1 The idea
A glow imitates light scattering in the lens: take only what’s above a threshold, blur it at several sizes, and add it back (plus). Intensity sets the strength.
One glow over the whole frame near the end is better than one per element, because a real lens scatters the whole image at once.
2 Hands-on in the mini compositor
3 Deep dive
What a glow really is
Lens glass, dust on the lens and the air all scatter a little light off its path. Every point gets a faint skirt, but you only notice it around things far brighter than their surroundings, like sparks near 62 over desert sand around 0.12.
A real lens skirt is not one Gaussian. It is many sizes stacked: a strong narrow core and a very faint long tail. So a believable glow is built from several blur sizes added together.
In our lab, Glow takes what is above the threshold, blurs it at 3 sizes (size, 2×, 4×), averages them and multiplies by intensity. Same idea as Nuke’s Glow, simpler
Three knobs: what you see and why
Threshold picks what glows. At 1 only values past screen white glow. In the desert shot nothing in the plate reaches 1: the brightest cloud edges, about 0.68, stay clean and only the sparks glow. Drop the threshold to 0.5 and those cloud edges start to bloom; by 0.3 about 17% of the sky glows, so the frame goes milky.
| Knob | What it does | Too much looks like |
|---|---|---|
| threshold | Chooses what glows | (too low) whole frame blooms, blacks lift |
| size | How far the light spreads | Wide fog with no core |
| intensity | How much light is added back | Blown-out white, detail gone |
Energy: plus, not screen, and don’t add the core twice
Nuke’s Merge screen is A+B−AB only while both inputs are at or below 1. Once one goes above 1 it returns the larger value (max). Screen a glow onto sparks at 62 and the cores get no brighter at all; the added light is lost. For light, use plus and control the amount with intensity.
If the glow still contains the original image, plussing it doubles the core. Add only the skirt: (glow − source), clamped so it never goes below 0, then plus.
Before you trust any glow gizmo, test it on a flat Constant. Blurring a flat image changes nothing, so any rise is hidden gain. We measured one gizmo turning a flat 0.5 into about 2.9: almost 6 times the energy.
- 1Key the hot cores
- 2Glow at several sizes
- 3glow − source, clamp ≥ 0
- 4Merge plus
Build it in levels, place it at the end
A good glow usually has at least two levels: a tight strong one from the hottest cores, and a very wide, very faint one. The wide layer lifts the blacks around lights slightly, which makes CG feel shot through a lens. Check it with the viewer gain up.
A glow happens in the lens, so nothing in the scene can sit in front of it. It follows the bright things in the picture, so it goes before any camera shake (see O5). Add it once over the whole frame near the end of the comp, and never stencil or hold out after it; cut things before. For a flash so bright the camera can’t hold it, real footage goes washed out and flat. Lift it with gamma, not gain: gain scales every value by the same ratio, so the contrast stays and the frame never goes flat the way real footage does.
Dial it until it looks right, then try half. Glow is very easy to overdo
4 In real work
In Nuke
- Keyer (luminance) with the range pushed above 1 to pick only the HDR cores → Premult
- Glow: its tolerance works like our threshold; effect only gives the glow alone. Or build your own from several Blur sizes plussed together.
- Merge minus (A = glow, B = source) → Clamp with minimum 0; make sure the maximum doesn’t cut at 1.
- Merge plus onto the comp, output set to rgb so alphas don’t add up.
- Test a gizmo on a Constant of 0.5 and sample the result; if it adds more than you meant, Grade it down.
Common mistakes → the fix
- ✗ A separate glow on every element, all stacked✓ One glow over the frame near the end; split off only things that are truly extreme.
- ✗ Screening a glow onto HDR light✓ Screen falls back to max above 1. Use plus at a lower intensity.
- ✗ Adding the whole glow, source included, so the core doubles✓ Add only (glow − source), clamped at 0.
- ✗ Trusting a gizmo’s defaults without measuring✓ Measure it on a flat Constant first and compensate with a Grade.
- ✗ Using gain for a blown-out explosion flash✓ Lift with gamma and pull saturation down; real overexposure is flat and washed out.
Before you send the shot
- One main glow over the whole frame, near the end
- Sky and ordinary bright surfaces don’t bloom (threshold not too low)
- Glow is plussed, skirt only, core not doubled
- No stencil or holdout after the glow
- With viewer gain up, the lift around lights is smooth, with no rings
5 Check yourself
Where should the glow go?
🔒 Answer the question above and this card opens
1. You screen a glow onto sparks near 62 and the spark cores don’t get any brighter. Why?
2. A flat Constant of 0.5 goes into a glow gizmo (glow only) and comes out near 2.9. What does that tell you?
Where the numbers come from: Computed in your browser. Saturation, Multiply, Clamp, Invert, Keymix, Copy and Grade match Nuke 17.0v2 (checked). Blur, Defocus, Glow, Sharpen, ChromaticAberration, Vignette, LensDistort, Grain, Erode, Toe, the Keyer and Despill are teaching versions, not identical to Nuke’s nodes. Transform here is also a teaching version: bilinear filtering, no concatenation, cropped at the frame edge. Images are our own Blender renders (the green screen is fully code-built; the desert scene uses Gaea 2.3 hills, a Poly Haven CC0 HDRI sky, Poly Haven CC0 rocks and grass, Quixel sand and cliff textures and a Mixamo rig for the robot, with Quixel and Mixamo used in the renders only).