Srcset resource selection use a geometric mean to determine resource.

Source/core/html/parser/HTMLSrcsetParser.cpp

 /*
  * Copyright (C) 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 303 matching lines @@
 {
     if (attribute.isNull())
         return;
 
     if (attribute.is8Bit())
         parseImageCandidatesFromSrcsetAttribute<LChar>(attribute, attribute.characters8(), attribute.length(), imageCandidates, document);
     else
         parseImageCandidatesFromSrcsetAttribute<UChar>(attribute, attribute.characters16(), attribute.length(), imageCandidates, document);
 }
 
+static int selectionLogic(Vector<ImageCandidate>& imageCandidates, float deviceScaleFactor, bool ignoreSrc)
+{
+    unsigned i = 0;
+    unsigned next;
+    float nextDensity;
+    float currentDensity;

[Mike West, 2014/10/22 13:41:39]

Why do you define `next`, `nextDensity`, and `currentDensity` outside the loop?

+    float geometricMean;
+
+    for (; i < imageCandidates.size() - 1; ++i) {
+        next = i + 1;
+        if (ignoreSrc) {
+            if (imageCandidates[i].srcOrigin())
+                continue;
+            if (imageCandidates[next].srcOrigin()) {
+                ++next;

[Mike West, 2014/10/22 13:41:39]

Should this be a loop? Or do you really only want to skip one?

+                if (next >= imageCandidates.size())
+                    break;
+            }
+        }
+
+        nextDensity = imageCandidates[next].density();
+        if (nextDensity < deviceScaleFactor)
+            continue;
+
+        currentDensity = imageCandidates[i].density();
+        geometricMean = sqrt(currentDensity * nextDensity);
+        if (deviceScaleFactor >= geometricMean)
+            return next;
+        break;
+    }
+    return i;
+}
+
 static ImageCandidate pickBestImageCandidate(float deviceScaleFactor, float sourceSize, Vector<ImageCandidate>& imageCandidates)
 {
     const float defaultDensityValue = 1.0;
     bool ignoreSrc = false;
     if (imageCandidates.isEmpty())
         return ImageCandidate();
 
     // http://picture.responsiveimages.org/#normalize-source-densities
     for (Vector<ImageCandidate>::iterator it = imageCandidates.begin(); it != imageCandidates.end(); ++it) {
         if (it->resourceWidth() > 0) {
             it->setDensity((float)it->resourceWidth() / sourceSize);
             ignoreSrc = true;
         } else if (it->density() < 0) {
             it->setDensity(defaultDensityValue);
         }
     }
 
     std::stable_sort(imageCandidates.begin(), imageCandidates.end(), compareByDensity);
 
-    unsigned i;
-    for (i = 0; i < imageCandidates.size() - 1; ++i) {
-        if ((imageCandidates[i].density() >= deviceScaleFactor) && (!ignoreSrc || !imageCandidates[i].srcOrigin()))
-            break;
-    }
+    unsigned i = selectionLogic(imageCandidates, deviceScaleFactor, ignoreSrc);
 
-    if (imageCandidates[i].srcOrigin() && ignoreSrc) {
-        ASSERT(i > 0);
-        --i;
-    }
     float winningDensity = imageCandidates[i].density();

[Mike West, 2014/10/22 13:41:40]

Perhaps ASSERT that `i` ended up in a valid range?

 
     unsigned winner = i;
     // 16. If an entry b in candidates has the same associated ... pixel density as an earlier entry a in candidates,
     // then remove entry b
     while ((i > 0) && (imageCandidates[--i].density() == winningDensity))
         winner = i;
 
     return imageCandidates[winner];
 }
 
@@ skipping 32 matching lines @@
     Vector<ImageCandidate> imageCandidates;
     imageCandidates.append(srcsetImageCandidate);
 
     if (!srcAttribute.isEmpty())
         imageCandidates.append(ImageCandidate(srcAttribute, 0, srcAttribute.length(), DescriptorParsingResult(), ImageCandidate::SrcOrigin));
 
     return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates).toString();
 }
 
 }