Apple Color in the .mga format. When you are ready to send the edited sequence to another application for color grading, it’s best to remove any LUTs, color-correction settings or filters that you’ve applied. Colorists working on any of the standard systems will have no trouble in restoring a gorgeous image with plenty of grading latitude. For instance, it’s not uncommon to see a slight green cast to the Log C image, which is easily corrected by adjusting channels or printer light values, depending on what controls the grading system offers.

In the case of Apple Color, you can load the correct ARRI LUT (import display LUT from the file menu). From that point on, all color corrections are made with this LUT applied. Most of the time, 80 percent of the work is already done for you once the LUT is there. When you render the final corrections, Color prompts you whether you want the LUT “baked in” with the renders. Typically you’d want to do this, unless you have other LUTs you might wish to use with these render files for other destinations, like digital cinema projection.

ARRI has introduced a wonderful way of working with this Apple ProRes workflow. The easiest high-end post solution with the benefits of a wide dynamic range in a highly malleable image. It’s no wonder that ALEXA has become the camera to beat for this year’s TV production season.

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