Blog Series: Hardware

If you have been following our social media and press releases, then you might be aware that we have just released a new version of our Tessera software, known as 3.2 to its friends. One of the key features of this new version is 3D LUT Import. What on earth is a 3D LUT and why should I care about it?

Before we go into exactly what 3D LUTs are, it is worth looking at what a common or garden LUT is. LUT (pronounced ‘lut’) is an acronym that stands for Look Up Table. Within the context of image editing, film and video, a LUT is used to remap the input colour values of source pixels to new output values based on data contained within the LUT.

In layman’s terms, a LUT can be considered as a kind of colour preset that can be applied to images or footage. But what does that mean in practical terms?
LUTs are used for a variety of colour correction tasks, both technical and creative:

Technical LUTs:

• Display calibration: LUTs are used when calibrating monitors or other display devices for colour critical work such as editing or grading
• Colour space conversion: LUTs that convert from different colour spaces, e.g. converting Rec 2020 to Rec 709
• Camera LUTs: LUTs that are often created by the camera manufacturer to convert camera logarithmic formats that digital cinema cameras typically capture footage in (which look very washed out to the eye) into Rec 709 or Rec 2020

Creative LUTs:

LUTs can also be used creatively: for colour enhancement, creating monochrome or sepia effects, boosting shadows and highlights, or replicating the look of a particular film emulsion – the possibilities are endless.

LUTs can also be both creative and technical, such as those created by DIT (digital imaging technicians) and stored into ‘LUT boxes’ connected to on-set monitors that combine a camera LUT with a creative LUT. This is helpful for giving some idea to the film crew on-set of what the camera is capturing might end up looking like when it has been graded in post-production.

There are two major kinds of LUT: 1D and 3D.

A 1D LUT on its own, when applied, can only change one input value to one output value. This means that it can only be used for changes in luminance such as gamma correction, contrast, brightness and black and white levels.

However, commonly 3x 1D LUTs are used together: one for each colour channel. When the LUTs are applied to a source, the input red, green and blue values of each pixel, are mapped to new output values. The LUTs adjust the brightness of each of the red, blue and green channels independently of each other, depending on the values contained within the LUTs. This means they can only control gamma, brightness, contrast, black and white levels.

Whilst 1D LUTs are useful for many colour correction tasks such as overall colour balancing and gamma correction, they lack the precision required for more complex non-linear colour correction such as film stock emulation and colour grading.

This where 3D LUTs come in. 3D LUTs map red, green and blue to three axes of a 3-dimensional cube. Colour values can be adjusted relative to each other, which allows any colour to be mapped to any other colour. This allows far more sophisticated colour correction, like changes in hue and saturation.

A 1D LUT has a lookup table of values based on the bit depth. So, a 10-bit 1D LUT has 1024 values, and 3 1D LUTs have 3 x 1024 values – or 3072 values.

Mapping the 10-bit colours in 3-dimensions means 1024 x 1024 x 1024 or over 1 billion values, clearly significantly bigger that the 1D LUT approach.

This means that storing a 3D LUT in this way would mean a massive file size with implications for storage and performance. However, unlike the majority of 1D LUTs, 3D LUTs do not store an output value for each input value. They use a lattice of values, so a given input value exists at a point within the lattice.

To calculate the output value, the value is interpolated – typically using trilinear or tetrahedral interpolation. The interpolation method used is very important to prevent visual artefacts. The size of the lattice and the bit depth of the 3D LUT determine the precision of the 3D LUT. Lattice sizes can be 17x17x17, 33 x 33 x 33 or 65 x 65 x 65 or higher, and bit depths can be 8-bit, 10-bit, 12-bit or 32- bit (floating point).

3D LUTs are not only an extremely precise method of colour correction but also a very efficient one. 3D LUTs are used extensively in post-production in grading and finishing, display calibration, and on-set grading, as well as converting from camera log formats to Rec 709 or Rec 2020.

We’re pleased to now be able to offer all those benefits of 3D LUTs with the ability to import them directly into your LED processor. We support any valid Adobe .cube file, and 3D LUTs can be pushed via IP control for more integrated workflows. This enables even more precise colour control for power users, and we hope it will be particularly useful for accurately recreating coloured “looks” and on-set grading on virtual production sets.

Learn more on our page about 3D LUT Import.

When you first get to use the Brompton Tessera system, you may not be aware of all the options available to make your LED screen work optimally for your particular environment.

