Merge pull request opencv#17386 from l-bat:tf_clamp_subgraph

* Added ClipByValue subgraph

* Return const nodes

Author: l-bat

modules/dnn/src/tensorflow/tf_graph_simplifier.cpp

@@ -725,6 +725,21 @@ class PReLUSubgraph : public TFSubgraph
     bool negativeScales;
 };
 
+class ClipByValueSubgraph : public TFSubgraph
+{
+public:
+    ClipByValueSubgraph()
+    {
+        int input = addNodeToMatch("");
+        int maxValue = addNodeToMatch("Const");
+        int minimum = addNodeToMatch("Minimum", input, maxValue);
+        int minValue = addNodeToMatch("Const");
+        addNodeToMatch("Maximum", minimum, minValue);
+
+        setFusedNode("ClipByValue", input, minValue, maxValue);
+    }
+};
+
 void simplifySubgraphs(tensorflow::GraphDef& net)
 {
     std::vector<Ptr<Subgraph> > subgraphs;
@@ -749,6 +764,7 @@ void simplifySubgraphs(tensorflow::GraphDef& net)
     subgraphs.push_back(Ptr<Subgraph>(new PReLUSubgraph(false)));
     subgraphs.push_back(Ptr<Subgraph>(new FlattenProdSubgraph()));
     subgraphs.push_back(Ptr<Subgraph>(new ResizeBilinearSubgraphDown()));
+    subgraphs.push_back(Ptr<Subgraph>(new ClipByValueSubgraph()));
 
     for (int i = 0; i < net.node_size(); ++i)
     {

modules/dnn/test/test_tf_importer.cpp

@@ -977,6 +977,11 @@ TEST_P(Test_TensorFlow_layers, tf2_dense)
     runTensorFlowNet("tf2_dense");
 }
 
+TEST_P(Test_TensorFlow_layers, clip_by_value)
+{
+    runTensorFlowNet("clip_by_value");
+}
+
 TEST_P(Test_TensorFlow_layers, tf2_prelu)
 {
     if (backend == DNN_BACKEND_INFERENCE_ENGINE_NN_BUILDER_2019)