More about ProRes RAW

A few weeks ago I wrote a two-part post – HDR and RAW Demystified. In the second part, I covered Apple’s new ProRes RAW codec. I still see a lot of misinformation on the web about what exactly this is, so I felt it was worth an additional post. Think of this post as an addendum to Part 2. My apologies up front, if there is some overlap between this and the previous post.

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Camera raw codecs have been around since before RED Digital Camera brought out their REDCODE RAW codec. At NAB, Apple decided to step into the game. RED brought the innovation of recording the raw signal as a compressed movie file, making on-board recording and simplified post-production possible. Apple has now upped the game with a codec that is optimized for multi-stream playback within Final Cut Pro X, thus taking advantage of how FCPX leverages Apple hardware. At present, ProRes RAW is incompatible with all other applications. The exception is Motion, which will read and play the files, but with incorrect default – albeit correctable – video levels.

ProRes RAW is only an acquisition codec and, for now, can only be recorded externally using an Atomos Inferno or Sumo 19 monitor/recorder, or in-camera with DJI’s Inspire 2 or Zenmuse X7. Like all things Apple, the complexity is hidden under the surface. You don’t get the type of specific raw controls made available for image tweaking, as you do with RED. But, ProRes RAW will cover the needs of most camera raw users, making this the raw codec “for the rest of us”. At least that’s what Apple is banking on.

Capturing in ProRes RAW

The current implementation requires a camera that exports a camera raw signal over SDI, which in turn is connected to the Atomos, where the conversion to ProRes RAW occurs. Although no one is very specific about the exact process, I would presume that Atomos’ firmware is taking in the camera’s form of raw signal and rewrapping or transforming the data into ProRes RAW. This means that the Atomos firmware would require a conversion table for each camera, which would explain why only a few Sony, Panasonic, and Canon models qualify right now. Others, like ARRI Alexa or RED cameras, cannot yet be recorded as ProRes RAW. The ProRes RAW codec supports 12-bit color depth, but it depends on the camera. If the SDI output to the Atomos recorder is only 10-bit, then that’s the bit-depth recorded.

Until more users buy or update these specific Atomos products – or more manufacturers become licensed to record ProRes RAW onboard the camera – any real-word comparisons and conclusions come from a handful of ProRes RAW source files floating around the internet. That, along with the Apple and Atomos documentation, provides a pretty solid picture of the quality and performance of this codec group.

Understanding camera raw

All current raw methods depend on single-sensor cameras that capture a Bayer-pattern image. The sensor uses a monochrome mosaic of photosites, which are filtered to register the data for light in the red, green, or blue wavelengths. Nearly all of these sensors have twice as many green receptors as red or blue. At this point, the sensor is capturing linear light at the maximum dynamic range capable for the exposure range of the camera and that sensor. It’s just an electrical signal being turned into data, but without compression (within the sensor). The signal can be recorded as a camera raw file, with or without compression. Alternatively, it can also be converted directly into a full-color video signal and then recorded – again, with or without compression.

If the RGGB photosite data (camera raw) is converted into RGB pixels, then sensor color information is said to be “baked” into the file. However, if the raw conversion is stored in that form and then later converted to RGB in post, sensor data is preserved intact until much later into the post process. Basically, the choice boils down to whether that conversion is best performed within the camera’s electronics or later via post-production software.

The effect of compression may also be less destructive (fewer visible artifacts) with a raw image, because data, rather than video is being compressed. However, converting the file to RGB, does not mean that a wider dynamic range is being lost. That’s because most camera manufacturers have adopted logarithmic encoding schemes, which allow a wide color space and a high dynamic range (big exposure latitude) to be carried through into post. HDR standards are still in development and have been in testing for several years, completely independent of whether or not the source files are raw.

ProRes RAW compression

ProRes RAW and ProRes RAW HQ are both compressed codecs with roughly the same data footprint as ProRes and ProRes HQ. Both raw and standard versions use a variable bitrate form of compression, but in different ways. Apple explains it this way in their white paper: 

“As is the case with existing ProRes codecs, the data rates of ProRes RAW are proportional to frame rate and resolution. ProRes RAW data rates also vary according to image content, but to a greater degree than ProRes data rates. 

With most video codecs, including the existing ProRes family, a technique known as rate control is used to dynamically adjust compression to meet a target data rate. This means that, in practice, the amount of compression – hence quality – varies from frame to frame depending on the image content. In contrast, ProRes RAW is designed to maintain constant quality and pristine image fidelity for all frames. As a result, images with greater detail or sensor noise are encoded at higher data rates and produce larger file sizes.”

