BaseGrade and the evolution of colour grading

A rough understanding of the evolution of colour correction helps in the evaluation of basegrades. The origins of colour grading as we know it today go back a long way, to the early days of television and cinema. For TV, video signals from television cameras or from a film scanner have always had to be levelled or corrected. The profession of colourist originated in the telecine, where film material was converted into a pleasing video signal.

VideoGrade

The technicians provided the colourist with four basic technical parameters with which he could process the video signal: Lift, Gamma, Gain and Saturation. These are still among the most popular grading tools today. In Baselight, they can be found in the VideoGrade Operator. With Lift, which is sometimes also called Pedestal, the colourist adjusts the black level and with Gain the white level. Experienced Photoshop users will find it easy to visualise the resulting gradation curve: Lift sets the starting point (bottom left) and Gain sets the end point (top right). When working with VideoGrade, these are the most important reference points. Gamma is technically a power function whose only parameter is the exponent. The gamma function determines the curvature of the gradation curve between the two end points.

These three parameters can be adjusted not only for brightness, but for all three colour channels of the video signal (RGB). However, the operator is not usually presented with individual controls for red, green and blue, but one for the brightness signal and a two-dimensional one for the colour component. This results in the basic structure of all grading panels: three spheres, which adjust the colour in two dimensions, and a rotating ring around or next to them for the luma setting. Lift is on the left, gain on the right and gamma in the centre. Colloquially, these three parameters are often referred to as shadows, mid-tones and highlights.

A very popular and solid working method used by video style colourists is to first adjust the black level and white point of the image. This involves balancing all three channels in both black and white just before the clipping points. The brightest point in the image is then pure white, i.e. without a colour cast and with maximum brightness, and the darkest point is pure black. This is often referred to as “clean” black and white. The gamma parameter is then set. This regulates the “airiness” and “heaviness” of the image. In other words, a combination of brightness and contrast. If you want to colour the image, for example, you often do this via the gamma, as black and white then remain “clean”.

Baselight provides two modes for VideoGrade. The standard mode is RGB. In YCbCr mode, the luma channel is processed in isolation. Changes in brightness then have no effect on the colour and saturation of the image. Over time, VideoGrade has also become the most important tool for telecine-style colourists. The colourist manually converts an image from a colour space with a high contrast range, such as log coding, into the output colour space. VideoGrade is therefore now not only applied to images in a video colour space, as originally intended, but also to images in a log colour space.

Lift, Gamm, Gain is probably the most frequently used grading operator in the video sector at present. However, it originates from a time when the handling of specular highlights, for example, was of secondary importance. Burned-out windows or overbright skies were tolerated as long as the faces were recognisable. A soft clip, i.e. a smoother transition to overshoot, is not possible with VideoGrade alone. Over time, video colourists therefore developed various techniques to meet this aesthetic requirement. Gradation curves, luma keys, blend modes or dedicated soft clip operators were used for example.

Another disadvantage becomes apparent in VFX workflows. Contemporary compositing works most realistically in a scene-linear colour space. The pixel values are proportional to the photons on set. Pregrading is nevertheless often helpful so that the basic brightness and white balance are correct and the individual shots in a sequence match each other. Unfortunately, lift, gamma and gain destroy the scene linearity and make VideoGrade unusable for this type of VFX pregrading.

FilmGrade

Long before the telecine colourists, the profession of the film light setter emerged. They influenced the look of a cinema film via the intensity of the copy lights and the chemical processes. With the introduction of the digital intermediate process around the year 2000, the new profession of DI colourist emerged. This person processes cinema images digitally before they are exposed on film material and copied onto print material.

