Shape the halation

What it is, briefly

On film, light bright enough to matter passes through the emulsion, hits the base behind it, and reflects back up into the layers it already exposed. The result is a soft ring of extra exposure around anything very bright. It skews warm because red light penetrates furthest and scatters back most readily, which is why the bleed around a headlight looks orange rather than white.

You can approximate this by hand in Resolve — key the highlights, blur them, tint them warm, add the result back on a layer mixer. That works, and it is worth building once to understand the shape of it. Two things below are not reachable that way, though: separating the spill from the highlight that made it, and letting the three colour channels spread by different amounts. Both are what make the effect read as film rather than as a glow.

The controls

Threshold — what counts as a highlight

This picks the region the effect is allowed to start from, and it is the control to set first. It is not a hard cutoff: the effect fades in gradually below the number you set and is fully on just above it, and brighter pixels always contribute more than dimmer ones. So a threshold of 0.775 does not mean nothing under 0.775 is touched — it means the falloff is centred there.

Set it high. If mid-tone skin is entering the mask, the whole face will start to bloom and the shot will read as foggy rather than filmic. Turn on Show Halation Mask and raise the threshold until only the things you would actually call highlights are lit.

Radius and Softness — how far, and how gradually

Radius sets how far the bleed spreads, from 1 to 30. Softness does not change the distance; it changes how gradual the falloff is across that distance. They interact — softness scales the sampling used for the spread — so a large radius with low softness gives a tight, defined bleed, and the same radius with high softness gives a diffuse one.

The usual mistake is reaching for radius when the problem is softness. A wide, hard-edged glow looks like a filter. A tighter, softer one looks like light.

Edge Isolation — the one nobody guesses

This is the most useful control here and the least obvious from its name. At 0, the whole bright region blooms, including the highlight itself — the sky gets brighter as well as bleeding into the trees. Turn it up and the effect keeps only the spill that lands beyond the highlight's own edge, so the sky stays where it was and only the boundary glows.

That distinction is most of the difference between halation and a plain glow. If your highlights are already exposed the way you want and you do not want them lifted, this is the control that lets you have the bleed without the brightening. Around the middle of its range is a good place to start.

Colour Tint and Green Fringe

The tint defaults to a strong red-orange, which is the honest starting point — real halation is red-dominant. Green fringe then does something more specific: it makes the three channels spread by different amounts, with red reaching furthest, green staying tighter, and blue confined nearly to the core. Raise it and you get colour separation across the bleed rather than a single flat tint.

It is off by default and it is easy to overdo. A little produces the subtle green-to-red transition you see on real film around a very bright source. A lot produces something that looks like a chromatic aberration problem.

Model, Channel Spread, Tail and Specular Drive

Plugin version 1.7.0 added a second halation model and made it the default. Classic spreads one mask and is what every release before it did. Film gives red, green and blue their own blur widths and an exponential tail rather than a Gaussian one, which is closer to how the dye layers actually scatter light — Channel Spread sets how far apart those widths are, Tail how long the falloff runs, and Specular Drive how much extra weight anything brighter than white carries into the bloom.

Green Fringe still applies in both models and still does what it says below. Channel Spread is the physical version of the same idea — the difference is that Green Fringe reshapes one mask after the fact, while Channel Spread blurs the three channels by different amounts in the first place.

Amount — last, and lower than you think

Set everything else with the mask on, then turn the mask off and bring the amount up from zero until you can just barely tell. Then stop. The effect is added on top of the image rather than blended with it, so it climbs quickly, and an amount you can point at has almost always gone too far.

A real starting point

These are the values from the graded frame on the front page of this site — a golden-hour exterior with a bright sky behind the subject. They are not a preset and they will not be right for your shot, but they are a real set of numbers rather than round ones invented for an example.

Getting it right rather than obvious

• Set the threshold with the mask on, before anything else. Everything downstream depends on it being right.
• Raise softness before you raise radius. Distance is rarely the problem; abruptness usually is.
• Use edge isolation when the highlights are already where you want them and you only want the bleed.
• Check it on a dark scene and a bright one. Values tuned on a night exterior will be far too strong on a daylight interior.
• Watch it in motion. Halation on a moving highlight is much more visible than on a still frame, which is where most people over-cook it.

Every control above is the halation module in Helios Grade, our film-look plugin for Resolve's Color page, and so is the mask view — being able to see what an effect is touching before committing to it is most of the reason it exists. There is a full breakdown of this grade if you want the other nine modules too. It runs free and watermarked with no expiry.