Improved the way the feature vector cache is used within the structural svm

solver.  This makes some things, such as the structural_object_detection_trainer,
significantly faster.

dlib/svm/structural_svm_problem.h

@@ -24,7 +24,7 @@ namespace dlib
     public:
 
         cache_element_structural_svm (
-        ) : prob(0), sample_idx(0) {}
+        ) : prob(0), sample_idx(0), last_true_risk_computed(std::numeric_limits<double>::infinity()) {}
 
         typedef typename structural_svm_problem::scalar_type scalar_type;
         typedef typename structural_svm_problem::matrix_type matrix_type;
@@ -66,19 +66,23 @@ namespace dlib
 
         void separation_oracle_cached (
             const bool skip_cache,
-            const scalar_type& cur_risk_lower_bound,
+            const scalar_type& saved_current_risk_gap,
             const matrix_type& current_solution,
             scalar_type& out_loss,
             feature_vector_type& out_psi
         ) const
         {
-            if (!skip_cache && prob->get_max_cache_size() != 0)
+            const bool cache_enabled = prob->get_max_cache_size() != 0;
+
+            // Don't waste time computing this if the cache isn't going to be used.
+            const scalar_type dot_true_psi = cache_enabled ? dot(true_psi, current_solution) : 0;
+
+            if (!skip_cache && cache_enabled)
             {
                 scalar_type best_risk = -std::numeric_limits<scalar_type>::infinity();
                 unsigned long best_idx = 0;
 
 
-                const scalar_type dot_true_psi = dot(true_psi, current_solution);
 
                 // figure out which element in the cache is the best (i.e. has the biggest risk)
                 long max_lru_count = 0;
@@ -95,7 +99,11 @@ namespace dlib
                         max_lru_count = lru_count[i];
                 }
 
-                if (best_risk - cur_risk_lower_bound > prob->get_epsilon())
+                // Check if the best psi vector in the cache is still good enough to use as
+                // a proxy for the true separation oracle.  If the risk value has dropped
+                // by enough to get into the stopping condition then the best psi isn't
+                // good enough. 
+                if (best_risk + saved_current_risk_gap-prob->get_epsilon() > last_true_risk_computed)
                 {
                     out_psi = psi[best_idx];
                     lru_count[best_idx] = max_lru_count + 1;
@@ -106,11 +114,13 @@ namespace dlib
 
             prob->separation_oracle(sample_idx, current_solution, out_loss, out_psi);
 
-            if (prob->get_max_cache_size() == 0)
+            if (!cache_enabled)
                 return;
 
             compact_sparse_vector(out_psi);
 
+            last_true_risk_computed = out_loss + dot(out_psi, current_solution) - dot_true_psi;
+
             // if the cache is full
             if (loss.size() >= prob->get_max_cache_size())
             {
@@ -167,6 +177,7 @@ namespace dlib
         mutable std::vector<scalar_type> loss;
         mutable std::vector<feature_vector_type> psi;
         mutable std::vector<long> lru_count;
+        mutable double last_true_risk_computed;
     };
 
 // ----------------------------------------------------------------------------------------
@@ -200,7 +211,7 @@ namespace dlib
 
         structural_svm_problem (
         ) :
-            cur_risk_lower_bound(0),
+            saved_current_risk_gap(0),
             eps(0.001),
             verbose(false),
             skip_cache(true),
@@ -315,7 +326,8 @@ namespace dlib
                 cout << endl;
             }
 
-            cur_risk_lower_bound = std::max<scalar_type>(current_risk_value - current_risk_gap, 0);
+            saved_current_risk_gap = std::max<scalar_type>(current_risk_value - current_risk_gap, 0);
+            saved_current_risk_gap = current_risk_gap;
 
             bool should_stop = false;
 
@@ -401,7 +413,7 @@ namespace dlib
         ) const 
         {
             cache[idx].separation_oracle_cached(skip_cache, 
-                                                cur_risk_lower_bound,
+                                                saved_current_risk_gap,
                                                 current_solution,
                                                 loss,
                                                 psi);
@@ -409,7 +421,7 @@ namespace dlib
     private:
 
 
-        mutable scalar_type cur_risk_lower_bound;
+        mutable scalar_type saved_current_risk_gap;
         mutable matrix_type psi_true;
         scalar_type eps;
         mutable bool verbose;