Upstream script to get "salient" image from webpage.

chrome/browser/resources/get_salient_image_url.js

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/*
+ * Looks on the page to find the image that could be the most representative
+ * one.
+ */
+(function() {
+  // Extract the referrer policy from the page.
+  var referrerPolicy = 'default';
+  var metaTags = document.getElementsByTagName('meta');
+  for (var i = 0; i < metaTags.length; ++i) {
+    if (metaTags[i].name.toLowerCase() == 'referrer') {
+      referrerPolicy = metaTags[i].content.toLowerCase();
+      break;
+    }
+  }
+
+  // See what to use for JSON. Some pages use a library that overrides JSON.
+  var jsonEncoder = JSON.stringify;
+  if (!jsonEncoder)
+    jsonEncoder = JSON.encode;
+
+  // First look if there is an Open Graph Image property available.
+  var ogImage = document.querySelector('meta[property=\"og:image\"]');
+  if (ogImage) {
+    // Checks that the url in ogImage has a path that contains more than just a
+    // simple '/'.
+    var url = ogImage.content;
+    var location = document.createElement('a');
+    location.href = url;
+    if (location.pathname.length > 1) {
+      return jsonEncoder({
+        'imageUrl': url,
+        'referrerPolicy': referrerPolicy
+      });
+    }
+  }
+
+  // Iterates through the images on the page, find the largest one that doesn't
+  // look like a banner.
+  var maxPointSize = 0;
+  var maxImage = null;
+
+  var images = document.getElementsByTagName('img');
+  for (var i = 0; i < images.length; i++) {
+    var currentImage = images[i];
+    var aspectRatio = currentImage.width / currentImage.height;
+    if (aspectRatio >= 2.4 || aspectRatio <= 0.5)
+      continue;  // Skip weirdly shaped images. Those are ads or headers.
+    var pointSize = currentImage.width * currentImage.height;
+    if (pointSize > maxPointSize) {
+      maxPointSize = pointSize;
+      maxImage = currentImage;
+    }
+  }
+
+  // Only keep images larger than 320*160.
+  if (maxPointSize <= 51200.0 || maxImage === null)
+    return '';
+
+  return jsonEncoder({
+    'imageUrl': maxImage.src,
+    'referrerPolicy': referrerPolicy
+  });
+})();