add cv::flipND; support onnx slice with negative steps via cv::flipND

modules/core/include/opencv2/core.hpp

@@ -1102,6 +1102,13 @@ around both axes.
 */
 CV_EXPORTS_W void flip(InputArray src, OutputArray dst, int flipCode);
 
+/** @brief Flips a n-dimensional at given axis
+ *  @param src input array
+ *  @param dst output array that has the same shape of src
+ *  @param axis axis that performs a flip on. 0 <= axis < src.dims.
+ */
+CV_EXPORTS_W void flipND(InputArray src, OutputArray dst, int axis);
+
 enum RotateFlags {
     ROTATE_90_CLOCKWISE = 0, //!<Rotate 90 degrees clockwise
     ROTATE_180 = 1, //!<Rotate 180 degrees clockwise

modules/core/src/matrix_transform.cpp

@@ -6,6 +6,8 @@
 #include "opencl_kernels_core.hpp"
 #include "opencv2/core/detail/dispatch_helper.impl.hpp"
 
+#include <algorithm> // std::swap_ranges
+
 namespace cv {
 
 ////////////////////////////////////// transpose /////////////////////////////////////////
@@ -812,6 +814,49 @@ void flip( InputArray _src, OutputArray _dst, int flip_mode )
         flipHoriz( dst.ptr(), dst.step, dst.ptr(), dst.step, dst.size(), esz );
 }
 
+static void
+flipNDImpl(uchar* data, const int* shape, const size_t* step, int axis)
+{
+    int total = 1;
+    for (int i = 0; i < axis; ++i)
+        total *= shape[i];
+
+    int shape_at_axis = shape[axis];
+    size_t step_at_axis = step[axis];
+    size_t offset = 0;
+    size_t offset_increment = axis == 0 ? 0 : step[axis - 1];
+    for (int i = 0; i < total; ++i, offset += offset_increment)
+        for (int j = 0, k = shape_at_axis - 1; j < shape_at_axis / 2; ++j, --k)
+            std::swap_ranges(data + offset + j * step_at_axis,
+                             data + offset + j * step_at_axis + step_at_axis,
+                             data + offset + k * step_at_axis);
+}
+
+void flipND(InputArray _src, OutputArray _dst, int _axis)
+{
+    CV_INSTRUMENT_REGION();
+
+    Mat src = _src.getMat();
+
+    // verify axis
+    int ndim = src.dims;
+    CV_CheckLT(_axis, ndim, "flipND: given axis is out of range");
+    CV_CheckGE(_axis, -ndim, "flipND: given axis is out of range");
+    int axis = (_axis + ndim) % ndim;
+
+    // in-place flip
+    _src.copyTo(_dst);
+
+    // return the src if it has only one element on the flip axis
+    const auto shape = src.size.p;
+    if (shape[axis] == 1)
+        return ;
+
+    // call impl
+    Mat dst = _dst.getMat();
+    flipNDImpl(dst.ptr(), dst.size.p, dst.step.p, axis);
+}
+
 void rotate(InputArray _src, OutputArray _dst, int rotateMode)
 {
     CV_Assert(_src.dims() <= 2);

modules/core/test/test_arithm.cpp

@@ -2201,6 +2201,72 @@ INSTANTIATE_TEST_CASE_P(Arithm, TransposeND, testing::Combine(
     testing::Values(perf::MatType(CV_8UC1), CV_32FC1)
 ));
 
+class FlipND : public testing::TestWithParam< tuple<std::vector<int>, perf::MatType> >
+{
+public:
+    std::vector<int> m_shape;
+    int m_type;
+
+    void SetUp()
+    {
+        std::tie(m_shape, m_type) = GetParam();
+    }
+};
+
+TEST_P(FlipND, basic)
+{
+    Mat inp(m_shape, m_type);
+    randu(inp, 0, 255);
+
+    int ndim = static_cast<int>(m_shape.size());
+    std::vector<int> axes(ndim*2); // [-shape, shape)
+    std::iota(axes.begin(), axes.end(), -ndim);
+    auto get_flipped_indices = [&inp, ndim] (size_t total, std::vector<int>& indices, int axis)
+    {
+        const int* shape = inp.size.p;
+        size_t t = total, idx;
+        for (int i = ndim - 1; i >= 0; --i)
+        {
+            idx = t / shape[i];
+            indices[i] = int(t - idx * shape[i]);
+            t = idx;
+        }
+
+        int _axis = (axis + ndim) % ndim;
+        std::vector<int> flipped_indices = indices;
+        flipped_indices[_axis] = shape[_axis] - 1 - indices[_axis];
+        return flipped_indices;
+    };
+
+    for (size_t i = 0; i < axes.size(); ++i)
+    {
+        int axis = axes[i];
+        Mat out;
+        cv::flipND(inp, out, axis);
+        // check values
+        std::vector<int> indices(ndim, 0);
+        for (size_t j = 0; j < inp.total(); ++j)
+        {
+            auto flipped_indices = get_flipped_indices(j, indices, axis);
+            switch (inp.type())
+            {
+            case CV_8UC1:
+                ASSERT_EQ(inp.at<uint8_t>(indices.data()), out.at<uint8_t>(flipped_indices.data()));
+                break;
+            case CV_32FC1:
+                ASSERT_EQ(inp.at<float>(indices.data()), out.at<float>(flipped_indices.data()));
+                break;
+            default:
+                FAIL() << "Unsupported type: " << inp.type();
+            }
+        }
+    }
+}
+
+INSTANTIATE_TEST_CASE_P(Arithm, FlipND, testing::Combine(
+    testing::Values(std::vector<int>{5, 10}, std::vector<int>{2, 3, 4}),
+    testing::Values(perf::MatType(CV_8UC1), CV_32FC1)
+));
 