The source material was also usually a film scan. The digital intermediate gradually replaced the analogue intermediate process and thus the creative part of determining the light in the copying plant. Film is the all-determining factor in the DI process, as you can only produce colours that can also be reproduced on film material. The analogue process up to the cinema copy is therefore simulated live in grading using a so-called Film Print Emulation LUT. The obligatory film LUT, which is used in the preview output, means that the image reacts differently to the colourist’s inputs than in telecine or video mode.
The manufacturers have developed new grading tools for DI processing, which are based on analogue light determination with copy lights. In Baselight it is called “FilmGrade”. FilmGrade is designed for processing images in the Cineon-Log colour space. After the colour correction, a conversion to a display colour space takes place, for example classically via a LUT or, since Baselight 4.4, via shaders with truelight colour spaces.

FilmGrade consists of a total of six tools, which are divided into two tabs. The main page consists of Exposure, Contrast and Saturation. The second page consists of Shadows, Midtones and Highlights. All tools offer adjustment options via a ball and a rotating ring. The most important tool is Exposure, which can also be adjusted in RGB copy light steps, so-called printer points, via the blackboard panel. The colourist uses Exposure, also known as Offset in other grading programs, to adjust the brightness and the sphere to adjust the colour of the image. Film-style purists try to work with exposure as much as possible, as this simulates an analogue light setting and the image remains very natural. The contrast in all colour channels and the scene linearity are retained when changing the exposure, for example. Shadows, midtones and highlights do not correspond to lift, gamma and gain, as the individual areas are limited by pivot points. If, for example, the shadows are lifted or coloured using Shadows, this is only done up to a defined point in the curve.

With Lift, on the other hand, the entire image is processed, but the shadows are the strongest. In film workflows, the LAD grey test field approximately in the middle of the curve is the anchor point both for calibration and for the FilmGrade colourist. It was defined by Kodak and is a little darker than 18% medium grey. Full black and white are difficult to determine on film because the curve is very flat. This is why a film-style colourist, unlike a video-style colourist, is not so concerned with achieving 100% white or black. With this concept, a soft clip is already active via the simulation of the print material. The visual impression takes centre stage. This grading concept is therefore more natural than VideoGrade.

There are also fundamental problems with this concept. Although controlling the brightness via Exposure works quite naturally, it is not exactly the same as changing the camera aperture or the ISO value. This is due to the log coding used, which does not define the black level exactly to zero, for example. The detailed reasons are beyond the scope of this article.

FilmGrade is now used not only on Cineon log data, but also on any type of log coding such as LogC from Arri. On the output side, not only film emulations but also modern approaches such as ACES are used. This brings us to future-proofing: log coding can only store a limited dynamic range. The Cineon curve, for example, was no longer sufficient for the high contrast range of the Alexa camera. This is why Arri developed the LogC curve. Future camera generations and HDR displays will require further adjustments, which may involve compromises. This also applies to the VideoGrade operator, which was originally only designed for video signals with a standard dynamic range.

BaseGrade – under the bonnet

The time is ripe for a next-generation colour grading operator. Filmlight uses neither a video signal nor film material as its foundation, but algorithms that are modelled on human perception. The dynamic range is not limited by a technical format such as Rec. 709 or Cineon coding, but is ready for the future characterised by HDR.

BaseGrade works identically in every working colour space and therefore always feels the same to the colourist, regardless of the camera used. However, in addition to a sensible colour setup in the scene settings, this also requires the correct keywording of the material, which is usually done automatically. BaseGrade autonomously converts the image into a linear colour space in which the original brightness ratios of the scene on set prevail, as in linear compositing. The user is not aware of the colour space conversion; the next operator in the stack receives the image in the defined working colour space again.

BaseGrade uses an internal colour model based on Lab, which consists of a pure lightness component L and two colour components a and b. The colour plane spanned by a and b has been distorted by the developers with regard to colour correction so that colour and saturation changes have the same visual effect in all colour areas and work feels more intuitive. Four parameters affect the entire image: Flare, Balance, Contrast and Saturation. In addition to these global parameters, BaseGrade divides the image into brightness zones. This is immediately reminiscent of Ansel Adams’ legendary zone system, which, according to the developers, also served as a source of inspiration.

