1 The idea
Layer apps (After Effects, Photoshop) stack images; the upper layer sits over the lower one, and a blend mode such as Normal or Add is a kind of merge. Node apps (Nuke, Fusion, Natron, the Blender compositor) write the same steps as a chain of nodes.
Change the layers in the Layers panel and watch the node graph follow. Normal = over, Add = plus, Opacity = mix. For a small shot both are the same; once you reuse one image in several places or branch the work, nodes show it more clearly.
2 Hands-on in the mini compositor
3 Deep dive
Same maths, drawn differently
Most blend modes in a layer app have a twin among Nuke’s Merge operations. Know the Merge formulas and you can read any app.
| Layer app | Nuke | Formula |
|---|---|---|
| Normal | Merge over | A + B·(1−a) |
| Add | Merge plus | A + B |
| Multiply | Merge multiply | A·B |
| Opacity | Merge mix knob | B + mix·(op − B) |
| Track matte (alpha) | Merge mask | B·a |
Top of the stack = A, below = B
A layer stack reads bottom to top. In Nuke every Merge takes B, what is behind, and A, what goes on top. Five layers become four Merges running down the B pipe.
An adjustment layer is a colour node, such as a Grade, placed after the Merges. It affects everything combined up to that point.
- 1Plate (B)
- 2Over robot
- 3Plus sparks
- 4Grade (adjustment)
- 5Write
Opacity is a mix
The Merge mix blends the result back toward B: B + mix·(op − B). For over at mix m that is m·A + B·(1 − m·a), the same as fading the element to m, just like the opacity of a Normal layer.
For plus at mix m you get B + m·A, the same as Add at opacity m.
Where each one shines
Nodes shine when you branch: one robot Read feeds both the robot and its reflection, you fix it once at the source, you can view any node, and the data flow is visible.
Layers shine in short motion-design work where timeline timing matters. Fusion, Natron and the Blender compositor think in nodes like Nuke; in Blender, Alpha Over plays over and Mix (Add) plays plus.
Whenever you switch apps, check first how it handles premultiplied alpha
4 In real work
In Nuke
- Each layer → a Read (or Constant/Roto) on its own branch
- Blend mode → Merge operation (over, plus, multiply…)
- Opacity → the Merge mix knob
- Adjustment layer → a Grade below the Merges, on the main pipe
- Track matte → Merge mask, or Copy the alpha then Premult
- Precomp → a Group, or a Write read back in
Common mistakes → the fix
- ✗ Plugging the plate into A and the element into B✓ Over then lays the plate on top. B is always what is behind.
- ✗ Using screen for light because that is the habit from layer apps✓ In Nuke, once either input goes above 1, screen returns max(A, B) instead of adding, so the plate’s light is lost. Use plus for HDR light.
- ✗ Duplicating a Read every time you reuse an image✓ Branch from one Read (with a Dot); one fix updates every use.
- ✗ Putting an adjustment-style Grade on an element’s branch✓ On a branch it only touches that element; for the whole picture, place it after the Merge.
Before you send the shot
- Every Merge has what is behind in B
- Operations fit the element (light = plus, solid = over)
- Reused images branch from one Read
- Whole-picture grades sit after the Merges on the main pipe
5 Check yourself
You need the same robot image in two places (the robot and its reflection). Which way is more direct?
🔒 Answer the question above and this card opens
1. A Normal layer at 50% opacity matches what in Nuke?
2. An adjustment layer at the top of the stack affects every layer. Where does the Grade go in Nuke?
Where the numbers come from: Merge, Grade, Premult and Unpremult here match Nuke 17.0v2 in all 109 test cases (numbers sampled from real Nuke). Blur and Transform are simplified: close, not identical to Nuke. All images are Blender renders of our own scene: a code-built layout with hills made as Gaea 2.3 terrains (Community edition, built headless), a Poly Haven CC0 HDRI sky, Poly Haven (CC0) rocks and dry grass, Quixel sand and cliff textures, and a robot on a Mixamo rig (Quixel and Mixamo are used in the renders only; no source files ship).