黒色の頂点から傾きを取得し、元の位置に戻す。

#target photoshop
preferences.rulerUnits = Units.PIXELS;

function cTID(s) { return app.charIDToTypeID(s); }
function sTID(s) { return app.stringIDToTypeID(s); }
function RGBc(r, g, b) {
  var color = new ActionDescriptor();
  color.putDouble(cTID("Rd  "), r);
  color.putDouble(cTID("Grn "), g);
  color.putDouble(cTID("Bl  "), b);
  return color;
}
function selectColorRange(color1, color2) {
  var desc = new ActionDescriptor();
  desc.putInteger(cTID("Fzns"), 0);
  desc.putObject(cTID("Mnm "), cTID("RGBC"), color1);
  desc.putObject(cTID("Mxm "), cTID("RGBC"), color2);
  executeAction(cTID("ClrR"), desc, DialogModes.NO);
}
function selectRange(x, y, w, h) {
  var dx = w + x;
  var dy = h + y;
  return [[x, y], [dx, y], [dx, dy], [x, dy]];
  // doc.selection.select(range);
}


var util = {
  log: function(str, mode) {
    var f = new File("D:/backup/photoshop-script/test.log");
    if (mode == undefined) { mode = "a"; }
    f.open(mode);
    f.write(str + "\r\n");
    f.close();
    f = null;
  },
  logPos: function(msg, x, y, mode) {
    var s = msg + ": (" + x + "," + y + ")";
    this.log(s, mode);
  },
  point1: function(selection) {
    var b = selection.bounds;
    var p1 = {
      x: b[0].as("px"),
      y: b[1].as("px"),
    };
    var p2 = {
      x: b[2].as("px"),
      y: b[3].as("px"),
    };
    var startPos = {
      x: p2.x,
      y: p2.y,
    };

    var sample = app.activeDocument.colorSamplers.add([startPos.x, startPos.y]);
    var isFlag = false;
    var findPos = { x: 0, y: 0 };

    var nums = [];
    for (var i = 0; i < p2.x - p1.x; ++i) {
      var pos = { x: startPos.x - i, y: startPos.y };
      sample.move([pos.x, pos.y]);

      var hex = sample.color.rgb.hexValue;
      if (hex != "FFFFFF") {
        findPos.x = pos.x;
        findPos.y = pos.y;
        break;
      }
    }
    sample = null;

    return findPos;
  },

  point2: function(selection) {
    var b = selection.bounds;
    var p1 = {
      x: b[0].as("px"),
      y: b[1].as("px"),
    };
    var p2 = {
      x: b[2].as("px"),
      y: b[3].as("px"),
    };
    var startPos = {
      x: p1.x,
      y: p2.y,
    };

    var sample = app.activeDocument.colorSamplers.add([startPos.x, startPos.y]);
    var isFlag = false;
    var findPos = { x: 0, y: 0 };

    var nums = [];
    for (var i = 0; i < p2.y - p1.y; ++i) {
      var pos = { x: startPos.x, y: startPos.y - i };
      sample.move([pos.x, pos.y]);
      var hex = sample.color.rgb.hexValue;
      if (hex != "FFFFFF") { 
        findPos.x = pos.x;
        findPos.y = pos.y;
        break;
      }
    }
    sample = null;

    return findPos;
  },
  selectionRotate: function (pos1, pos2) {
    var radian = Math.atan2(pos2.y - pos1.y, pos2.x - pos1.x);
    var degree = (radian * 180.0) / Math.PI;
    return degree;
  },

  t1: function() {
    var dir = "D:/backup/photoshop-script/images/";
    var name = "500x500.png";
    var file = new File(dir + name);
    var doc = app.open(file);
    app.activeDocument.flatten();
    selectColorRange(
      RGBc(0.0, 0.0, 0.0),
      RGBc(0.0, 0.0, 0.0)
    );

    var pos1 = this.point1(app.activeDocument.selection);
    var pos2 = this.point2(app.activeDocument.selection);
    this.logPos("pos1", pos1.x, pos1.y);
    this.logPos("pos2", pos2.x, pos2.y);
    var rot = this.selectionRotate(pos2, pos1);
    app.activeDocument.selection.rotate(-rot);
    // app.activeDocument.selection.invert();
    // app.activeDocument.selection.cut();
    // app.activeDocument.trim()
    // app.activeDocument.selection.deselect();

    selectColorRange(
      RGBc(0.0, 0.0, 0.0),
      RGBc(0.0, 0.0, 0.0)
    );
    var b = app.activeDocument.selection.bounds;
    var re = {
      x: b[0].as("px"), 
      y: b[1].as("px"), 
      dx: app.activeDocument.width.as("px"), 
      dy: app.activeDocument.height.as("px")
    };
    var range = selectRange(re.x, re.y, re.dx, re.dy);
    app.activeDocument.selection.translate(-re.x, -re.y);
    app.activeDocument.selection.deselect();
  },
};

util.t1();