ProRes RAW and HDR do not depend on each other

One of my gripes, when watching some of the ProRes RAW demos on the web and related comments on forums, is that ProRes RAW is being conflated with HDR. This is simply inaccurate. Raw applies to both SDR and HDR workflows. HDR workflows do not depend on raw source material. One of the online demos I saw recently immediately started with an HDR FCPX Library. The demo ProRes RAW clips were imported and looked blown out. This made for a dramatic example of recovering highlight information. But, it was wrong!

If you start with an SDR FCPX Library and import these same files, the default image looks great. The hitch here, is that these ProRes RAW files were shot with a Sony camera and a default LUT is applied in post. That’s part of the file’s metadata. To my knowledge, all current, common camera LUTs are based on conversion to the Rec709 color space, not HDR or wide gamut. If you set the inspector’s LUT tab to “none” in either SDR or HDR, you get a relatively flat, log image that’s easily graded in whatever direction you want.

What about raw-specific settings?

Are there any advantages to camera raw in the first place? Most people will point to the ability to change ISO values and color temperature. But these aren’t actually something inherently “baked” into the raw file. Instead, this is metadata, dialed in by the DP on the camera, which optimizes the images for the sensor. ISO is a sensitivity concept based on the older ASA film standard for exposing film. In modern digital cameras, it is actually an exposure index (EI), which is how some refer to it. (RedShark’s Phil Rhodes goes into depth in this linked article.)

The bottom line is that EI is a cross-reference to that camera sensor’s “sweet spot”. 800 on one camera might be ideal, while 320 is best on another. Changing ISO/EI has the same effect as changing gain in audio. Raising or lowering ISO/EI values means that you can either see better into the darker areas (with a trade-off of added noise) – or you see better highlight detail, but with denser dark areas. By changing the ISO/EI value in post, you are simply changing that reference point.

In the case of ProRes RAW and FCPX, there are no specific raw controls for any of this. So it’s anyone’s guess whether changing the master level wheel or the color temp/tint sliders within the color wheels panel is doing anything different for a ProRes RAW file than doing the same adjustment for any other RGB-encoded video file. My guess is that it’s not.

In the case of RED camera files, you have to install a camera raw plug-in module in order to work with the REDCODE raw codec inside of Final Cut Pro X. There is a lot of control of the image, prior to tweaking with FCPX’s controls. However, the amount of image control for the raw file is significantly more for a REDCODE file in Premiere Pro, than inside of FCPX. Again, my suspicion is that most of these controls take effect after the conversion to RGB, regardless of whether or not the slider lives in a specific camera raw module or in the app’s own color correction controls. For instance, changing color temperature within the camera raw module has no correlation to the color temperature control within the app’s color correction tools. It is my belief that few of these actually adjust file data at the raw level, regardless of whether this is REDCODE or ProRes RAW. The conversion from raw to RGB is proprietary with every manufacturer.

What is missing in the ProRes RAW implementation is any control over the color science used to process the image, along with de-Bayering options. Over the years, RED has reworked/improved its color science, which theoretically means that a file recorded a few years ago can look better today (using newer color science math) than it originally did. You can select among several color science models, when you work with the REDCODE format. 

You can also opt to lower the de-Bayering resolution to 1/2, 1/4, 1/8, etc. for a RED file.  When working in a 1080p timeline, this speeds up playback performance with minimal impact on the visible resolution displayed in the viewer. For full-quality conversion, software de-Bayering also yields different results than hardware acceleration, as with the RED Rocket-X card. While this level of control is nice to have, I suspect that’s the sort of professional complication that Apple seeks to avoid.

The main benefit of ProRes RAW may be a somewhat better-quality image carried into post at a lower file size. To get the comparable RGB image quality you’d need to go up to uncompressed, ProRes 4444, or ProRes 4444 XQ – all of which become very taxing in post. Yet, for many standard productions, I doubt you’ll see that great of a difference. Nevertheless, more quality with a lower footprint will definitely be welcomed.

People will want to know whether this is a game-changer or not. On that count, probably not. At least not until there are a number of in-camera options. If you don’t edit – and finish – with FCPX, then it’s a non-starter. If you shoot with a camera that records in a high-quality log format, like an ARRI Alexa, then you won’t see much difference in quality or workflow. If you shoot with any RED camera, you have less control over your image. On the other hand, it’s a definite improvement over all raw workflows that capture in image sequences. And it breathes some life into an older camera, like the Sony FS700. So, on balance, ProRes RAW is an advancement, but just not one that will affect as large a part of the industry as the rest of the ProRes family has.

(Note – click any image for an enlarged view. Images courtesy of Apple, FilmPlusGear, and OffHollywood.)

©2018 Oliver Peters

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