Using Prelight to realise your vision - without drowning in colour science

Prelight
James Milne, Filmlight
Post Production
August 4th 2017 at 12:56PM : By

James Milne, FilmLight’s lead developer on Prelight, the company's onset grading software, talks about how Prelight works and it's potential advantages for DP's as well as graders

When the industry moved to digital cameras, the temptation was to think that it would be easy to view what you were shooting on screen, and that the workarounds from the old days of film shoots – where cameras had to be modified to fit in a video assist so directors and DoPs had some idea of what they had just shot – were gone.

We quickly discovered that it was not that simple.

Digital cinematography cameras shoot with far greater dynamic range than can be accommodated on a video screen. If you just take the camera output, it looks very flat; to get a viewable picture, you have to apply a mathematical transform to the signal, which converts colour values in the camera’s raw format to colour values in the display format. To do this the processor uses a look-up table, and so we have all become familiar with the idea of the LUT.

The other huge shift from film days, where colour controls were limited, is that LUTs then moved on from a simple transfer to becoming part of the creation of the look of the movie. Fast forward to today, where the look is created in the grading suite by the colourist, using powerful tools like Baselight.

So where does the DoP now sit in this process? DoPs often take reference stills in the pre-production stage of a project. Using Photoshop, they will modify these to the grade the director is seeking. These are invaluable tools, yet they are often used simply as visual references that then have to be recreated from scratch in the grading suite.

 

Prelight

 

Once production begins, their craft is in using light and lenses to make pictures that underline, and often drive, the story forward. These skills are what make it art and not a YouTube video. And to be able to concentrate on making those beautiful pictures, they need to be able to see something on set that is quite close to the finished image. But the danger is that this comes at the cost of getting too involved with the colour science, which may take them away from their primary objective. While the DIT can help a little, they too have a demanding role in data wrangling and do not necessarily have the skills or knowledge in grading.

Rather than just imposing a LUT on an image so the DoP can see that it is framed correctly and that there are no serious lighting errors, what would be ideal would be a visual way of setting a look on set that is simple, not time-consuming, and most importantly can be done without specialist knowledge of colour management.

The need is for an application, too, that can capture the underlying colour transformations in the original reference still, then apply them to the output of a digital cinematography camera in real time. As well as making the scene match the pre-visualisation as it is shot, the grading information should be saved and delivered to the colourist upstream, as the foundation of the grade.

Other workflows will see the director, DoP and colourist working on the look of the movie in pre-production, perhaps with the benefit of some test shoots. At that point, the colourist (backed up by the post house’s colour scientists) can define the LUTs to be used, and set some basic looks. As with the reference stills, these can be used on set so that the video assist output of the camera has the foundations, at least, of the finished grade.

To achieve this, there has to be consistency in the way that the grade is imposed, and some degree of control over the display devices, so you are making a reasonable comparison. There is no point setting a look on set if the monitor you are using has a very different performance to projection. Similarly, there is no point in setting details on set if there is no way of guaranteeing that they will be precisely carried through to the final grade.

The ASC CDL was an attempt at making a portable format, but in truth it is a limited set of instructions. In practice, it puts a brake on creativity rather than enhancing workflows. But the need for complete and accurate capture of grading decisions from the camera to the final deliverables is vital.

The FilmLight approach is to use a common file format that describes every aspect of the grade. This format describes not just colour decisions but also windows, vignettes, shading, position and all the other tools the colourist has. The BLG format (Baselight Linked Grade) operates on the basis of cumulative metadata: nothing is baked into the raw footage until the point of rendering the final deliverables. Any decision can be altered or rolled back at any time.

To meet the needs of DoPs and DITs on set, FilmLight has now taken its colour science and the BLG format and wrapped it into a simple package, called Prelight. This has the full grading power of Baselight, along with all its colour science, but in a very simple shell so that it can be used on set without getting in the way of the shoot.

There are two versions. One – Prelight – is a free download, and gives the DIT the ability to load looks and tweak the grade using stills. The full version – Prelight ON-SET– puts a lot of power on set.

First, it applies the look to the camera output in real time, either through an external device or, in the case of cameras like the ALEXA SXT, in the camera itself. Because it incorporates all of the Baselight colour science, it can impose one transformation for the display device – there might be an HDR screen for playback on set – and another for the HD output, so the material going to editorial looks right, too.

Once connected, the DIT and DoP can do as much or as little as they want. They can simply accept the LUTs and maybe any looks pre-set by the colourist. They can choose to match to reference stills. Or, if they want to, they can adjust the grade, using either on-screen controls or a simple panel plugged in to the computer.

Similarly, the basic functionality can be performed on a Mac laptop, or a more powerful computer can be used to give access to the full functionality. The DITs notes on the shoot, like circle takes, can be added to the Prelight timeline for transfer to the post pipeline. The people using the system have the choice on how much functionality to use on any given project.

Essentially, this gives reassurance – and, if required, control – to the DoP that what is coming out of the camera at the point of shooting will achieve the right look. And it does it without the need for any knowledge of the underlying technology, nor the need to have a skilled colourist on site. It simply allows the DoP to focus completely on realising the vision.

Prelight will be enhanced by the 5.0 colour tools when Baselight 5.0 is released, and customers will be upgraded to 5.0 at no cost as soon as it is available. This means that users will be able to access the Base Grade—a new primary grading operator for modern colour workflows and HDR—as well as other colour tools such as midtone contrast and gamut compression.