Halation

Halation is the film's own glow: bright highlights bleeding a colored mist into the dark around them, the way light scatters back through a film's base and fogs the emulsion. Heeler draws it after Depth of Field and the Lens Flare, in scene light, so the profile rolls the bloom into the highlights the way it would have on film.

Controls

• Threshold is the brightness a highlight must reach to bloom, in stops above middle gray. Lower it and dimmer lights join in.
• Background gain keeps the effect to high-contrast borders: halation is hard to see against a bright field, so the higher this is, the darker the surroundings must be for the bloom to show.
• By depth weights the bloom by distance on the depth plane, so far lights bloom more (or less) than near ones.
• View isolated regions, the eye in the section, shows only what will bloom, white on black, so the thresholds are tuned against exactly that.
• Spread is how far the light leaks; Diffusion runs the halo from a hard visible ring to a soft blended gradient.
• Format scales the spread the way the film gauge would have: 8mm and 16mm are enlarged more for viewing, so their halation reads larger; 65mm reads tighter.
• Hue and Saturation set the glow's color; the red-orange of film by default, zero saturation for the white mist. Blue compensation keeps the bloom visible on blue highlights, like skies, that would neutralize it.
• Strength is the bloom's brightness and Mix how much of it shows.
• Bloom adds a wider, neutral glow around the same sources, the diffusion-filter look, with its own radius.

Halation on a perfectly clean digital frame can look pasted on; the Grain section sits beside it for that reason, and the lens presets set both.