Bringing Film into the Digital Age:
Optimizing the DI Workflow
by Simon Cuff

Advances in technology over the last decade have allowed work at film resolution – 2k and even 4k pixels per line – to be handled digitally. This new post production process is generally known as digital intermediate, and has transformed the creativity of the movie-making process while at the same time offering an improved workflow. The article traces the history and explains how an all-embracing architecture can bring real benefits through parallel working. In addition, it also explains how this same data-based architecture will also take over from video-based post production.

Introduction

Digital intermediate is the post production process, independent of acquisition and delivery formats. The vast majority of movie projects are still captured on film, although digital cameras such as the Grass Valley Viper are beginning to have some impact. Similarly, digital cinema is starting to gain momentum, with delivery to cinemas as data files rather than cans of celluloid. This article is only concerned with the post production process.

Digital intermediate as a workflow grew out of earlier experiments with digital film, which were largely concerned with visual effects. The majority of work was done by in-house teams in the major studios, creating bespoke systems in response to demands from directors.
This early work was used only for specific visual effects with the results written back to film for a traditional cut. At that time storage costs were prohibitive and computing power insufficient to consider using digital workstations to finish a movie.

Digital Film Parameters

It is worth defining digital film resolution at this stage. The object has to be to capture all the richness of image on the original celluloid and preserve it throughout the post production process. No amount of operational benefits in digital intermediate will outweigh a visible reduction in movie image quality.
An analysis of the grain structure of a modern film stock, as defined by its modulation transfer characteristic, suggests that a scanning resolution of 4096 pixels across the full width of the 35mm film frame will capture all the detail. The vertical component is defined by the desired aspect ratio: for a 4:3 full frame 35mm (Super 35) it is 3072 lines, for movie widescreen 1.85:1 it is 2214 lines, and so on. This defines 12,582,912 as the maximum number of samples per frame.

There is a third dimension to consider, the depth of colour. Film stocks have rather more power to resolve colour than video systems, and it is generally recognised that 14 bits per colour are required to capture all of the subtlety.

12,582,912 pixels by 14 bits per colour by three colours by 24 frames per second (for movies) clearly represents a very large amount of data indeed, even given today’s low-cost disk arrays and wide data paths. For this reason, some compromises are frequently made.

Click on the above image for a more detailed view

4096 pixels across the film frame is known in shorthand as 4k resolution. This remains the gold standard, and the Digital Cinema Initiative, the grouping of the seven leading Hollywood studios, has made it a requirement for digital delivery systems. The majority of productions, however, accept 2k resolution – half the pixels in each direction for one quarter of the data – as the pragmatic choice, and the majority of the movies released which have gone through the digital intermediate process were done at 2k.

Second, it can be demonstrated that using logarithmic rather than linear quantisation, the full colour resolution can be captured in 12 bits per colour rather than 14. A number of facilities approximate still further here, settling for 10 bit logarithmic colour depth, again without apparent visual impairment.

These parameters are incorporated into a number of file formats, of which the most widely used is DPX, originally developed by Kodak for its Cineon system and now extended and ratified by SMPTE. The DPX format is frame-based – each frame of the film is a separate file. It supports different resolutions and colour depths and, importantly, includes space for metadata as part of its header. Widespread adoption of DPX has allowed considerable flexibility and portability in digital intermediate systems and facilities.

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