GRADING - THE COLOUR CHALLENGE

Digital Print System
Visual effects and post production house, The Lab Sydney, has developed a digital print system used for the first time on the Paramount feature film 'Tomorrow When the War Began'. As the post production and lead visual effects vendor on director Stuart Beattie's film, The Lab Sydney provided 230 visual effects shots, rushes, scanning, digital intermediate grade and hosted the editorial team in-house on three Avid suites.

Head of Digital Intermediate Al Hansen conceived and created a colour calibration process he named the Digital Print System, or DPS. With the support of the film's cinematographer, Ben Nott, ACS, Al started work and running tests on a process to completely colour calibrate all levels of the production, ensuring that the images from rushes to post maintained the characteristics of the final 35mm film deliverables.

"Ben understood the process and saw the advantage of being able to view all images in relation to the final film print as well savings in time and money on the rushes transfer,' said Al. Ben also recognised the importance of planning for the film's final release while still in pre-production. Once they had convinced the producers the post workflow was achievable, they gained approval to run it.

Reference Point
Because rushes colourists often work without accurate reference, they may have a subjective influence on the look of the images. Ben said, "Al's system lifts the decision from the dailies colourist and returns control over fine trims of light and shade and colour bias to the cinematographer or Director of Photography. I get a better understanding of the colour negative and I'm more disciplined about exposing it.

"From the beginning I only had to deal with the imagery in the film log space, and look at exposures and colour reproduction within the latitude of the print stocks. The images in dailies, although compressed, were only ever viewed as they would be interpreted by a film print. The result is an offline edit with consistent exposure from cut to cut. That is important not only to the editor and director, who deal with the film in this mode for months before the DI, but also to screenings for Studio Executives and Distributors, which require very little tweaking."

Al said, "Because the film is going to be scanned at high resolution and show exactly what has been shot, it's important that it be as similar to the rushes as possible. In any case, it forms the starting point for the creative grade. When it matches the rushes, everyone's notes and ideas can be applied quite easily."

Video Rushes
Formerly when rushes were printed on film each day, they naturally had a 'film look', and most DPs typically just wanted a one-light grade that was set up prior to and rolled out throughout the shoot, keeping the look of the film in the hands of the DP. The grader would determine the one light grade via charts the DP had shot and apply it to all rushes. The DP could closely monitor and correct his lighting and exposure daily, with no outside intervention.

But since the advent of non-linear editing, productions can use cheaper, quicker video rushes instead of 35mm print rushes. It has become the norm for feature film rushes to be transferred to tape or Avid via telecine, which has no calibration parameters and means the director, DOP and editor may not be looking at accurate representations of what was actually shot. Grading rushes, often shot to shot, has become an important step to compensate for lack of calibration on telecine, opening the potential for error and a misunderstanding of the traditional film laboratory process.

Video Aim Density
Hansen and another colourist developed a system, in use on film and TV productions since the 1990s, called Video Aim Density that calibrates the telecine to the camera and film. "I specifically invented Video Aim Density calibration to work just like the Laboratory Aim Density that Kodak had created 30 years ago. Without either systems, the DOP could lose track of what they were seeing in the rushes, among the many other variables onset. By having this system accurately in place, we can track other problems down."

Until now, the Video Aim Density and DPS had never been combined. The DPS calibrates the telecine to the Northlight 2 Scanner, and is viewed under a print emulation Look Up Table as log-in video. In effect, for 'Tomorrow When the War Began', the DPS emulated on the DP's on-set monitor and digital rushes what the film was going to look like on 35mm print.

Hansen treated the entire rushes process like a DI in which everyone was using the same LUT and working within the same parameters as the colourist and DP. All variables across the production had to be eliminated to ensure an accurate workflow.

3D LUT and Calibration
Since film is still the main deliverable Hansen felt it was important to work with the actual 3D LUT that would be used in the final post, consisting of the exact process the final images would go through as well as having the actual characteristics of film. Al said, "I had asked Tony Poriazis, Head of Digital Imaging at Digital Pictures where the final record out to film would be done, to build the LUT and assist in the film-out process. We brought him on board at the beginning and worked backwards.

"The LUT is the absolutely accurate, 3D viewing simulation of the film path that the DI file will take once we are ready to record to film. It depends on the ARRI recorder and how its parameters are set, the DI stock we will use and which laboratory we will process at. These choices needed to be made first before we could make the LUT, and I needed the LUT before I could calibrate at my end.

