1 The idea
Transform moves every channel at once, alpha included. Our robot carries its shadow in alpha, so the shadow follows, which is fine on flat ground. Scale the robot a lot, though, and the shadow scales with it while the contact point slides, and the robot’s size no longer fits where it stands in the plate’s perspective.
Rotation and scale happen around the frame centre (the pivot); in Nuke you can set the center yourself.
2 Hands-on in the mini compositor
3 Deep dive
Transform moves every channel together
r, g, b and alpha all move by the same amount. Our robot carries its shadow in alpha, so the shadow always moves, scales and rotates with it.
Sliding along flat ground is fine. Rotating turns the shadow too, so it no longer points away from the plate’s sun. A big scale keeps the shadow in proportion with the robot, but the contact point slides (next section) and the robot’s size stops fitting the spot it stands on in the plate’s perspective.
| Knob | What it does | Watch out |
|---|---|---|
| translate | Moves in x and y | Part-pixel values cause a resample (see T2) |
| rotate | Turns about the center | The shadow turns away from the sun |
| scale | Scales about the center | Position slides towards or away from the center |
| center | The pivot point | Fixed at frame centre in this lab |
The pivot maths
Scaling by s about a center c sends a point p to c + s·(p − c). This lab is 640 px wide, so the center is at x = 320.
If the robot’s feet are at x = 200 (120 px from the center) and you scale by 0.8, the feet land at 320 − 96 = 224: 24 px towards the middle without touching translate. The same happens vertically, which is how the feet leave the ground.
- 1Put the center at the feet
- 2Scale
- 3Feet stay planted (for a size change only; to push it farther away, see T3)
Whatever sits on the center doesn’t move when you scale or rotate
The ground contact is what the eye checks first
Viewers judge whether an object “sits in the shot” by where it touches the ground. Feet floating or sinking by a few pixels look wrong straight away, even when colour and light are perfect.
In the desert plate, use rocks at a similar distance as reference for size, sharpness and where they meet the ground.
In a shot with camera movement
Track the plate with a Tracker and pick a reference frame, the frame with zero movement. Place the robot with a separate Transform on that frame, before the match-move Transform exported from the Tracker.
Keep those two Transforms back to back so Nuke merges them into a single resample (see the next lesson).
4 In real work
In Nuke
- Transform: set the center on the point that must stay put, such as the feet, before touching scale or rotate.
- Work on premultiplied CG so the edges filter cleanly; straight colour picks up fringes at the alpha edge.
- Tracker: set the reference frame, then export a match-move Transform.
- Order: Read robot → placement Transform → match-move Transform → Merge (over) onto the plate.
Common mistakes → the fix
- ✗ Scaling with the center left mid-frame, then fixing the floating feet with translate every time✓ Move the center to the feet first; they stay planted as you scale.
- ✗ Rotating the robot to tilt or straighten it while its shadow is in alpha✓ The shadow turns away from the sun. For a big change, ask for a re-render or a separate shadow layer. (A 2D rotate only turns the image in the frame plane; it can’t make the robot face another way.)
- ✗ Scaling CG far above the size it was rendered at✓ Stretched pixels go soft. Ask for a render at the size you need.
Before you send the shot
- Feet sit exactly on the ground for the whole shot
- Robot size agrees with rocks at a similar distance
- The shadow still points the way the plate’s light says
- Placement and match-move Transforms sit back to back with nothing between
5 Check yourself
You scale the CG with a Transform and its feet leave the ground. Why?
🔒 Answer the question above and this card opens
1. The center is at x = 320. Where does a point at x = 200 go after a 0.8 scale?
2. You rotate the robot 30° and the shadow looks wrong. Why?
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).