Make DNN Crop layer match Caffe default offset behavior

and add parametric unit test for crop layer.

modules/dnn/src/layers/crop_layer.cpp

@@ -110,27 +110,13 @@ public:
         }
 
         crop_ranges.resize(dims, Range::all());
-        for (int i = 0; i < dims; i++)
+        for (int i = start_axis; i < dims; i++)
-        {
-            if( i < start_axis )
-                continue;
-
-            if (!offset.empty()) //normal case
         {
             if (offset_final[i] < 0 || offset_final[i] + inpSzBlob.size[i] > inpBlob.size[i])
-                    CV_Error(Error::StsBadArg, "invalid crop parameters");
+                CV_Error(Error::StsBadArg, "invalid crop parameters or blob sizes");
 
             crop_ranges[i] = Range(offset_final[i], offset_final[i] + inpSzBlob.size[i]);
         }
-            else //detect offset automatically so that cropped image is center of original one
-            {
-                if (inpSzBlob.size[i] > inpBlob.size[i])
-                    CV_Error(Error::StsBadArg, "invalid output blob size");
-
-                int cur_crop = (inpBlob.size[i] - inpSzBlob.size[i]) / 2;
-                crop_ranges[i] = Range(cur_crop, cur_crop + inpSzBlob.size[i]);
-            }
-        }
     }
 
     void forward(InputArrayOfArrays inputs_arr, OutputArrayOfArrays outputs_arr, OutputArrayOfArrays internals_arr)

modules/dnn/test/test_layers.cpp

@@ -715,4 +715,87 @@ INSTANTIATE_TEST_CASE_P(Layer_Test, Scale_untrainable, Combine(
 /*conv fusion*/  testing::Bool()
 ));
 
+typedef testing::TestWithParam<tuple<Vec4i, Vec4i, int, int, int> > Crop;
+TEST_P(Crop, Accuracy)
+{
+    Vec4i inpShapeVec = get<0>(GetParam());
+    Vec4i sizShapeVec = get<1>(GetParam());
+    int axis = get<2>(GetParam());
+    int numOffsets = get<3>(GetParam());
+    int offsetVal = get<4>(GetParam());
+    const int inpShape[] = {inpShapeVec[0], inpShapeVec[1], inpShapeVec[2], inpShapeVec[3]};
+    const int sizShape[] = {sizShapeVec[0], sizShapeVec[1], sizShapeVec[2], sizShapeVec[3]};
+
+    // Create a network with two inputs. Crop layer crops a first input to
+    // the size of a second one.
+    // See http://caffe.berkeleyvision.org/tutorial/layers/crop.html
+    Net net;
+
+    LayerParams lp;
+    lp.name = "testCrop";
+    lp.type = "Crop";
+    lp.set("axis", axis);
+    if (numOffsets > 0)
+    {
+        std::vector<int> offsets(numOffsets, offsetVal);
+        lp.set("offset", DictValue::arrayInt<int*>(&offsets[0], offsets.size()));
+    }
+    else
+        offsetVal = 0;
+    int id = net.addLayerToPrev(lp.name, lp.type, lp);
+    net.connect(0, 1, id, 1);
+
+    Mat inpImage(4, inpShape, CV_32F);
+    Mat sizImage(4, sizShape, CV_32F);
+    randu(inpImage, -1, 1);
+    randu(sizImage, -1, 1);
+
+    std::vector<String> inpNames(2);
+    inpNames[0] = "cropImage";
+    inpNames[1] = "sizImage";
+    net.setInputsNames(inpNames);
+    net.setInput(inpImage, inpNames[0]);
+    net.setInput(sizImage, inpNames[1]);
+
+    // There are a few conditions that represent invalid input to the crop
+    // layer, so in those cases we want to verify an exception is thrown.
+
+    bool shouldThrowException = false;
+    if (numOffsets > 1 && numOffsets != 4 - axis)
+        shouldThrowException = true;
+    else
+        for (int i = axis; i < 4; i++)
+            if (sizShape[i] + offsetVal > inpShape[i])
+                shouldThrowException = true;
+
+    Mat out;
+    if (shouldThrowException)
+    {
+        ASSERT_ANY_THROW(out = net.forward());
+        return;
+    }
+    else
+        out = net.forward();
+
+    // Finally, compare the cropped output blob from the DNN layer (out)
+    // to a reference blob (ref) that we compute here.
+
+    std::vector<Range> crop_range;
+    crop_range.resize(4, Range::all());
+    for (int i = axis; i < 4; i++)
+        crop_range[i] = Range(offsetVal, sizShape[i] + offsetVal);
+
+    Mat ref(sizImage.dims, sizImage.size, CV_32F);
+    inpImage(&crop_range[0]).copyTo(ref);
+    normAssert(out, ref);
+}
+
+INSTANTIATE_TEST_CASE_P(Layer_Test, Crop, Combine(
+/*input blob shape*/    Values(Vec4i(1, 3, 20, 30)),
+/*cropsize blob shape*/ Values(Vec4i(1, 3, 10, 12)),
+/*start axis*/          Values(0, 1, 2),
+/*number of offsets*/   Values(0, 1, 2, 4),
+/*offset value*/        Values(3, 4)
+));
+
 }