Interlace combing
Also called: comb artifacts, interlacing artifacts, mouse teeth, field tearing
Interlace combing is the toothed edge on moving objects when interlaced video is shown as full frames. See why it happens and how to approximate it.
WHAT CAUSES IT
Interlace combing is the pattern of alternating shifted lines along moving edges that appears when two video fields are woven together into one frame. Interlaced video, such as 59.94 fields per second NTSC, records the odd numbered lines at one moment and the even numbered lines about 1/59.94 of a second later. A television shows the fields in sequence, so the eye merges them. A digital file or a progressive display that stores the two fields together as one frame displays both moments at once. Anything that moved between the two samples is drawn at two positions, one on each set of lines, which creates a comb of horizontal teeth along vertical edges. The size of the teeth equals the distance the object moved in one field time. A wrong field order, top field first versus bottom field first, makes the problem worse by swapping the time order, causing motion to stutter back and forth. Deinterlacing filters remove combs by interpolating one field or blending both.
WHAT IT LOOKS LIKE
Horizontal teeth along the edges of moving objects, like the teeth of a comb. A person walking across the frame has sawtooth edges on the leading and trailing sides of their body, and the width of the teeth grows with speed. Pans make the whole picture comb at every vertical edge, while static areas look sharp. Text scrolling across the screen turns into horizontal dashes. Wrong field order gives a juddering look in which motion jumps backward every other field. The picture can look sharp on still frames but combs severely when paused during action.
WHERE YOU SEE IT
Standard definition and 1080i interlaced footage played on a progressive display or exported without deinterlacing: DV camcorder tapes, broadcast recordings, DVDs of television, and sports and news archives. It is a technical mistake in modern video, though many people recognize it as a sign of amateur or archival conversion. Editors add it to give digital footage the feel of a bad transfer, or in a faked mistake that sells an older workflow.
RECREATE IT
STATIC DRIFT
CONTROLS STATIC BREAK, DRIFT, STATIC COMPOSITE
No control copies the two-field time offset exactly, so this is an approximation that suits moving shots. STATIC BREAK and DRIFT displace lines sideways, so keep both low and aim for a fine alternating shift on moving edges, not a broad tear. Use STATIC COMPOSITE to keep the effect subtle. It reads best on a pan or a fast moving subject, where the real artifact appears.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Choose footage with movement; combing needs motion. A locked-off still frame will not show it. 2. Pre-compose the footage and apply STATIC DRIFT. 3. Start with STATIC BREAK and DRIFT near their minimum. 4. Raise them in small steps until alternating lines on moving vertical edges look shifted, and static areas still look clean. 5. If the whole frame is shifting, the settings are too high; real combing only appears where something moved. 6. Use STATIC COMPOSITE to reduce the strength if the result looks like a tear instead of teeth. 7. Zoom to 200 percent to judge the structure, since combing is a fine-scale detail. 8. If you have real interlaced footage, consider leaving the fields unprocessed during delivery so the genuine artifact appears naturally. 9. Be honest in your expectations: this is an approximation intended for stylistic work, not a technical field simulator.
QUESTIONS
- What causes combing in interlaced video?
- The two fields of a frame are captured at different moments. When both are woven into one picture, a moving object appears at two positions on alternate lines, and the mismatch looks like a comb along its edges. The teeth are a record of motion, and their width tells you how far the subject moved in a single field time.
- How do I remove interlace combing?
- Deinterlace the footage with a method that suits the material. Bob or field-separation methods keep full motion at the cost of vertical detail, while blending hides combs but adds ghosting. Setting the correct field order first is essential. For archival work, deinterlace once, at the start of the chain, rather than letting several tools each guess.
- Is combing the same as horizontal tearing?
- No. Combing is a regular alternating line pattern tied to the two fields and appears only on moving edges. Horizontal tearing is an irregular displacement of groups of lines caused by sync loss. You can tell them apart by looking at static areas: combing leaves them untouched, tearing disturbs the whole band.
- Do modern cameras produce combing?
- Most record progressive video, so combing does not occur. Interlaced modes still exist in some broadcast cameras. If you see combs in a progressive project, the footage was interlaced and was interpreted incorrectly. Streaming and phone video is nearly always progressive, so combing now mostly signals archival or broadcast sources.