【脚本示例】在2个图层位置中间创建1个空对象

/**
 * Creates a null positioned at the average position of all selected layers.
 * 在2个图层位置中间创建1个空对象 （引导层）
 *
 * Modifiers:
 *  - Hold SHIFT to set to a fixed average at current time, vs dynamic expression
 *  - 不按shift键 可以绑定动态表达式
 * 
 * 
 * @author Zack Lovatt <zack@zacklovatt.com>
 * @version 0.4.0
 */
 (function setToAvgPosition() {
    var useFixed = ScriptUI.environment.keyboardState.shiftKey;

    var comp = app.project.activeItem;

    if (!(comp && comp instanceof CompItem)) {
      alert("Please select a composition!");
      return;
    }

    var layers = comp.selectedLayers;

    if (layers.length < 2) {
      alert("Select at least 2 layers!");
      return;
    }

    app.beginUndoGroup("Set to Average Position");

    var avgNull = comp.layers.addNull();
    avgNull.name = "Average Position Null";
    avgNull.guideLayer = true;
    avgNull.label = 14;

    if (useFixed) {
      var sum = [0, 0];

      for (var ii = 0, il = layers.length; ii < il; ii++) {
        var layer = layers[ii];
        sum += layer.position.valueAtTime(comp.time, false);
      }

      var avg = sum / layers.length;

      avgNull.position.setValue(avg);
    } else {
      var firstLayerIndex = layers[0].index;
      var lastLayerIndex = layers[layers.length - 1].index;

      avgNull.position.expression = [
        "var firstLayerIndex = " + firstLayerIndex + ";",
        "var lastLayerIndex = " + lastLayerIndex + ";",
        "",
        "var sum = [0, 0];",
        "var numLayers = lastLayerIndex - firstLayerIndex + 1;",
        "",
        "for (var ii = firstLayerIndex; ii <= lastLayerIndex; ii++) {",
        "  var layer = thisComp.layer(ii);",
        "  sum += layer.position;",
        "}",
        "",
        "sum / numLayers;",
      ].join("\n");
    }

    app.endUndoGroup();
  })();