Fix crash when truth center is outside of the image (#2471)

dlib/dnn/loss.h

@@ -3682,7 +3682,7 @@ namespace dlib
                 tensor& grad = layer<TAG_TYPE>(sub).get_gradient_input();
                 DLIB_CASSERT(input_tensor.num_samples() == grad.num_samples());
                 DLIB_CASSERT(input_tensor.num_samples() == output_tensor.num_samples());
-                const double input_area = input_tensor.nr() * input_tensor.nc();
+                const rectangle input_rect(input_tensor.nr(), input_tensor.nc());
                 float* g = grad.host();
 
                 // Compute the objectness loss for all grid cells
@@ -3709,7 +3709,7 @@ namespace dlib
                             double best_iou = 0;
                             for (const yolo_rect& truth_box : *truth)
                             {
-                                if (truth_box.ignore)
+                                if (truth_box.ignore || !input_rect.contains(center(truth_box.rect)))
                                     continue;
                                 best_iou = std::max(best_iou, box_intersection_over_union(truth_box.rect, pred.rect));
                             }
@@ -3724,7 +3724,7 @@ namespace dlib
                 // Now find the best anchor box for each truth box
                 for (const yolo_rect& truth_box : *truth)
                 {
-                    if (truth_box.ignore)
+                    if (truth_box.ignore || !input_rect.contains(center(truth_box.rect)))
                         continue;
                     const dpoint t_center = dcenter(truth_box);
                     double best_iou = 0;
@@ -3780,7 +3780,7 @@ namespace dlib
                             const double th = truth_box.rect.height() / (anchors[a].height + truth_box.rect.height());
 
                             // Scale regression error according to the truth size
-                            const double scale_box = options.lambda_box * (2 - truth_box.rect.area() / input_area);
+                            const double scale_box = options.lambda_box * (2 - truth_box.rect.area() / input_rect.area());
 
                             // Compute the gradient for the box coordinates
                             g[x_idx] = scale_box * (out_data[x_idx] - tx);