Add heuristics to skip thumbnail generation when it's unnecessary.

chrome/common/thumbnail_score.h

Index: chrome/common/thumbnail_score.h
diff --git a/chrome/common/thumbnail_score.h b/chrome/common/thumbnail_score.h
index 30bcf8fb6049b538e7cdd02a3fbb72038a93e94a..ee32cad415d73c30d1505cb995172f0d10b97319 100644
--- a/chrome/common/thumbnail_score.h
+++ b/chrome/common/thumbnail_score.h
@@ -6,6 +6,7 @@
 #define CHROME_COMMON_THUMBNAIL_SCORE_H_
 #pragma once
 
+#include <string>
 #include "base/time.h"
 
 // A set of metadata about a Thumbnail.
@@ -27,6 +28,9 @@ struct ThumbnailScore {
   // Tests for equivalence between two ThumbnailScore objects.
   bool Equals(const ThumbnailScore& rhs) const;
 
+  // Returns string representation of this object.
+  std::string ToString() const;
+
   // How "boring" a thumbnail is. The boring score is the 0,1 ranged
   // percentage of pixels that are the most common luma. Higher boring
   // scores indicate that a higher percentage of a bitmap are all the
@@ -67,6 +71,10 @@ struct ThumbnailScore {
   // How bad a thumbnail needs to be before we completely ignore it.
   static const double kThumbnailMaximumBoringness;
 
+  // We consider a thumbnail interesting enough if the boring score is
+  // lower than this.
+  static const double kThumbnailInterestingEnoughBoringness;
+
   // Time before we take a worse thumbnail (subject to
   // kThumbnailMaximumBoringness) over what's currently in the database
   // for freshness.
@@ -80,4 +88,9 @@ struct ThumbnailScore {
 bool ShouldReplaceThumbnailWith(const ThumbnailScore& current,
                                 const ThumbnailScore& replacement);
 
+// Checks whether we should generate a new thumbnail. For instance, we
+// don't have to generaet a new thumbnail if the current thumbnail is new
+// and interesting enough.
+bool ShouldGenerateThumbnail(const ThumbnailScore& current);

[brettw, 2011/01/26 05:55:20]

Can you move this to the ThumbnailScore class? There are already some functions
there and it would be nice to avoid more global functions. You may also want to
consider a new name for the new location, since it has a slightly different
meaning being a class member. Perhaps like ShouldBeReplaced or
ShouldConsiderUpdating?

[satorux1, 2011/01/26 08:14:46]

Agreed. Changed accordingly. Chose ShouldConsiderUpdating.

+
 #endif  // CHROME_COMMON_THUMBNAIL_SCORE_H_