The Tessera software includes default options such as ChromaTune, Dark Magic, Advanced Genlock with Phase Offset, and OSCA (On Screen Colour Adjustment), as well as features enabled per panel type such as High Frame Rate (up to 144Hz) and Studio Mode.

These features will allow you to achieve better low-brightness performance, improved on-camera performance and superior overall visual colour accuracy.

With ChromaTune, you’ll be able to tweak an individual colour with colour replace, or you can tweak an array of 12 colours and black &white with the 14-way colour corrector, to get the image on screen looking the way your client wants it to look. With OSCA, you can correct variances between different batches of LED panels or within the same batch that have aged differently. Apart from colour matching, OSCA also allows you to correct the seam brightness of adjacent panels and modules.

Dark Magic and Studio Mode are designed to improve low-brightness performance. In the case of Dark Magic, using temporal and spatial dithering can remove banding in dark areas when you have reduced the brightness of your panels or when you have content with gradients from bright to dark. On panels with Studio Mode implemented, you can also improve low-brightness performance by reducing the current gains of the LEDs and capping the brightness of the panel at a pre-determined level, thus increasing the number of steps of brightness available when the panel brightness is lower.

Since our 2.3.1 release, we introduced Advanced Genlock with Phase Offset and Frame Rate Multiplication. Using this, you can improve on-camera performance to ensure the screen refresh is in sync with the camera sensor’s frame capture period. In some cases, Frame Rate Multiplication will improve the chance of the camera sensor picking up the entire frame.

No matter your requirements, we are sure that Brompton has a feature that can assist in delivering a world-class production. If ever in any doubt, our technical support team are available 24/7 to answer your questions on support@bromptontech.com.

New virtual production methods using LED walls as backdrops for filming are still comparatively new, but there are many great benefits that may mean traditional green screens become a thing of the past. Of course, as with any new technology there are also some shortcomings and pitfalls. We’ll talk through them all in this blog.

Some of the pros of using LED walls instead of traditional green screens:

• Capture VFX in-camera
  • Directors and DOPs can now see the full picture through the lens, live on set, and work with the
    technical team to make real-time adjustments pre-shooting

• Realistic reflections
  • Previously the bane of post-production, now reflections are immediately accurately captured on
    set, reflecting the “world” around the actors on all set and costumes

• Realistic ambient lighting
  • LED panels deliver dynamic and nuanced coloured light, creating a much more lifelike environment
    to be captured in camera, saving time in grading later on

• Ease time pressure on-set
  • Capture perfect outdoor lighting in the real world, and have it on-set for as many hours as
    needed in a controlled environment

• Immersive on-set experience
  • Create a seamless blend of physical set with pre-shot or VFX environments, so that for the first
    time actors and the creative team can see and react to a CGI world in real time

• Shorter post-production time
  • Most of the CGI work is done pre-shooting, and the realistic lighting and reflections mean the
    post-production process is considerably streamlined

 

And some cons you might not be aware of:

• Can be expensive
  • Depending on the scope of the shoot, LED green screen replacement can be expensive, involving several LED panels and a quality LED processor to ensure you don’t run into any of the other issues listed below

• Camera picking up moiré
  • Keeping the screen separated from the foreground and ensuring a suitably shallow depth of field is essential to avoid unsightly moiré effects

• Visual artefacts
  • It is possible to get interactions between particular camera technology and the refresh patters of LED screens, which can be unexpected (such as the potential for scan mux lines when a camera is tilted up or down)

• Dropped or doubled frames
  • Poor quality processing without proper genlocking can cause issues with sync between the screens and the camera’s shutter

• Latency
  • If using the LED screen as part of a virtual production setup using camera tracking, you need a powerful processor capable of extremely low latency, otherwise the footage on the screen won’t sync with the movements of the camera

• Colour accuracy
  • Cameras sense colour differently to the human eye, so it is important that your processing provides the required level of colour accuracy and the tools to tweak the colour output

• Careful set lighting
  • Traditional set lighting needs to be adapted to balance with the emissive lighting coming from the LED screen, as well as minimising light spill onto any screen surfaces

The best news here is that most of the cons of replacing green screens with LED screens can be overcome with careful planning, diligent camera tests pre-shoot, and – crucially – quality LED screens with powerful processing behind them.

Brompton has a team of technical support engineers who are highly experienced with setups using LED screens as green screen replacements for virtual studios and virtual production on film and TV sets. If you’d like to know more, get in touch at: info@bromptontech.com