At first glance, there are few parallels to existing tools in Baselight or comparable video grading software such as Resolve. Raw development in Adobe Lightroom comes closest to working with BaseGrade. However, BaseGrade not only offers brightness controls for each zone, but also colour and saturation. In addition, each zone area can be fine-tuned via pivot and falloff. All exposure settings and pivot points are specified in f-stops. This also comes close to human perception and helps photographers and cameramen to understand them. Stops are also a widely recognised and established unit.

Baselight 5.0 

Users are provided with new builds every few weeks, but now version 5 is finally in the starting blocks. The significance of this release becomes clear when you consider that the last full version jump from 3.O to 4.O was more than seven years ago (2OO9).

Although BaseGrade is the most significant innovation in the upcoming version, it is of course not the only one. For example, the software will be greatly expanded in the finishing area with a dedicated blue/green screen keyer, perspective tracker and warper as well as a grid warper and a paint tool. In the plug-in area, support for the Autodesk Flame matchbox shader format and GPU acceleration for OFX will be added. 

CGI renderings with WSP and normal maps can be relit in Baselight 5 or individual objects can be graded separately. And the powerful colour management has been further developed with a special focus on HDR and has been structured more clearly for the user. DP will be looking at further new features in a future issue. 

The colourist can now give the camera operator direct, meaningful feedback, such as: “I have made the entire image half a stop brighter” or “I have lowered the highlights by one stop”. If a Dailies colourist works with BaseGrade, it is even conceivable that such feedback – like the copy light report in the past – will help the cameraman when working on set. In any case, communication is simplified, especially when the cameraman cannot be physically present during grading.
BaseGrade’s reference point is medium grey, as found on 18% grey cards. The brightness zones are defined from there in f-stops. A correction of three f-stops up or down is the maximum in standard mode and is sufficient. For extreme cases, up to six f-stops can be corrected in “Extended mode”.

User interface

The user interface is based on VideoGrade and FilmGrade. Three main parameters at the top, which are mapped to the three sphere-ring combinations on the blackboard panel. Below this is a visualisation of the current grade as a gradation curve, framed by other parameters such as pivot points. The developers have divided BaseGrade into two tabs. The first page is called Dim/Balance/Light and the second Dark/Balance/Bright. The most important parameter, Balance, is permanently visible and mapped to the centre sphere/ring combination on the blackboard. Flare, Contrast and Saturation are also visible on both sides and can be adjusted via potentiometers. As with all other tools, the colourist can of course adjust everything in the user interface and on the blackboard to suit their individual requirements.

The developers have come up with something special for visualisation using a gradation curve. They superimpose a luma waveform display of the current image over the curve. This means that the colourist can always see which parts of the image he is currently working on and how he should readjust the pivot points if necessary
Pivot points should be readjusted.

Practical test

A little familiarisation time is needed to find your way around. However, the curiosity of having a revolutionary tool at your fingertips makes it easy to get started, after which the results motivate you to continue.
Firstly, the promise of the exact aperture scale is checked. And indeed, increasing the balance by one f-stop is exactly the same as doubling the ISO value in the raw developer. And this applies to all tested cameras from Arri, RED and Sony. The best thing about this is that no raw material is required. For example, if you are working with a mixed ProRes and ArriRAW timeline and all raw shots have been pushed by one f-stop, you can now apply the exact same correction to all shots. A cumbersome switch to the De-Bayer settings is no longer necessary and does not bring any qualitative advantages compared to working with BaseGrade.

Adjusting from shot to shot seems to be another ideal task for the tool. Many colourists divide their grade into a base correction per shot and the creative look. Used as the first layer in the stack, BaseGrade’s scene-linear functionality makes it easier to compensate for exposure differences. Shots with diffuse lens flares, which appear in the image as a raised black, are a common problem when equalising. Even technically high-quality lenses such as the master primes show this effect, especially in scenes with light-coloured backgrounds. If you tried to remove the flare using Lift, for example, the entire image would change and the grade would no longer fit correctly. The flare parameter helps in these cases as it corresponds to optical stray light. If the exposure is basically correct, the differences in the black level can be corrected well using flare.

