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A CUT ABOVE THE REST

From STORAGE Magazine Vol 6, Issue 2 - March 2006
 

The days of a blockbuster movie being edited by cutting up strips of film and gluing them together are long gone. Now advanced techniques, driven by technology, play a huge part - and the outcome can be sensational
 

Many of the films highlighted as examples of great cinematography have gone through a digital production process that, a short time ago, would have been beyond contemplation. Just look at films like ‘The Constant Gardener’ and ‘Goal!’, as well as those with complex special effects, such as ‘Die Another Day’ and the Harry Potter films, and the scale of the breakthrough is all too evident.

The demands of the film industry are phenomenal, in data terms alone. Each pixel stored uses a 10-bit RGB value (packed into a 32-bit int), with the data stored as a DPX file. Film is shot using an aspect ratio of 4:3 (meaning there are three-quarters the number of pixels vertically as there are horizontally), and is edited in this format, although it is cropped into a widescreen format for projection at the cinema.

Today, film is usually mastered at 2k (2048x1556 pixels), which requires around 12M bytes per frame, but the industry is rapidly moving towards 4k (4096x3112 pixels), requiring about 50M bytes a frame. With the emergence of 4k capture, the resolution and quality of digital film matches that of the original 35mm motion picture negative.

Any post-production system must operate in real time, even at the highest resolutions, something that not only requires powerful processing, but also needs a storage system that can stream data in and out at high rates. At 24 frames per second, film data needs to be streamed at 1.22GB/s. However, if changes are made and written back, even a simple colour change will demand a total I/O bandwidth of 2.44GB/s. Storage capacities also need to be huge. With each frame's images requiring 50MB, a 1hr 20min film - consisting of 150,000 frames - would take up around 7.5TB of storage.

One of the leading companies in the industry is FilmLight, a British company founded by people from within the post-production community who felt they could develop better 'digital film' products for intermediate and post-production. The systems need to offer a range of features, such as colour management; the ability to use film shot at different resolutions and in different formats; and a database to manage the many clips of film that are shot in the process of making a movie.

Rather than developing a fully custom hardware platform, FilmLight chose commodity PC hardware for their Baselight systems. Like many companies building a platform dedicated to a particular function, they decided that the best operating system would be Linux, running on a conventional dual-processor PC.

The system allows performance to scale by adding additional computers. This is achieved by using FilmLight's own video engine technology that combines the output of the cards into a single image, allowing each system to operate fully independently.
Three products are available in the Baselight family - the Baselight, Baselight Four and Eight. The main difference between the three products is the number of processing nodes, with the latter systems offering real-time processing of 2K and 4K images respectively.

To maintain real-time operation, each node of the system has its own directly connected RAID array. With huge volumes of data to be stored - the FilmLight systems offer up to 48TB - the cost per GB of SCSI technology was prohibitive. SATA hard drives, however, offer high reliability, low cost per GB and vast densities. SATA drives also provide high-performance sequential reads and writes.

For film editing, with data being streamed to and from the disc, sequential R/W performance is the metric that really matters. Each node uses two 8-channel RAID controllers; managing a total of 16 SATA drives, which are configured into 4 arrays, with software managing the striping of data across the discs. This produces a read bandwidth of 600MB/s per node or - put another way - a stunning 4.8GB/s across an 8-node system.

A typical post-production facility might include several BaseLight systems, as well as a Truelight system for cinema-quality digital playback. All of the digitised film data is stored on a parallel version of the XFS filesystem, with project metadata stored in a relational database, which also makes use of SATA RAID. Both the digital film data and the project metadata can be shared between any number of users. Transferring copies is made easy, as the Linux nodes and the parallel file system are configured to appear as standard NFS volumes.

FilmLight chose 3ware SATA RAID controllers from AMCC for the Baselight range of products. These controllers offered the highest performance available, open-source drivers and the benefits of true hardware RAID. In particular, the low CPU overhead was a key factor. FilmLight also chose the Multi-lane connection system developed by AMCC, which uses a single cable to support four drives, improving airflow in the system and increasing reliability.

By choosing a commodity PC hardware platform, a Linux OS and AMCC SATA RAID cards, FilmLight has produced an economic, flexible and highly scalable platform. With film credits ranging from ‘Love Actually’ and ‘Thunderbirds’ to ‘Resident Evil’ and ‘Troy’, Baselight has already become the system of choice for many film studios. By using standardised products, the system is easily upgradeable to take advantage of higher capacity SATA drives as they are introduced, offering the opportunity
of even better films in the future.

"FilmLight has a long experience of pulling together the most advanced technology to meet the requirements for the film industry's post-production process. We are pleased that SATA could meet our demands," says Steve Chapman, director of FilmLight.

 "Supporting the IO rate of digital film colour grading is not easy, but, together with our Baselight architecture, AMCC's 3ware controllers have provided industry-leading performance in a cost-effective solution." ST