Auto exposure

since 0.4.3 Post-processing

Open the live demo · Read the source · View on GitHub

A camera with a fixed exposure blows out a bright room and turns a dark one to mud. Auto exposure measures how bright the scene actually is and moves the exposure toward a value that puts the middle of the frame at a chosen grey, the way a camera's own metering does.

Step 1: Where it starts #

The very first frame has nothing to measure yet, so it is drawn at the plain exposure setting. Everything after that is the meter's answer.

exposure: seedExposure,

Step 2: Turn the meter on #

AutoExposureSettings.enabled runs a small pass that reads the scene's own brightness. target is the grey it aims the middle of the histogram at, 0.18 by default, the same middle grey a photographer's light meter uses. speedUp and speedDown are how many stops a second the exposure is allowed to move.

autoExposure: AutoExposureSettings(
  enabled: true,
  target: target,
  speedUp: speed,
  speedDown: speed,
),

Move the view around this scene while the app is running and the exposure follows: step to face the bright floor and the picture darkens to compensate, step toward a dark corner and it brightens. A single still picture cannot show that motion, which is why this page's own check only holds the first frame to what Step 1 says it should be.

Step 3: Read what the frame used #

FrameResult.exposure is the number the composite actually multiplied by, whether it came from the setting or from the meter. On the first frame it is exactly the seed.

if (!passRan(frame, 'luminance')) {
  throw StateError('the luminance pass did not run: ${frame.skipped}');
}
if ((frame.exposure - seedExposure).abs() > 1e-6) {
  throw StateError(
    'the first frame should wear the seed exposure, since nothing has '
    'been metered yet; it reported ${frame.exposure}',
  );
}

Tip. Raise Adaptation speed and the exposure of a running scene snaps to a new value in a fraction of a second. Lower it and a walk from a bright room to a dark one takes a visible moment to catch up, the way an eye does.