Digital Film Supplement
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DIGITAL FILM SUPPLEMENT Editor: Bob Pank Introduction What started over a decade ago with the use of digital effects and offline in film has grown into the much wider use of digital technology to embrace the whole scene-to-screen process. For many, Avid’s Film Composer was their first encounter with any form of digital film, and today it is commonplace for the offline decision-making process. Certainly the use of equipment like Kodak’s Cineon, Quantel’s Domino and later Discreet’s Inferno have transformed the whole use of effects shots in motion pictures – even to the extent of changing the way pictures are made. Initially, only the effects shots were put through the film-digital-film processes. Now, thanks to further developments in many areas such as digital storage, scanning, processing and projection, whole features are handled as Digital Intermediates to produce digital masters at full film resolution. As the technology gaps are plugged events are moving rapidly so that digital pictures and sound can carry all the way from scene to screen. This is already happening with a number of feature films, including Star Wars Episode 2: Attack of the Clones, shot on digital cameras and digital all the way through to projection in digital, as well as traditional, cinemas. Famously, George Lucas said at NAB 2001‘I will never make another film… on film again.’ Clearly he is convinced of the benefits of all-digital production. The digital scene-to-screen path comprises three distinct areas: capture/shooting, post production and distribution/projection – otherwise know as Digital Cinema. Any part of this process is interchangeable with the celluloid medium, thus a film shoot, digital post and film distribution is increasingly common. To a degree the choice depends on budgets, the required look and the distribution/cinema network that is available. 156 The use of digital technology through the ‘film’ chain is democratising the ‘film’ features business and changing the art. It is already possible to digitally shoot on DV and edit ‘films’ for very low budgets and, with digital projection, these can be shown without ever touching actual film. Digital projection itself opens the door to new possibilities for cinema operators to show a whole new swathe of digital material including live events. Major feature productions are turning to Digital Intermediates and some are using HD cameras, rather than film, for shooting where the benefits include the immediacy and interactivity of shoot and review, to ensure a good take. After a century of celluloid the whole business is now changing. Quite rightly, the first attempts at applying digital technology involved at least equalling the quality of 35mm camera negative film. However, what cameras record and the detail that is seen in cinemas are two different things – raising the question of what is film quality? Digital projection has already shown the benefits of digital techniques and the whole digital film business is moving forward. To understand the achievements of celluloid film and the potential of digital film, this section includes some detail about the nature of both and their scene- to-screen path. As digital technology offers viable alternatives and increasing advantages over film, there is a crossover and exchange of terminology which is best understood by having some knowledge of both areas. Here, the emphasis is on the ‘digital intermediate’ or‘digital lab’ process, rather than acquisition and exhibition. 157 Tutorial 1 FILM BASICS The film process is designed for capturing images from scenes that will be edited and copied for eventual projection onto hundreds or thousands of big cinema screens. This has been in operation for over 100 years and so has been developed and refined to a very high degree to precisely meet these objectives. The film stocks used in cameras have a characteristic that allows them to capture a very wide range of scene brightness with good colour saturation to provide wide latitude for colour correction after processing. Intermediate stocks used to copy the one original negative are designed to be as faithful to the original as possible. Print stocks provide the high contrast needed to produce a bright and good contrast image on the projector screen to overcome the background illumination in the theatre. Television is different in many ways. For example, the results are always instantly viewable and are delivered, sometimes live, to millions of smaller screens. The sensors used in video cameras do not presently have the wide dynamic range of film and so shooting with them has to be more carefully controlled as the ability to correct exposure faults later is more restricted. Also the viewing conditions for video are different from cinema. Not many of us sit in a darkened room to watch television, so the images need to be brighter and more contrasted than for film. The three different basic types of film stock used – camera negative, intermediate and print – each have very specific jobs. Camera negative records as much detail as possible from the original scene, both spatially and in range of light to make that original detail eventually available on a multitude of internegatives from which are produced thousands of release prints for projection. The Film Lab Between the camera negative and the print there are normally two intermediate stages: Interpositive and Internegative. At each point more copies are made so that there are a large number of internegatives from which to make a much larger number of release prints. The object of these intermediate stages is purely to increase the numbers of negatives to print as clearly the precious and unique camera negative would be effectively destroyed with so much handling. The intermediate materials, interpositive and internegative, are exactly the same and designed to make, as near as possible, exact copies for each stage (with each being the negative of the previous stage). For this requirement the material has a gamma of 1. But the release print is not just a film representation of the shot scenes: editing, visual effects, and grading – not to mention audio work – must take place between. This mainly works in parallel with the film processing path – partly to reduce handling the negative. 158 Film post production Camera Camera negatives Rush prints viewed and checked. Initial grading decided Negatives cut from Edited copy of rush prints Soundtrack scene list used to make master scene list Master interpositives Internegatives produced Release prints produced produced & distributed to after final grading figure production labs agreed Camera negative is printed to make the rush prints which provide the first viewing of the shot material. Note that this will be at least several hours after the shoot so hopefully all the good takes came out well! The first edit decisions about what footage is actually required are made from the rush prints and with the aid of offline editing. The negative cutter has the responsibility of cutting the unique footage according to the scene list. Initial grading is applied as the cut negative is transferred to interpositive. Should there be any further need of grading, instructions for this are sent with the internegatives to the print production labs. Any need of dissolves rather than cuts, or more complex visual effects, will require work from the optical printer or, increasingly, a digital film effects workstation. Grading or Timing Grading is the process of applying a primary colour correction to the film copying process. The original camera negative may contain lighting errors which will mean that scenes shot on different days or times during the day need to look the same but simply do not. By effectively controlling the colour of the light used to copy the negative to one of the intermediate stages these errors can be much reduced to produce a scene-to- scene match. Grading is carried out on a special system equipped with a video monitor displaying the current frame from the negative loaded onto it. Three controls provide settings of the red, green and blue ‘printer’ light values that adjust the amount of each of the three lights used to image the frame. These adjustments allow the operator to balance the colour and brightness of the scenes in the movie. 159 This results in a table of corrections linked to the edge code of the original negative. This table may be stored on floppy disk or paper tape and used to control the optical printer making the copy. Most processing laboratories subscribe to a standard definition of the settings but this does not mean that setting defined at one processing lab can be used at another. The photochemical process is very complex and individual labs will vary. However they all aim toward a standard. The ‘neutral’ value for RGB printer lights is represented typically as between 25, 25, 25 to 27, 27, 27 – depending on which lab is used. To print an overexposed negative will require higher values and 1 underexposed lower values. A change of 1 in the value represents /12 of a stop adjustment in exposure. Differential adjustments of the values provides basic colour correction. See also: Basic optical measurements (Appendix) 160 Entries 10-bit log This refers to a way that film image information is transformed into digits and stored. It uses 10-bit data and so can describe 210 or 1024 discrete numbers, or levels: 0–1023 for each of the R, G and B planes in the images. However, as all electronic light sensors are linear, they produce an output proportional to the light they see, in this case, representing the transmittance of the film. This means a large portion of the numbers describes the black and dark areas, and too few are left for the light areas where ‘banding’ could be a problem – especially after digital processing.