Projector gate weave

Visible effect

The complete projected image shifts sideways

Projector gate weave makes the complete projected image move side to side relative to a stable screen aperture. Objects, grain, scratches, and all other detail share the same lateral displacement: the film frame is misregistered as one rigid element rather than locally distorted.

The movement may be subtle and intermittent. Worn or incorrectly adjusted lateral guides, clearance between perforations and locating parts, damaged perforations, film shrinkage, or changing transport tension can let successive frames settle at slightly different horizontal positions.

Physics

Lateral registration is established at the gate

The projector transport delivers each film frame to the aperture, but the gate and its lateral guides determine the frame’s final side-to-side position while it is illuminated. Clearance, wear, contamination, perforation damage, shrinkage, or changing tension can let successive frames settle at different x positions.

Sprockets, rollers, and loop geometry can contribute through tension and film condition, so the transport is part of the causal chain. The visible registration error is nevertheless imposed where the film is located in the gate; that is why this effect belongs to the projector’s gate-and-intermittent-movement zone.

Mathematics

A bounded frame-held x offset

Registration frequency fw selects held state k. A deterministic signed hash h, parameterized by seed s, produces a lateral offset bounded by amplitude Ax. Division by output width W converts pixels to normalized texture coordinates; y remains unchanged.

The piecewise-random model uses a small, quantized horizontal offset. Real weave can be correlated, periodic, biased, or dominated by a particular damaged perforation, so the controls are a teaching approximation rather than a projector specification.

Shader

One rigid lateral coordinate remap

The runtime computes one signed offset for each held registration state. The shader applies that same x displacement to every output coordinate and keeps y untouched. Where the shifted image leaves the fixed aperture, the playground reveals black gate area instead of stretching edge pixels.

// WHAT: Move a projected film image laterally beneath a fixed gate.
// HOW: Inverse-map every destination pixel by the same frame-held x offset.
// WHY: Projector weave is rigid lateral registration error, not vertical film
//      slip, a frame-line transition, or a local geometric deformation.
vec2 projectorGateWeaveSourceCoordinate(
  vec2 destinationUv01,
  float offsetPixels,
  vec2 destinationSize
) {
  return destinationUv01 + vec2(
    offsetPixels / max(destinationSize.x, 1.0),
    0.0
  );
}

Why these steps are here

1. Quantise time. The film does not swim continuously within a held state.
2. Choose one bounded offset. Every feature moves together as a rigid frame.
3. Normalize by width only. Amplitude is expressed in visible pixels and affects x alone.
4. Inverse-map the coordinate. Each destination pixel performs one source lookup without forward-warp holes.
5. Keep the gate fixed. A narrow black reveal makes relative film motion visible.
6. Leave y unchanged. Vertical slip and frame-line exposure belong to Projector frame slip.

Notes

• The model isolates projection-time lateral registration. A weave already recorded by the camera or added during printing/scanning can coexist and cannot be separated from the image by appearance alone.
• Large amplitudes are provided for diagnosis and teaching; healthy theatrical projection should be far subtler.
• The black edge reveal represents the fixed aperture. A real screen mask, overscan, or cropped scan may hide it.
• Rotation, vertical jump, blur during movement, film buckle, and projector vibration are outside this isolated effect.

References