Moved label_to_ignore into loss_multiclass_log_per_pixel_ and also cleaned up a

few minor things.

dlib/dnn/loss.h

@@ -1531,14 +1531,15 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+    class loss_multiclass_log_per_pixel_
+    {
+    public:
+
         // In semantic segmentation, if you don't know the ground-truth of some pixel,
         // set the label of that pixel to this value. When you do so, the pixel will be
         // ignored when computing gradients.
         static const uint16_t label_to_ignore = std::numeric_limits<uint16_t>::max();
 
-    class loss_multiclass_log_per_pixel_
-    {
-    public:
 
         // In semantic segmentation, 65535 classes ought to be enough for anybody.
         typedef matrix<uint16_t> training_label_type;
@@ -1565,12 +1566,15 @@ namespace dlib
             const float* const out_data = output_tensor.host();
 
             // The index of the largest output for each element is the label.
-            const auto find_label = [&](long sample, long r, long c) {
+            const auto find_label = [&](long sample, long r, long c) 
+            {
                 uint16_t label = 0;
                 float max_value = out_data[tensor_index(output_tensor, sample, r, c, 0)];
-                for (long k = 1; k < output_tensor.k(); ++k) {
+                for (long k = 1; k < output_tensor.k(); ++k) 
+                {
                     const float value = out_data[tensor_index(output_tensor, sample, r, c, k)];
-                    if (value > max_value) {
+                    if (value > max_value) 
+                    {
                         label = static_cast<uint16_t>(k);
                         max_value = value;
                     }
@@ -1578,10 +1582,13 @@ namespace dlib
                 return label;
             };
 
-            for (long i = 0; i < output_tensor.num_samples(); ++i, ++iter) {
+            for (long i = 0; i < output_tensor.num_samples(); ++i, ++iter) 
+            {
                 iter->set_size(output_tensor.nr(), output_tensor.nc());
-                for (long r = 0; r < output_tensor.nr(); ++r) {
-                    for (long c = 0; c < output_tensor.nc(); ++c) {
+                for (long r = 0; r < output_tensor.nr(); ++r) 
+                {
+                    for (long c = 0; c < output_tensor.nc(); ++c) 
+                    {
                         // The index of the largest output for this element is the label.
                         iter->operator()(r, c) = find_label(i, r, c);
                     }

dlib/dnn/loss_abstract.h

@@ -808,12 +808,17 @@ namespace dlib
                 EXAMPLE_LOSS_LAYER_.  In particular, it implements the multiclass logistic
                 regression loss (e.g. negative log-likelihood loss), which is appropriate
                 for multiclass classification problems.  It is basically just like
-                loss_multiclass_log_ except that it lets you define matrix output instead
-                of scalar.  It should be useful, for example, in semantic segmentation where
-                we want to classify each pixel of an image.
+                loss_multiclass_log_ except that it lets you define matrix outputs instead
+                of scalar outputs.  It should be useful, for example, in semantic
+                segmentation where we want to classify each pixel of an image.
         !*/
     public:
 
+        // In semantic segmentation, if you don't know the ground-truth of some pixel,
+        // set the label of that pixel to this value. When you do so, the pixel will be
+        // ignored when computing gradients.
+        static const uint16_t label_to_ignore = std::numeric_limits<uint16_t>::max();
+
         // In semantic segmentation, 65535 classes ought to be enough for anybody.
         typedef matrix<uint16_t> training_label_type;
         typedef matrix<uint16_t> output_label_type;
@@ -850,7 +855,7 @@ namespace dlib
             except it has the additional calling requirements that:
                 - sub.get_output().num_samples() == input_tensor.num_samples()
                 - sub.sample_expansion_factor() == 1
-                - all values pointed to by truth are < sub.get_output().k() (or std::numeric_limits<uint16_t>::max() to ignore)
+                - all values pointed to by truth are < sub.get_output().k() or are equal to label_to_ignore.
         !*/
 
     };

dlib/test/dnn.cpp

@@ -2243,7 +2243,7 @@ namespace
                     DLIB_TEST(truth < num_classes);
                     ++truth_histogram[truth];
                     if (ignore(generator)) {
-                        ytmp(jj, kk) = label_to_ignore;
+                        ytmp(jj, kk) = loss_multiclass_log_per_pixel_::label_to_ignore;
                     }
                     else if (noise_occurrence(generator)) {
                         ytmp(jj, kk) = noisy_label(generator);