Dark-region electrical noise

// WHAT: Add electrical-looking fluctuations that are most visible in shadows.
// HOW: Generate one deterministic random value from pixel position and a
// time-step number, then multiply it by an inverse-luminance shadow weight.
// WHY: Dark current and readout noise compete with a weak target signal; the
// same absolute fluctuation becomes less noticeable as useful signal rises.

// This integer mixer is only a compact pseudorandom-number generator. Its bit
// shifts and hexadecimal constants have no camera-tube meaning: they repeatedly
// scramble nearby integer inputs so adjacent pixels do not receive similar noise.
uint vidiconHashUint(uint value) {
  value ^= value >> 16u;
  value *= 0x7feb352du;
  value ^= value >> 15u;
  value *= 0x846ca68bu;
  value ^= value >> 16u;
  return value;
}

float vidiconNoiseHash(uvec2 pixel, uint timeState) {
  // Give x, y, and time different large multipliers before the final mixer.
  uint seed = pixel.x * 0x8da6b343u;
  seed ^= pixel.y * 0xd8163841u;
  seed ^= timeState * 0xcb1ab31fu;

  // Convert the complete unsigned 32-bit range to approximately [0, 1].
  return float(vidiconHashUint(seed)) * (1.0 / 4294967295.0);
}

vec3 darkRegionElectricalNoise(
  vec3 source,
  vec2 pixel,
  float time,
  float amount,
  float shadowBias,
  float rate
) {
  float luminance = dot(source, vec3(0.299, 0.587, 0.114));

  // Weight is near one in black and falls toward zero in bright regions.
  // shadowBias controls how quickly the fluctuation disappears with luminance.
  float shadowWeight = pow(
    1.0 - clamp(luminance, 0.0, 1.0),
    max(0.25, shadowBias)
  );

  // Holding an integer state avoids unrelated random noise on every display
  // refresh. rate selects how many new noise states appear per second.
  uint timeState = uint(floor(time * max(1.0, rate)));
  float signedNoise = vidiconNoiseHash(uvec2(pixel), timeState) * 2.0 - 1.0;

  return source + vec3(signedNoise * amount * shadowWeight);
}