The Ouch of 4K Post

df_4kpost_sm4K is the big buzz. Many in the post community are wondering when the tipping point will be reached when their clients will demand 4K masters. 4K acquisition has been with us for awhile and has generally proven to be useful for its creative options, like reframing during post. This has been possible long before the introduction of the RED One camera, if you were shooting on film. But acquiring in 4K and higher is quite a lot different than working a complete 4K post production pipeline.

There are a lot of half-truths surrounding 4K, so let me tackle a couple. When we talk about 4K, the moniker applies only to frame dimensions in pixels, not resolution, as in sharpness. There are several 4K dimensions, depending on whether you mean cinema specs or television specs. The cinema projection spec is 4096 x 2160 (1.9:1 aspect ratio) and within that, various aspects and frame sizes can be placed. The television or consumer spec is 3840 x 2160 (16:9 or 1.78:1 aspect ratio), which is an even multiple of HD at 1920 x 1080. That’s what most consumer 4K TV sets use. It is referred to by various labels, such as Ultra HD, UHD, UHDTV, Quad HD, 4K HD and so on. If you are delivering a digital cinema master it will be 4096 pixels wide, but if you deliver a television 4K master, it will be 3840 pixels wide. Regardless of which format your deliverable will be, you will most likely want to acquire at 4096 x 2304 (16:9) or larger, because this gives you some reframing space for either format.

This brings us to resolution. Although the area of the 4K frame is 4x that of a 1080p HD frame, the actual resolution is only theoretically 2x better. That’s because resolution is measured based on the vertical dimension and is a factor of the ability to resolve small detail in the image (typically based on thin lines of a resolution chart). True resolution is affected by many factors, including lens quality, depth of field, accuracy of the focus, contrast, etc. When you blow up a 35mm film frame and analyze high-detail areas within the frame, you often find them blurrier than you’d expect.

The brings us to post. The push for 4K post comes from a number of sources, but many voices in the independent owner-operator camp have been the strongest. These include many RED camera owners, who successfully cut their own material straight from the native media of the camera. NLEs, like Adobe Premiere Pro CC and Apple Final Cut Pro X, make this a fairly painless experience for small, independent projects, like short films and commercials. Unfortunately it’s an experience that doesn’t extrapolate well to the broader post community, which works on a variety projects and must interchange media with numerous other vendors.

The reason 4K post seems easy and viable to many is that the current crop of 4K camera work with highly compressed codecs and many newer computers have been optimized to deal with these codecs. Therefore, if you shoot with a RED (Redcode), Canon 1DC (Motion-JPEG), AJA Cion (ProRes), BMD URSA (ProRes) and Sony F55 (XAVC), you are going to get a tolerable post experience using post-ready, native media or by quickly transcoding to ProRes. But that’s not how most larger productions work. A typical motion picture or television show will take the camera footage and process it into something that fits into a known pipeline. This usually means uncompressed DPX image sequences, plus proxy movies for the editors. This allows a base level of color management that can be controlled through the VFX pipeline without each unit along the way adding their own color interpretation. It also keeps the quality highest without further decompression/recompression cycles, as well as various debayering methods used.

Uncompressed or even mildy compressed codecs mean a huge storage commitment for an ongoing facility. Here’s a quick example. I took a short RED clip that was a little over 3 minutes long. It was recorded as 4096 x 2304 at 23.976fps. This file was a bit over 7GB in its raw form. Then I converted this to these formats with the following results:

ProRes 4444 – 27GB

ProRes HQ (also scaled to UHD 3840 x 2160) – 16GB

Uncompressed 10-Bit – 116GB

DPX images (10-bits per channel) – 173GB

TIFF images (8-bits per channel) – 130GB

As you can see, storage requirement increase dramatically. This can be mitigated by tossing out some data, as the ProRes444 versus down-sampled ProResHQ comparison shows. It’s worth noting that I used the lower DPX and TIFF color depth options, as well. At these settings, a single 4K DPX frame is 38MB and a single 4K TIFF frame is 28MB.

For comparison, a complete 90-100 minute feature film mastered at 1920 x 1080 (23.976fps) as ProRes HQ will consume about 110-120GB of storage. UHD is still 4x the frame area, so if we use the ProRes HQ example above, 30x that 3 min. clip would give us the count for a typical feature. That figure comes out to 480GB.

This clearly has storage ramifications. A typical indie feature shot with two RED cameras over a one-month period, will likely generate about 5-10TB of media in the camera original raw form. If this same media were converted to ProRes444, never mind uncompressed, your storage requirements just increased to an additional 16-38TB. Mind you this is all as 24p media. As we start talking 4K in television-centric applications around the world, this also means 4K at 25, 30, 50 and 60fps. 60fps means 2.5x more storage demands than 24p.

The other element is system performance. Compressed codecs work when the computer is optimized for these. RED has worked hard to make Redcode easy to work with on modern computers. Apple ProRes enjoys near ubiquitous playback support. ProRes HQ even at 4K will play reasonably well from a two-drive RAID-0 stripe on my Mac Pro. Recode plays if I lower the debayer quality. Once you start getting into uncompressed files and DPX or TIFF image strings, it takes a fast drive array and a fast computer to get anything approaching consistent real-time playback. Therefore, the only viable workflow is an offline-online editorial system, since creative editorial generally requires multiple streams of simultaneous media.

This workflow gets even worse with other cameras. One example is the Canon C500, which records 4K camera raw files to an external recorder, such as the Convergent Design Odyssey 7Q. These are proprietary Canon camera raw files, which cannot be natively played by an NLE. These must first be turned into something else using a Canon utility. Since the Odyssey records to internal SSDs, media piles up pretty quickly. With two 512GB SSDs, you get 62 minutes of record time at 24fps if you record Canon 4K raw. In the real world of production, this becomes tough, because it means you either have to rent or buy numerous SSDs for your shoot or copy and reuse as you go. Typically transferring 1TB of data on set is not a fast process.

Naturally there are ways to make 4K post efficient and not as painful as it needs to be. But it requires a commitment to hardware resources. It’s not conducive to easy desktop post running off of a laptop, like DV and even HD has been. That’s why you still see Autodesk Smokes, Quantel Rio Pablos and other high-end systems dominate at the leading facilities. Think, plan and buy before you jump in.

©2014 Oliver Peters