Projector frame slip

Visible effect

The gate sees across a film-frame boundary

A correctly framed projector holds one complete film image inside the illuminated gate. If the strip stops above or below that registered position, the picture moves vertically and the opaque gap between frames enters the screen. Beyond that frame line, the audience sees part of the neighbouring film frame.

The error may remain as a fixed misframing, drift as loop geometry or transport becomes unstable, or jump when the intermittent movement loses registration. Unlike camera weave, this displacement is introduced while an existing film element is being reproduced.

Physics

The aperture is fixed while the film stops in the wrong place

A projector shutter should block the lamp while an intermittent mechanism advances the strip, then reopen only after the next frame is stationary in the gate. The upper and lower loop isolate that pulsed motion from continuously rotating sprockets. Incorrect framing, damaged perforations, lost loop geometry, or intermittent-movement trouble can leave the strip displaced when illumination resumes.

The horizontal bar is not an electronic blanking interval. It is the optically opaque area between two photographic frames. Once that boundary enters the gate, the remaining portion must come from a genuinely neighbouring frame on the film strip.

Mathematics

A live one-texture approximation of the film strip

Signed drift v, measured in frames per second, advances the strip linearly from registration; changing its sign reverses travel, while zero keeps the strip registered. Fractional coordinate yf wraps the live source vertically. Mask L creates the opaque frame line of height h.

In this real-time one-texture approximation, fract(y+Δ) repeats the same live frame on both sides of the line. It is temporally smooth because every region samples the current source. A physically complete offline model would instead decode and index genuine adjacent frames.

Shader

Single-texture repeated-frame coordinate mapping

The mapping returns a wrapped in-frame coordinate and frame-line coverage. The runtime uploads one live source texture per render, so the entire displaced image retains the same frame cadence.

// WHAT: Approximate projector misframing with a vertically repeated source.
// HOW: Offset film-space y, wrap it with fract(), and expose the opaque frame
//      line at each repeated boundary.
// WHY: This is a bounded one-texture implementation. A faithful offline
//      film strip would supply genuinely adjacent decoded frames instead.
vec3 projectorFrameSlip(
  vec2 uv,
  float drift,
  float frameLineHeight,
  float time
){
  float slip=drift*time;
  float filmY=fract(uv.y+slip);
  float boundaryDistance=min(filmY,1.0-filmY);
  float frameLine=1.0-smoothstep(
    0.0,
    max(0.0001,frameLineHeight),
    boundaryDistance
  );
  return vec3(uv.x,filmY,frameLine);
}

Why these steps are here

1. Shift in film space. Elapsed time and slip drift move the strip vertically through a fixed gate.
2. Advance linearly in time. Signed slip drift sets both speed and direction without artificial oscillation.
3. Wrap with fract. The approximation repeats one live image across the boundary.
4. Mask the physical frame line. The separator is opaque film area, not an arbitrary overlay.
5. Sample the current texture. Every output region updates at the source frame rate.
6. Keep x unchanged. Horizontal weave is a separate lateral-registration effect, not part of frame slip.

Notes

• All playground sources use the repeated-frame approximation without introducing a low-frame-rate retained region.
• A complete offline implementation should supply exact previous and next decoded frames rather than wrapping one frame.
• Severe transport faults may also introduce blur, shutter streaking, tearing, focus change, or film damage; those are outside this isolated registration model.
• Digital “rolling frame” transitions resemble the geometry but do not establish a mechanical projector cause.

References