How does colour grading work?

Straight out of a cinema camera, footage looks flat: grey blacks, dull colours, no punch. That is on purpose. The camera records in log, a curve that gives every stop of light (every doubling of brightness) about the same number of code values. A sunny street has a sky many times brighter than a face in shade, and log keeps both.

A normal video picture, Rec.709, is made for screens. It spends its codes where our eyes look, and runs out just 2.5 stops above a grey card. Anything brighter, like the sky, is clipped to flat white forever. A log file from a cinema camera keeps about 7.8 stops above grey. That extra room is why cameras shoot log (see CineCameraClear for dynamic range).

To watch log you need a LUT, a lookup table. A 1D LUT is a list: for each input level, an output level, one curve per channel. A 3D LUT is a cube of colours: for a grid of input colours (say 33 × 33 × 33) it stores the output colour, and the graphics card blends between the nearest points. A 3D LUT can do things a 1D one cannot, like changing saturation or turning the camera's wide colours into screen colours. The cube of dots on the stage is that LUT, moving every colour to its new place.

The first job in any grade is the primary correction: make the shot look natural and balanced. Colourists do it with three colour wheels, one for each part of the picture.

Lift moves the blacks and shadows. Gamma moves the middle tones, where faces usually sit. Gain moves the highlights. Turning a wheel's ring makes that part brighter or darker. Dragging its centre towards a colour tints that part: warm shadows, cool highlights, or a green cast taken out of the middle. Next to them sit exposure, white balance (temperature and tint), contrast and saturation.

Screens and eyes lie, so colourists trust the scopes. The waveform plots brightness from 0 at the bottom to 100 at the top, for every column of the picture. The RGB parade shows red, green and blue side by side: a neutral grey has all three at the same height. The vectorscope is a colour wheel: distance from the centre is saturation, angle is hue. Every human skin tone, light or dark, lands close to one line, the skin-tone line. Push anything past 100 and it clips; push below 0 and the shadows are crushed. Colour temperature on set is explained in LightingClear, and how the eye sees colour in EyeClear.

A scene is cut from many shots, filmed hours apart, sometimes on different cameras. The sun moves, clouds pass, and every camera sees colour a little differently. If the shots are not matched, the audience feels a jump at every cut, even if they cannot say why. Matching is a huge part of a colourist's day. How the cuts themselves work is in EditClear.

Colourists match with the scopes, not by eye. They put the two shots side by side and compare the RGB parade: something that should be neutral grey must have red, green and blue at the same height in both. That fixes the balance. Then the exposure: the same grey card should sit at the same level on the waveform. Last, saturation and contrast: the same coloured object should sit the same distance out on the vectorscope.

A colour chart filmed at the start of each setup makes this much easier, because everyone knows exactly what colour each patch should be. Grading can match colour and brightness, but not everything: it cannot move a shadow that falls the wrong way.

Primaries change the whole picture. Secondaries change only part of it. There are two main tools.

A qualifier picks pixels by their colour: a range of hue, saturation and brightness (HSL). Choose the blue of the sky and only the sky turns deeper; choose the purple of a shirt and you can make it red for a continuity fix. Colourists check the selection as a matte, a black-and-white mask, to make sure nothing else sneaks in.

A power window is a shape, like a soft circle, drawn on the picture. Brighten inside it to lead the eye to a face, darken outside it to make a vignette. People move, so windows are tracked: the software follows a point from frame to frame and moves the window with it. VFX artists track in the same way (see VFXClear).

Every strong look must protect skin. Push a whole shot towards blue-green and faces go grey and ill. So colourists key the skin tones and hold them out of the push. The eye forgives a strange sky, but never a strange face.

After correction comes the look: the colour that tells you how to feel. Warm and golden, cold and steely, faded and old. Film makers plan it early with a palette (see ConceptArtClear) and the colourist builds it in the grade, often saving it as a creative LUT so every shot gets the same treatment.

The most famous modern look is teal and orange. It works because of complementary colours: orange and teal sit opposite each other on the colour wheel, and opposites make each other look stronger. Skin, of every shade, sits on the orange side. So if you push shadows, skies and backgrounds towards teal, the faces pop out. Watch the vectorscope squeeze every colour onto one line.

Other classic looks: bleach bypass (silvery, harsh, low colour), film print emulation (a gentle S-curve and blacks that never quite reach zero), day for night (filmed in daylight, darkened and turned blue), sepia and noir. Amélie (2001) is famous for its warm greens and golds. A look is only a starting point: colourists still fine-tune it shot by shot.

Colourists work in a grading suite: a dim room with neutral grey walls and a calibrated reference monitor. Our eyes adjust to whatever is around them, so a bright or coloured room would fool them. The HDR standard even says how bright the wall behind the screen should be: a dim, neutral grey of about 5 nits. The monitor is measured with a probe so that its grey really is grey. Projection in cinemas is its own story (see ProjectorClear, coming soon).

Brightness on a screen is measured in nits (candelas per square metre). Normal SDR video is graded for a white of 100 nits, and a cinema screen is only about 48. HDR TVs reach 1,000 nits or more, and HDR masters are often made on 1,000 to 4,000 nit monitors. In HDR a white shirt stays around 200 nits but the sun, lamps and sparkles get ten times more room, so they keep their detail and colour. That is why films are delivered in several versions: an HDR grade, an SDR trim and a cinema version.

Screens also differ in how saturated their colours can go: their gamut. On the CIE diagram, the horseshoe is every colour a human can see (how the eye does it is in EyeClear). Rec.709 covers about a third of it, DCI-P3 (cinema) more, and Rec.2020 about two thirds of the diagram's area (other ways of measuring put it nearer three quarters). India's digital intermediate industry took off in the early 2000s: Prime Focus says its 2003 grade of Qayamat was the first fully colour-graded Indian film, and today studios such as Prime Focus and Red Chillies grade films in-house.