Frame-gate shadow

Visible effect

A dark rectangular rim reveals the gate aperture

A frame-gate shadow is a narrow, rectangular darkening at the extreme edge of the image. Its straight sides and corner accumulation reveal the shape of the gate aperture; the centre of the picture remains essentially unchanged.

If film lifts or bows away from the gate, the aperture edge no longer lands as a perfectly sharp boundary on the emulsion. It forms a small penumbra instead. Changing film seating can also make the shadow density breathe from frame to frame.

Physics

The distance from aperture plate to emulsion controls the edge

The gate aperture masks the projected image to one frame while the pressure plate and guides keep film flat and registered. When the emulsion sits close to the aperture plate, the gate edge records as a crisp boundary. Curl, debris, pressure variation, or imperfect seating can increase that separation.

Because image-forming rays arrive across a range of angles, a gap lets the aperture edge cast a penumbra rather than an infinitely sharp line. The visible result follows the rectangular gate, and its density can change as successive film sections seat differently.

Mathematics

A high-power rectangular edge mask

Centered coordinate u reaches magnitude one at every frame edge. Softness s maps to an exponent range from 32 to 16. High powers stay near zero over most of the picture and rise rapidly only at the boundary.

The horizontal and vertical terms sum so corners accumulate both contributions. Amount a may vary slowly with seating amplitude q, frequency f, and phaseφ. The resulting mask reduces linear-light radiance L.

Shader

One fixed destination-space mask

The GLSL sums high-power horizontal and vertical axes over the selected exponent range. It evaluates the mask in destination coordinates so the gate stays fixed, then applies bounded transmission in linear light instead of subtracting an unbounded value from encoded RGB.

// WHAT: Darken a narrow rectangular region at the film-frame boundary.
// HOW: Raise centered coordinates to a high exponent and sum both axes.
// WHY: A frame gate casts an aperture-shaped edge shadow, unlike the broad
// radial brightness loss produced by natural optical vignetting.
float frameGateShadow(
  vec2 centeredUv,
  float amount,
  float softness
) {
  float exponent = 32.0 - 16.0 * clamp(softness, 0.0, 1.0);
  vec2 edge = pow(abs(centeredUv), vec2(exponent));
  return clamp(max(amount, 0.0) * (edge.x + edge.y), 0.0, 1.0);
}

vec3 applyFrameGateShadow(
  vec3 sourceSrgb,
  vec2 centeredUv,
  float amount,
  float softness,
  float effectMix
) {
  vec3 sourceLinear = pow(max(sourceSrgb, vec3(0.0)), vec3(2.2));
  float shadow = frameGateShadow(centeredUv, amount, softness);
  vec3 processedLinear = sourceLinear * (1.0 - shadow);
  vec3 processedSrgb = pow(
    max(processedLinear, vec3(0.0)),
    vec3(1.0 / 2.2)
  );
  return mix(sourceSrgb, processedSrgb, clamp(effectMix, 0.0, 1.0));
}

Why these steps are here

1. Centre the frame first. Absolute coordinates create four symmetric aperture edges.
2. Use a high exponent. The centre remains clear while the boundary rises rapidly.
3. Sum both axes. Straight sides meet with stronger corner density, matching a rectangular mask.
4. Keep the mask in destination space. Scene content and frame jitter may move beneath a mechanically fixed gate.
5. Multiply in linear light. A transmission loss darkens without producing negative channel values.

Notes

• The high-power edge equation uses a 16–32 exponent range, with strength and softness exposed as independent teaching controls.
• Bounded linear-light transmission avoids negative values and encoding-dependent colour shifts.
• Frame jitter moves the photographed image relative to the gate. Gate shadow describes the fixed boundary itself; combining them makes the stationary rim especially visible.
• Real gates may have asymmetric edges, rounded corners, machining marks, accumulated debris, or format-specific aperture proportions. A measured matte is preferable when those details matter.
• Do not remove a visible frame line automatically during restoration: it can be useful evidence for registration, stabilization, and historical framing.

References