1 The idea
Roto is drawing an alpha by hand. A soft edge (feather) stands in for the real object’s soft edge. Look at the a channel to see the feather as the band where alpha ramps from 1 to 0.
Match the feather to the softness of the real edge in the plate. Too hard looks like cut paper; too soft lets the old background show through.
2 Hands-on in the mini compositor
3 Deep dive
Feather is the alpha ramp
Switch the Viewer to the a channel and the shape has three zones: white inside (1), black outside (0), and a graded band between them. That band is the feather.
In this lab the feather band straddles the outline: half inside, half outside. A feather of 30 gives a band about 30 px wide, and the outline you drew sits exactly at alpha 0.5.
| Feather | What you see | What it means |
|---|---|---|
| Too hard | A crisp line, like cut paper | Sharper than the real edge in the plate |
| Right | You can’t tell there is a roto | Matches the real edge softness |
| Too soft | A ring of the old image shows round the edge | It reaches further than the real edge |
Read the softness from the plate, don’t guess
In the desert plate, a rock in focus has an edge only 1–2 px wide, while the far hills, out of focus, ramp over many pixels. One plate can need different feathers depending on distance and focus.
How to measure: zoom into the real edge and count the pixels from fully object to fully background. That count is the feather width you want.
- 1Zoom into the real edge
- 2Count the ramp pixels
- 3Set the same feather
- 4Wipe against the plate
Fast movement needs motion blur
A fast-moving object blurs along its direction of travel. The side edges can stay sharp while the edges facing the motion smear out. An even feather all round can’t copy that.
The right way is to animate the shape with the object, then turn on the roto’s motion blur with a shutter that matches the camera that shot the plate (a 180° shutter is 0.5 in Nuke), and compare the smear length with the plate.
Feather is for still edges, motion blur is for moving ones
A good roto is one nobody notices
Use the fewest points that still hold the outline. Fewer points move smoothly between keyframes, so the edge doesn’t chatter or boil in playback.
Split the object into pieces that move separately, such as upper arm, forearm and hand. Key the extreme poses first, then fix the frames in between. If there is something to track, track it and let the shape follow; it saves a lot of hand work.
4 In real work
In Nuke
- Roto: draw Beziers with few points, one shape per part that moves rigidly.
- Feather: use the shape’s feather value when the edge is evenly soft, or pull individual feather points where the softness varies.
- Tracker: track a feature on the object, then drive the Roto shape or layer transform from it.
- Turn on motion blur in the Roto and match the shutter to the plate.
- Check: view the alpha in the Viewer and wipe against the plate before using it as a mask for a Merge or Keymix, or a Copy into alpha.
Common mistakes → the fix
- ✗ Giving every shape a big feather “to make it seamless”✓ More feather than the real edge lets a ring of the old image through. For object edges, measure from the plate every time.
- ✗ Using a big feather instead of motion blur on something fast✓ Feather softens every side equally; motion blur smears along the direction of travel. Turn on the roto’s motion blur.
- ✗ One big shape with many points over the whole body, fixed every frame✓ Split it by joints, use few points and key the extremes first, so the edge doesn’t chatter.
- ✗ Checking the roto on one frame only✓ Play the whole shot, both the alpha and the comp. Chatter only shows in playback.
Before you send the shot
- Feather matches the real edge softness in each part of the plate
- Fast parts have motion blur, with the smear length matching the plate
- Full playback: no chatter, no slipping
- No ring of the old image round the edge in the comp
5 Check yourself
How much feather is right?
🔒 Answer the question above and this card opens
1. A rock in focus has a real edge about 2 px wide. Roughly what feather should you use?
2. A fast-swinging hand smears in one direction only. How do you roto it?
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).