1 The idea
A cleanup that looks clean on one still can fail once it plays: the patch boils, drifts off its track, or leaves a smooth spot with no grain. Two difference views catch these: against the original to see where the work is, and against the previous frame, played, to see boiling and frozen or missing grain.
Here are four versions (v1–v4) of a marker cleanup on a 24-frame shot. Switch the viewer between “Result”, “vs original ×8” and “vs previous frame ×8”, press Play, and watch both the picture and the bars under it. Then give each version a verdict.
outside the patch: difference to original = 02 Hands-on
Give all four versions the right verdict: Passes, Boils, Drifts or Smooth spot
Play each version in “vs previous frame ×8” and see whether the patch behaves differently from the grain around it. Then switch to “vs original ×8” and follow it to the last frame: does the changed area stay on the markers?
3 Deep dive
Difference to the original: where the work is
A Merge set to difference between the original plate and the result leaves values only where you worked. Everywhere outside the area you touched must be 0; anything showing up out there means some node touched the plate by accident.
This view also catches drift: played to the end, the changed area should stay on the markers. If it grows or slides away, the track has been lost.
Paint differences are usually tiny: push the viewer gain well up before you decide something is 0.
Difference to the previous frame: play it
This view compares each frame with the one before. Plate grain changes every frame, so the whole picture shimmers evenly. A good patch shimmers like its surroundings, no more and no less.
A single still in this view tells you little; you have to play it to see a patch flicker or sit unnaturally still.
| Fault | Which view | What it looks like |
|---|---|---|
| Boils | Difference to previous | The patch flickers harder than the grain around it, because each frame’s strokes differ |
| Smooth spot | Difference to previous | A still, dark hole in the dancing grain |
| Drifts | Difference to original | The changed area grows and moves off the markers |
| Passes | Both views | Stays on the markers; grain in the patch matches its surroundings |
Every frame, and the ends of the shot
Tracks often fail far from the reference frame, which is usually near the start or end of the shot. For a tracked or projected patch, toggle it on and off at the first, middle and last frames and look for drift. For the stretch where the old patch no longer holds, build a second patch and blend across from the old one to the new.
Once a stroke is done, move to the next frame and check that it is still there; a stroke meant to cover the whole shot needs its lifetime set to all frames. And after any fix, scan every frame rather than eyeballing a few.
- 1First frame
- 2Middle frame
- 3Last frame
- 4Play the whole shot
- 5Scan every frame
When the tracked feature leaves the frame
A fix tied to a track slides off as soon as the tracked feature leaves the picture. In this room’s drifting version the track is lost a few frames past the middle of the shot, something no still from early in the shot could ever show.
The fix is to fade the repair out where the feature is gone, or start a new patch with its own track and dissolve to it, then run the every-frame QC again.
A per-frame difference curve that starts climbing from one frame on is often the first sign of drift.
4 In real work
In Nuke
- Merge (difference) the original plate against the result, view it in the Viewer with the gain pushed up: outside the work area must be 0.
- TimeOffset the result by 1 frame and Merge (difference) it with itself to get the difference to the previous frame.
- If the patch comes from a FrameHold, step one frame off the held frame and check the strokes are still there.
- CurveTool (Avg Intensities) on the difference, inside a box around the patch, gives a per-frame difference curve.
- Render and open it in the Flipbook; play both the result and the difference views through to the last frame.
Common mistakes → the fix
- ✗ Judging on a single frame✓ Play it: boiling and drift only show in motion.
- ✗ Looking only at the result, never at a difference✓ Check both the difference to the original and the difference to the previous frame.
- ✗ Checking the middle of the shot but not the ends✓ Toggle the patch at the first, middle and last frames; tracks tend to fail at the ends.
- ✗ Gain on the difference view too low to see small changes✓ Push the viewer gain well up before calling anything 0.
Before you send the shot
- Difference to the original is 0 everywhere outside the area you touched.
- Difference to the previous frame, played: the patch neither flickers nor sits as a still hole.
- Patch toggled at the first, middle and last frames: no drift.
- Strokes meant for the whole shot are there on every frame.
- Every frame scanned, including both ends of the shot.
5 Check yourself
In the “vs previous frame” view, played, the grain dances all over the frame, but the patch is a still, dark hole. What does it mean?
🔒 Answer the question above and this card opens
1. Where should the difference between the original plate and a correct cleanup show values?
2. A tracked patch looks perfect on the middle frame. What next?
Where the numbers come from: All four versions are built in the browser from our own Blender renders (the same 24-frame markers shot as PC3): the good one uses the clean render on every frame, “Boils” adds a small random shift and brightness change per frame inside the patch, “Drifts” adds a growing offset from a fixed frame on, and “Smooth spot” leaves the grain out. These are teaching versions with our own grain; the flip-book plays in the browser (Nuke’s Flipbook plays rendered frames).