When copying corrections from shot to shot, the colourist should also pay attention to the flare value and readjust it if necessary. For example, if you copy from a scene with strong stray light, the black in the new shot may be completely washed out. With none of the other controls except Flare can you get it back to a decent level. However, if Flare is set correctly, the black can practically never be crushed. In BaseGrade, the shadows are pressed into a pleasant-looking compression and not clipped hard.

The Saturation slider is also pleasant to use. It is amazing that something as widespread as colour saturation could be improved even further. In direct comparison, BaseGrade behaves visually more evenly than existing implementations. Previously, primary colours such as red, for example, quickly became overweight in the image when saturation was increased. Not so with BaseGrade, the strength of the effect is distributed more evenly across the colour wheel. And the best thing about it is that it works in the same way when desaturating images.

BaseGrade shows its particular strengths through the zone model. The gradation of an image can now be modelled in great detail with just one operator. Previously, this required detailed and sometimes quite fiddly tweaking of the curves in CurveGrade or Keying, but now there is a more intuitive alternative. Extracting detail from a sky normally requires a luminance key on the highlights. With BaseGrade you can get surprisingly far without any secondary correction. Bright is lowered and by raising Light you can tease out the last details until just before clipping. If you initially set the correction too high, it is easier to find the right pivot points. You can then reduce to a realistic level.

If the colourist opts for extremely strong corrections, there is a risk of unsightly effects, a preliminary stage of solarisation, so to speak. Although BaseGrade prevents true solarisation, i.e. negative gradients in the gradation curve, the colourist can flatten the curve so much that the drawing is ruined in certain brightness zones. In these cases, a larger falloff, the transition area of the zone, usually provides a remedy, but at the same time reduces the effect slightly.

BaseGrade can also be convincing with a common technique in colour correction. Cross process toning is a popular stylistic device that makes images more interesting in terms of colour. This involves colouring the highlights and shadows in different shades. Complementary colours are often chosen for this: the shadows, for example, in cold turquoise blue and the highlights in warm orange. Colourists often use HLS keys so that they have better control over the effect. With BaseGrade and its zone model, you can precisely control the strength and colour tone of the effect. This will probably save a few layers in the timeline in the future.

On the subject of VFX pregrading: Flare is an artefact that occurs in the camera optics and distorts the scene linearity. With the help of BaseGrade, the real brightness conditions on set can be reconstructed using a correctly set flare value. Afterwards, Balance is used for complete scene-linear colour correction. The flare correction makes BaseGrade more suitable for scene-linear pre-grading than Exposure/Printer-Lights in FilmGrade.

Conclusion

BaseGrade is powerful, but also complex. A properly set up project is the be-all and end-all for correct functioning. If the colour spaces are not correct, it will not work properly. It is advisable to familiarise yourself with the tool slowly at first and then integrate it into your daily work bit by bit. A good way to start, for example, would be to make all saturation corrections with BaseGrade. The next step would be to use it for basic corrections from shot to shot, and so on.

The new grading concept could also be a good introduction to moving image colour correction for photographers with Lightroom experience and, thanks to the intuitive and aperture-based approach, also for cameramen.
The first few days with BaseGrade were very promising: the tool has the potential to create both more natural and possibly completely new looks. It also makes day-to-day work easier. The decisive factor will be how well it is accepted by users.

Andy Minuth is a graduate of the Stuttgart Media University. He then spent several years at CinePost in Munich, working his way up from junior to senior colourist. He is a beta tester for Baselight and currently lead colourist at 1OOO Volt in Istanbul. His work there focuses on commercials and cinema films. www.specular.xyz