Before shooting commenced, Al and Ben also had to do a complete calibration loop of the entire process, and then determine the format from a series of camera tests to choose which stocks to use. In order for the DOP to see an accurate result from his camera test, the calibration pipeline needed to be in place. Once he chose his stocks, the team at The Lab incorporated them into the system, and the DOP could add new stocks while shooting if necessary. Ben shot the film with ARRICAM ST and LTs and an ARRI 235, using Kodak 5201 50D and 5219 500T stocks.

Grading OPtions
Al said, "This system also leaves the option to grade and use a Colour Decision List, which carries the metadata through the process back into DI. You will be starting the grade from a point accurately referenced to what was shot, which makes it easy for the DP to explain what he wants. In the case of 'Tomorrow', we created pre-grades for looks and loaded them into the Gamma box that carried the LUT so the DOP could instantly show the director different looks or intensions. Ben didn't actually want us to grade his rushes because he was controlling everything in camera. If he wanted to alter the rushes image, he had the grades in the Gamma box."

To extend the DPS into other parts of the production, The Lab's VFX Supervisor and Lead Compositor on the film Tony Cole helped roll the system and LUT out to all the VFX vendors and the production VFX Supervisor Chris Godfrey, increasing transparency between the original and the completed files. He and Al made several trips to the Hunter Valley shoot location to make sure the monitors were calibrated and the system was working.

Into the Future
Likewise, it was also extended to editorial's Avid unity, to establish a non-linear colour calibrated workflow. Al said, "We could show cuts instantly to the production in our DI theatre straight off Avid or colour corrected through our Baselight grading system. The entire process remained non-linear until the final film out for distribution. The DPS is specific to the combination of the Northlight 2 scanner and Spirit Telecine. I could set it up on other systems but you need certain work flows in place to do it correctly.

"The Lab Sydney is completing post production on 'Burning Man' directed by Jonathan Teplitzky, including rushes, conform, film scanning and film deliverables, plus about 25 VFX shots. Again, Tony and I worked with the producer, director and DOP Garry Philips, who are using The Lab's DPS on their film across the entire post work flow. We are working on a similar system for digital cameras, a little more challenging due to all the different formats."

Say Nothing
Cutting Edge is among the first Australian post production facilities to work with the ARRI Alexa on a feature film project. Adrian Hauser, Cutting Edge Digital Intermediate and Rushes Colourist, has been working with DP Jules O'Loughlin on the upcoming film 'Say Nothing' to determine the best acquisition method and workflow for the Alexa, carrying out tests to compare the options. Cutting Edge is handling post production for the movie, developing a pipeline carrying the footage from rushes processing to 35mm film out. Rushes are being graded in a film out context and quality controlled daily on Cutting Edge's 5m digital projection screen.

Adrian said, "Aside from the ARRI D21, no other digital cinema camera footage I have graded has come as close as the Alexa to capturing the same subtle characteristics of colour and light that 35mm film captures. The extended latitude of the camera outside the photographed intent, and the colour rendition and tonal quality of the captured scene sets is impressive, such as the ability to render extremely subtle differences seen in skin tones."

Film Curve
Adrian said, "By its photographic nature the logarithmic encoding of light exposed on 35mm film gives the colourist a direct path for getting footage to film-out than digital footage. By using math to map the translation of scanned original camera negative images to print, an LUT can easily be built to give rushes transfers a direct relation to the final look of the film.

"Typically, video footage doesn't afford the Director and DP as much opportunity to control and achieve the look they want when delivering for film. The look is baked in. But the D-21 digital camera demonstrated ARRI's understanding of mapping visible light to emulate a film curve. Because it allowed the use of a more film-like LUT, rushes and the resulting DI, rushes could be graded with simpler, film-style tools.

To process the rushes, after camera testing, an initial production LUT was created for the LogC files. Rushes arrived nightly on portable drives and after being checked for data integrity, were laid up and visually checked again in the rushes timeline. The 4:4:4 ProRes files were then QC'd and graded on the 15 ft Digital Cinema screen at Cutting Edge.

"Grading the Alexa footage was a simple process using basic RGB Lab Style Printer Lights underneath the Production Lookup table," Adrian said. "On completion, Avid MXF files with appropriate burn-ins were then output directly to editorial. Working on the ARRI 4:4:4 native ProRes files and grading them with traditional colour tools made rushes really straightforward. Using simple photochemical style grading tools with the ARRI Log files lets the cinematographer comprehend the colour and density of the resulting rushes in a way that timed 35mm print dailies used to.

