Start Stop (gate) Values with Markers expression in After Effects

startstopgate.js

// === CONFIG ===
var offValue = 0;        // Value to use outside of active segments
var blendDur = 0.5;      // Duration (in seconds) for blending in/out
// ==============

var t = time;
var val = value;
var markers = thisLayer.marker; // change to thisComp.marker for compostion based maerkers
var segments = [];

// Collect all "start" and "stop" markers with time
for (var i = 1; i <= markers.numKeys; i++) {
    var m = markers.key(i);
    if (m.comment.toLowerCase() === "start") {
        segments.push({ t: m.time, type: "start" });
    } else if (m.comment.toLowerCase() === "stop") {
        segments.push({ t: m.time, type: "stop" });
    }
}

// Sort by time
segments.sort(function(a, b) {
    return a.t - b.t;
});

// Build active time ranges
var ranges = [];
var inSeg = false;
var segStart = thisLayer.inPoint;

for (var i = 0; i < segments.length; i++) {
    var s = segments[i];
    if (s.type === "start") {
        if (!inSeg) {
            segStart = s.t;
            inSeg = true;
        }
    } else if (s.type === "stop") {
        if (inSeg) {
            ranges.push([segStart, s.t]);
            inSeg = false;
        } else {
            // orphan stop
            ranges.push([thisLayer.inPoint, s.t]);
        }
    }
}

// Handle trailing open "start"
if (inSeg) {
    ranges.push([segStart, thisLayer.outPoint]);
}

// Check if time is in any range
var active = false;
var blend = 0;

for (var i = 0; i < ranges.length; i++) {
    var a = ranges[i][0];
    var b = ranges[i][1];
    if (t >= a - blendDur && t <= b + blendDur) {
        if (t < a) {
            // Blending in
            blend = ease(t, a - blendDur, a, 0, 1);
        } else if (t > b) {
            // Blending out
            blend = ease(t, b, b + blendDur, 1, 0);
        } else {
            blend = 1;
        }
        active = true;
        break;
    }
}

linear(blend, 0, 1, offValue, val);