Merge pull request opencv#17733 from l-bat:tiny_yolov4

* Supported yolov4-tiny

* Added comments

Author: l-bat

modules/dnn/src/darknet/darknet_io.cpp

@@ -363,6 +363,28 @@ namespace cv {
                     fused_layer_names.push_back(last_layer);
                 }
 
+                void setSlice(int input_index, int split_size, int group_id)
+                {
+                    int begin[] = {0, split_size * group_id, 0, 0};
+                    cv::dnn::DictValue paramBegin = cv::dnn::DictValue::arrayInt(begin, 4);
+
+                    int end[] = {-1, begin[1] + split_size, -1, -1};
+                    cv::dnn::DictValue paramEnd = cv::dnn::DictValue::arrayInt(end, 4);
+
+                    darknet::LayerParameter lp;
+                    lp.layer_name = cv::format("slice_%d", layer_id);
+                    lp.layer_type = "Slice";
+                    lp.layerParams.set("begin", paramBegin);
+                    lp.layerParams.set("end", paramEnd);
+
+                    lp.bottom_indexes.push_back(fused_layer_names.at(input_index));
+                    net->layers.push_back(lp);
+
+                    layer_id++;
+                    last_layer = lp.layer_name;
+                    fused_layer_names.push_back(last_layer);
+                }
+
                 void setReorg(int stride)
                 {
                     cv::dnn::LayerParams reorg_params;
@@ -717,6 +739,7 @@ namespace cv {
                     {
                         std::string bottom_layers = getParam<std::string>(layer_params, "layers", "");
                         CV_Assert(!bottom_layers.empty());
+                        int groups = getParam<int>(layer_params, "groups", 1);
                         std::vector<int> layers_vec = getNumbers<int>(bottom_layers);
 
                         tensor_shape[0] = 0;
@@ -725,10 +748,31 @@ namespace cv {
                             tensor_shape[0] += net->out_channels_vec[layers_vec[k]];
                         }
 
-                        if (layers_vec.size() == 1)
-                            setParams.setIdentity(layers_vec.at(0));
+                        if (groups > 1)
+                        {
+                            int group_id = getParam<int>(layer_params, "group_id", 0);
+                            tensor_shape[0] /= groups;
+                            int split_size = tensor_shape[0] / layers_vec.size();
+                            for (size_t k = 0; k < layers_vec.size(); ++k)
+                                setParams.setSlice(layers_vec[k], split_size, group_id);
+
+                            if (layers_vec.size() > 1)
+                            {
+                                // layer ids in layers_vec - inputs of Slice layers
+                                // after adding offset to layers_vec: layer ids - ouputs of Slice layers
+                                for (size_t k = 0; k < layers_vec.size(); ++k)
+                                    layers_vec[k] += layers_vec.size();
+
+                                setParams.setConcat(layers_vec.size(), layers_vec.data());
+                            }
+                        }
                         else
-                            setParams.setConcat(layers_vec.size(), layers_vec.data());
+                        {
+                            if (layers_vec.size() == 1)
+                                setParams.setIdentity(layers_vec.at(0));
+                            else
+                                setParams.setConcat(layers_vec.size(), layers_vec.data());
+                        }
                     }
                     else if (layer_type == "dropout" || layer_type == "cost")
                     {

modules/dnn/test/test_darknet_importer.cpp

@@ -627,6 +627,12 @@ TEST_P(Test_Darknet_layers, reorg)
     testDarknetLayer("reorg");
 }
 
+TEST_P(Test_Darknet_layers, route)
+{
+    testDarknetLayer("route");
+    testDarknetLayer("route_multi");
+}
+
 TEST_P(Test_Darknet_layers, maxpool)
 {
 #if defined(INF_ENGINE_RELEASE) && INF_ENGINE_VER_MAJOR_GE(2020020000)