1 The idea
Camera projection turns the camera that shot the plate into a “projector” that throws the image onto simple geometry lined up with the scene. Here that is one flat ground plane, one card for the hills and a far sky card. Then a second camera films it and can move.
The catch: the plate has a fixed number of pixels. The closer the new camera gets, the more the near ground gets magnified (stretched) until it goes soft. Anything without geometry under it, like the rocks, is painted flat onto the ground and smears as the camera moves.
stretch = 1 / σ_min( ∂plate px / ∂screen px )2 Hands-on
Move the camera until the worst stretch sits between 1.8× and 2.0×: the edge of the safe zone
Try the stretch map: green = as sharp as the plate, red = magnified more than 2×
3 Deep dive
A projector and a new camera
Two cameras do two jobs. The projection camera must match the plate camera exactly and stay frozen on the frame you project. The render camera is the one that moves.
If the projection camera moves with the render camera, the picture travels with it and never sticks to the geometry. Think of it like a tracker’s reference frame.
- 1Painted / chosen frame
- 2Projection camera (frozen)
- 3Project onto geometry
- 4Render with the moving camera
Reading the stretch number
Stretch is how many new screen pixels one plate pixel covers in its worst direction. 1× = as sharp as the plate; 2× = pixels blown up twice in that direction, and softness starts to show.
In the figure the nearest ground is about 4.4 m from the camera. Pushing in only about 1.4 m already stretches it close to 1.9×, because near ground is seen at a glancing angle and grows on both axes. A 2 m sideways move also reaches about 1.8× from the ground shearing.
| Push in | Worst stretch |
|---|---|
| 0 m | 1.00× |
| 0.5 m | ≈ 1.28× |
| 1.0 m | ≈ 1.60× |
| 1.4 m | ≈ 1.89× |
| 2.0 m | ≈ 2.36× |
Fixing resolution
A texture shown smaller on screen keeps all its detail; a texture shown bigger cannot invent new detail. So project from the frame where the camera is closest to the subject, or paint a texture finer than the plate (for example 2×) for the areas the camera will approach.
Soft things like fog, sky or a grade tolerate magnification and can stay low-res. Sharp surfaces close to camera need the pixels.
Project from the closest frame, not the first one
How too-simple geometry shows up
The rocks have no geometry under them, so they are projected flat onto the ground. Still, they look fine; moving, they lean and slide with the ground. In the figure the orange ring is where the rock should be and the cyan dot is where the projection puts it.
Either build rough geometry for the rock, or paint the rock out of the ground projection and give it its own layer.
4 In real work
In Nuke
- A Camera matching the plate: duplicate it for projection and freeze the copy on the projection frame
- Image to project + projection camera → Project3D
- Project3D → a Card or ReadGeo geometry lined up with the scene
- Gather in a Scene and render with ScanlineRender through the moving Camera
- Compare against the plate on the projection frame: it must match before you move anything
Common mistakes → the fix
- ✗ Using the same animated camera to project and to render✓ Use a separate frozen projection camera so the image sticks to the geometry
- ✗ Projecting from the first frame when the camera pushes in later✓ Project from the closest frame, or paint a finer texture
- ✗ Projecting raised objects onto a flat ground✓ Give them rough geometry, or paint them out and put them on their own layer
- ✗ Moving the camera beyond what the plate covered✓ Watch for areas outside the plate, then paint extra or limit the move
Before you send the shot
- On the projection frame the render matches the plate
- No sharp surface stretched past about 2× anywhere in the shot
- No outside-the-plate area shows in frame
- Raised objects don’t slide with the ground
5 Check yourself
The new camera pushes toward the ground and the near ground goes soft. What fixes the cause?
🔒 Answer the question above and this card opens
1. The projected image follows the camera around and never sticks to the ground. Likely cause?
2. A shot pushes in on a statue from frame 1 to 100. Which frame should you project from?
Where the numbers come from: The scene is rebuilt in your browser from our desert plate and its Blender depth pass (35 mm lens, 36 mm sensor, 1.6 m high, tilted 4° down). Parallax numbers use the pinhole formula: move the camera T m and a point Z m deep shifts ≈ 622·T/Z px in a 640-wide frame. The layer split, the “stretch” measure, the depth mesh and the automatic clean plate are teaching versions (object edges and rock removal are simplified), not Nuke output.