Curves, and the shape of contrast

What a curve is

A tone curve maps every input value to an output value. The horizontal axis is what comes in, the vertical is what goes out. A straight diagonal line changes nothing. Bend it and you have changed the relationship between input and output across the whole range.

The wheels give you three broad handles. A curve gives you as many as you want, which makes it both more powerful and easier to ruin an image with.

The S-curve

The most useful shape in colour work. Pull the bottom quarter of the line down slightly and the top quarter up slightly, leaving the middle on the diagonal.

What this does: darks get darker, lights get lighter, and the mid-tones — where faces live — get steeper, meaning small differences in the middle become larger differences in the output. That is why an S-curve reads as "more contrast" and also as "more detail in the faces", even though no detail was added.

Keep it gentle. Two control points, each moved a small amount. A steep S-curve clips both ends and produces the look of an over-processed phone photo.

The reverse S, or the faded look

Pull the bottom up and the top down. Shadows lift off black, highlights come down from white, and the image loses contrast and gains a matte, filmic quality.

This is the mechanism behind almost every "film emulation" and "faded" look you have seen. It is a small amount of lifted black and rolled-off white. Once you can see it as a curve, you can build any of those looks yourself rather than downloading one.

Where to put the control points

A practical constraint: do not put a control point where the important values live. A control point pins that value in place; the curve bends around it.

So for a shot where faces sit around 45% brightness, put your control points at roughly 20% and 80% and leave the region around 45 unpinned, so it can move as a whole rather than being anchored in the middle of the face.

Per-channel curves

Switch the curve to red, green or blue individually, and you are now shaping colour rather than brightness.

Lift the bottom of the blue curve and your shadows go blue. Pull down the top of the blue curve and your highlights go warm. Together, those two moves produce the cool-shadows-warm-highlights split that most commercial grading uses, and you have built it from two control points rather than a preset.

The warning: per-channel curves change saturation as well as hue, because saturation is the distance between channels. Small moves.

Hue-versus curves

Resolve's colour page has a set of curves that take hue as their input, and they solve problems the ordinary curves cannot:

• Hue vs Sat — select a hue range and change its saturation. The standard use is taking saturation out of an over-saturated red, or desaturating a distracting green sign without touching anything else.
• Hue vs Hue — shift one hue toward another. Used for pulling an orange-red skin tone toward a more natural orange, or moving a foliage green away from yellow.
• Hue vs Lum — change the brightness of one hue.
• Lum vs Sat — change saturation according to brightness. The standard use is pulling saturation out of the deep shadows, which stops noise in the darks looking coloured and is one of the cheapest ways to make cheap footage look better.

These are all in the free version, and Lum vs Sat on the shadows is worth applying almost every time.

Curves against wheels

When to use which:

• Wheels for the primary correction: white point, black point, mid-tones, balance. Broad and predictable.
• Curves for shaping: contrast, a specific tonal region, a colour relationship across the range.
• Hue-versus curves for isolating one colour without drawing a mask.

A common working grade is: wheels for the correction node, a gentle S-curve for the contrast node, Lum vs Sat for the shadows, and a hue-versus curve if one colour is misbehaving. That is a complete grade and none of it needs a plug-in.

How far to go

The reliable check is the before-and-after toggle, used frequently. Chromatic adaptation means you drift: twenty minutes into a grade your eye has accepted whatever you have been doing and you will keep pushing.

Second check: look at it tomorrow. Almost everybody's grade from last night is too warm and too saturated.

Today

Take one shot, apply a gentle S-curve with two control points, and toggle it on and off. Then apply a much stronger one and see where it starts to clip on the waveform.