Digital Interpretations
"Camera manufacturers have recently added their own interpretations of the Log curve to allow more flexibility in grading. Although these gave more capturable latitude, they still aren't ideal for to film workflows because they output images by extending and manipulating digital gain."
The way a digital camera records gain values is very different to the way film responds and exposes light on the film curve. On a digital recording, stock black will always remain black and as light is introduced the values above black become activated but won't necessarily map easily into a film log world. Film exposure, on the other hand, can record these values anywhere on the film curve.

Tonal Range
Adrian feels that ARRI have improved on the mapping of digitally recorded light to produce a Log output from the Alexa that fits very well into the Log Film workflow. He also finds that the Alexa makes a greater tonal range available than most digital cameras. This wide exposure latitude results in a camera that essentially has a speed of 800 ASA, which is very fast and sometimes requires extensive ND filtering on set to achieve a narrow filmic depth of field.

"This isn't only a problem in the Alexa," Adrian said. "All 'fast' digital cameras are similar. But if the DP were to cut the sensitivity down by 4 stops, for example, to improve the DOF, an unusual problem may become evident – infrared light that is invisible on set would become visible in the footage. ND filters only cut light in the visible spectrum. Thus if light is reduced with ND filters then, in effect, the amount of invisible infrared (near red) light, which the Alexa also captures, is increased and becomes visible in the processed footage. On 'Say Nothing', this characteristic was first noticed because of its effect on fabric dyes in some of the costumes, which looked much redder in the footage."

The director did, of course, want to see DOF in the footage and so, once filters were added and the stops adjusted, the look of skies, sunsets and vegetation, in particular, showed evidence of the camera's colour depth. For example, the trees and plants took on a warmer glow than is usual in Australian bush vegetation, which they got to like and called their 'Tuscan' colour scheme. Another advantage came in the ability to show detail across skies even when they formed a background light source behind other detail and character action.

Happy Accidents
The sensitivity is a result of ARRI's decision to lower the infrared cut-off to provide plenty of colour detail for skin tones. "A DP would need to control and be aware of it, and make it work for the project, and the production might consider adjusting colours on set to work with the camera," Adrian said. "ARRI suggests using the Tiffen T1 filter for IR, and also IRND filters. While experimenting with filters and light stops introduces some unknown factors into the shoot, lowering and softening the cut-offs has allowed for the 'happy accidents' that can occur when shooting on film. In most video footage, colours are typically flatter and working with them in post to look more filmic will always result in a 'forced effect'."

Colour detail in the images retained its natural looks under low lights very well. 'The sun never sets on the Alexa' was an expression the crew started to use when they found they could continue shooting into the evening. "Areas in shadowed parts of an image will maintain colour detail when shadows are lifted. Furthermore, while most video cameras tend to clip colours abruptly in the highlights of the image, the Alexa has a softer clipping point, keeping a lot more detail toward the clipping point.

Compression Testing
A few weeks before shooting commenced in mid-December 2010, Adrian and Jules began some compression and Dynamic Range Testing on the camera. Adrian has been working with 4:4:4 ProRes data, which was recorded on SX cards simultaneously with each of three other formats - HD Cam SR, Cinedeck uncompressed and Wavelet compressed. ARRIRAW was not available to them at the time of the test shoot.

The results obtained from panning shots across resolution trumpets on test charts found that all formats achieved an identical look to each other within about 5 per cent. The Cinedeck uncompressed, or raw, format had the least compression artefacts. Adrian said, "The ProRes broke up slightly in the extreme specular highlights at very high magnification of about 800 per cent but it would be more economical to work with, and 'Say Nothing' is not an effects driven film that needs extra compression detail within the specular highlights for FX artists to work with. While the HD Cam SR also looked good, it exhibited its own visible DCT compression matrices when zoomed up, and ProRes was better than this.

"It's true that higher resolutions can be obtained from some other digital cameras – such as 4K images from the RED - but what is of more interest to a filmmaker is how each pixel is interpreted, instead of how many. Thus colour handling is the Alexa's strength, where the aim is to create a sharp, cinematic image with film-like qualities."

Dynamic Range Testing
In a given lighting scenario, a DP usually adjusts the light in images within a window of seven stops of light. In post, this window could be shifted up or down in the footage to extend the latitude beyond the captured intent. The Alexa was tested at each stop above and below base sensitivity, E1 800, from -6 to +6. Even as far out as plus or minus 5, Adrian could still correct the image to within a reasonable threshold, maintaining latitude, showing the true original colours and fine gradations of light from the highlights down to shadow without big 'digital' leaps creating flattened areas.