Diffusion: highlight glow without blurring the whole image.
A soft glow around a window and an out-of-focus window are different effects. Halation's Diffusion control aims for the first: it keeps the base image and combines it with a blurred layer extracted from its bright regions. The entire image is not replaced by a blur.
This is a digital highlight effect. It can soften local contrast around lights, but it does not reproduce every optical property of a physical diffusion filter or correspond to a particular manufacturer's filter strength.
What the glass in front of the lens actually does
Physical diffusion filters alter light before it reaches the recording medium. Different designs balance highlight spread, contrast reduction and fine-detail softening differently. There is no single diffusion look or guarantee that every edge remains unchanged.
For example, Tiffen's Diffusion Guide describes Black Pro-Mist's highlight spill, while Digital Diffusion/FX reduces fine-detail resolution with much less visible highlight halation. Soft/FX uses another approach and can produce more widespread softening. The product, strength and scene all matter.
Halation's control focuses on the visible glow around bright regions. If you want to study a specific glass filter, compare photographs made with that filter; this slider's number cannot tell you which optical strength to buy.
What happens to detail
Keeping the unblurred base means the original edge positions remain in the composite. They can still look less distinct: glow brightens neighbouring dark regions, reducing the difference between them and the highlight. That is a change in local contrast, even though the base layer itself was not blurred.
Too much glow can obscure fine texture and flatten a face or skyline. It cannot restore missed focus, and it does not selectively protect eyes or hair. Inspect both the bright edge and nearby texture when deciding how much to keep.
Why this editor uses a screen blend
In the Halation pipeline, diffusion arrives at the last pass and is applied like this: the blurred highlight layer is multiplied by the Diffusion amount and a factor of 1.25, clamped to 0–1, then combined with the clamped base image using a screen blend. For one channel, the formula is 1 − (1 − base) × (1 − glow).
Screen has a property that matters here. It can only ever brighten. Feed it a black pixel in the diffusion layer and the base comes through untouched; feed it a bright one and the base is pushed toward white, but never past it. For example, a base value of 0.2 screened with a glow value of 0.1 becomes 0.28. A base value of 1 remains 1. These are gamma-encoded working values, not measurements of incoming light.
A linear mix would interpolate between the base and the blurred layer, which can darken a bright base pixel when the blur is darker. Screen was chosen here to keep this control brightening-only. It is an artistic blend, not an energy-conserving optical model; physical filters can also reduce highlight intensity and change contrast in ways this operation does not reproduce.
Threshold and Bleed radius control both effects
Diffusion in this tool shares its source layer with halation. Both effects read from the same blurred highlight extraction — the same threshold, the same blur radius, the same quarter-resolution buffer. They differ only in what happens afterwards: halation multiplies that layer by Bleed colour and adds it, diffusion screens it without adding that tint. The source layer still carries the colours of the graded highlights; untinted does not mean that its glow is always neutral white.
The practical consequence is that the Threshold and Bleed radius sliders, which sit visually under the halation controls, also govern diffusion. Raise the threshold and both effects retreat to the brightest parts of the frame. Widen the radius and both spread further. If you have been adjusting diffusion and finding it stubbornly localised, the threshold is the control you actually want.
Sharing this buffer is a design choice for a small, fast renderer. Film halation and lens diffusion have different physical causes and can be adjusted independently in other systems. Here, changing Threshold or Bleed radius affects both, so set the amount of the effect you are not judging to zero while comparing.
How far the presets push it
Diffusion runs on a 0–100 scale. These eight built-in looks use the following settings; the numbers are editor values, not optical filter grades:
- Silver — 8, and Nocturne — 10. Both are contrast-driven looks. Diffusion works against contrast, so it stays minimal.
- Cinematic — 12. Present, but yielding to the halation, which is the louder effect in that look.
- Golden and Ember — 22. A more visible glow for these warm looks.
- Komorebi and Tide — 26. The airiness of these looks is mostly this parameter.
- Bloom — 34, the highest of these eight. A stronger glow is part of its intended softness. Check fine detail as you adjust it.
Using it well
Try a window, sky or small light against a darker background. If the image changes very little, temporarily lower Threshold to see which regions are supplying the glow. If a broad milky veil appears instead of a local halo, raise Threshold or reduce Diffusion. Brightening everything further usually makes that veil stronger.
Start with Halation at zero, then compare Diffusion at zero and at your chosen amount while leaving the other controls unchanged. Adjust Bleed radius last. Check the sky edge, dark branches and fine textures at the size you intend to share, then restore Halation if you want its separate tinted contribution.
Try it on a real photo
Start with Bloom. Open Film and bring Diffusion down to zero, then raise it again. The comparison below shows the whole preset; this one-slider experiment isolates diffusion.
OriginalBloom
Then choose Replace to use your own photo or clip. New here? Read the photo guide or video guide.
References
Tiffen's Diffusion Guide compares the different optical effects of its filter families. Its Black Pro-Mist description also lists highlight and contrast reduction. Halation's screen blend is a digital approximation and is not a measured emulation of those products.