 TEST(Core_minMaxIdx, regression_9207_2)
 {

modules/dnn/src/layers/slice_layer.cpp

@@ -84,6 +84,34 @@ Range normalizeRange(const Range& input_range, int n)
     return range;
 }
 
+// TODO: support cv::Range with steps and negative steps to get rid of this transformation
+void tranformForNegSteps(const MatShape& inpShape, std::vector<std::vector<Range> >& sliceRanges, std::vector<std::vector<int> >& sliceSteps)
+{
+    // in case of negative steps,
+    // x of shape [5, 10], x[5:0:-1, 10:1:-3] <=> np.flip(x[1:5:1, 2:10:3], aixs=(0, 1))
+    // new_end_i = start_i + 1 > dim_i ? dim_i : start_i + 1
+    // new_start_i = end + 1
+    // new_start_i = new_end_i - 1 - ((new_end_i - 1 - new_start_i) / abs(step_i)) * abs(step_i)
+    int start, end, new_start, new_end, step;
+    for (int i = 0; i < sliceSteps[0].size(); ++i)
+    {
+        step = sliceSteps[0][i];
+        if (step > 0)
+            continue;
+
+        step = -step;
+        start = sliceRanges[0][i].start;
+        end = sliceRanges[0][i].end;
+        new_end = start >= inpShape[i] ? inpShape[i] : start + 1;
+        new_start = end + 1;
+        new_start = new_end - 1 - ((new_end - 1 - new_start) / step) * step;
+
+        sliceSteps[0][i] = step;
+        sliceRanges[0][i].start = new_start;
+        sliceRanges[0][i].end = new_end;
+    }
+}
+
 std::vector<std::vector<cv::Range> > finalizeSliceRange(const MatShape& inpShape, int& axis,
                                                         const std::vector<std::vector<cv::Range> >& inputSliceRanges)
 {
@@ -149,6 +177,24 @@ public:
             const DictValue &sizesOrEnds = params.has("size") ? params.get("size") : params.get("end");
             CV_Assert(begins.size() == sizesOrEnds.size());
 
+            if (params.has("steps"))
+            {
+                const DictValue &steps = params.get("steps");
+                sliceSteps.resize(1);
+                sliceSteps[0].resize(steps.size());
+
+                for (int i = 0; i < steps.size(); ++i)
+                {
+                    int step = steps.get<int>(i);
+                    CV_Assert(step != 0);
+                    if (step < 0)
+                        neg_step_dims.push_back(i);
+                    if (std::abs(step) > 1)
+                        hasSteps = true;
+                    sliceSteps[0][i] = step;
+                }
+            }
+
             sliceRanges.resize(1);
             sliceRanges[0].resize(begins.size(), Range::all());
             for (int i = 0; i < begins.size(); ++i)
@@ -166,26 +212,13 @@ public:
                 else
                 {
                     int end = sizeOrEnd;
-                    CV_Assert(end < 0 || end > start);  // End index is excluded.
+                    if (hasSteps && !neg_step_dims.empty() && sliceSteps[0][i] < 0)
+                        CV_Assert(end < 0 || end != start); // if current step is negative, end < start is allowed.
+                    else
+                        CV_Assert(end < 0 || end > start);  // End index is excluded.
                     sliceRanges[0][i].end = end;  // We'll finalize a negative value later.
                 }
             }
-
-            if (params.has("steps"))
-            {
-                const DictValue &steps = params.get("steps");
-                sliceSteps.resize(1);
-                sliceSteps[0].resize(steps.size());
-
-                for (int i = 0; i < steps.size(); ++i)
-                {
-                    int step = steps.get<int>(i);
-                    CV_Assert(step >= 1);
-                    if (step > 1)
-                        hasSteps = true;
-                    sliceSteps[0][i] = step;
-                }
-            }
         }
     }
 
@@ -193,11 +226,11 @@ public:
     {
 #ifdef HAVE_INF_ENGINE
         if (backendId == DNN_BACKEND_INFERENCE_ENGINE_NGRAPH)
-            return sliceRanges.size() == 1 && !hasSteps;
+            return sliceRanges.size() == 1 && !hasSteps && neg_step_dims.empty();
 #endif
 #ifdef HAVE_CUDA
         if (backendId == DNN_BACKEND_CUDA)
-            return !hasSteps;
+            return !hasSteps && neg_step_dims.empty();
 #endif
         return backendId == DNN_BACKEND_OPENCV || backendId == DNN_BACKEND_CANN;
     }
@@ -210,8 +243,13 @@ public:
         CV_Assert(inputs.size() == 1);
         MatShape inpShape = inputs[0];
 
+        std::vector<std::vector<int> > sliceSteps_ = sliceSteps;
+        std::vector<std::vector<cv::Range> > sliceRanges_ = sliceRanges;
+        if (hasSteps && !neg_step_dims.empty())
+            tranformForNegSteps(inpShape, sliceRanges_, sliceSteps_);
+
         int axis_rw = axis;
-        std::vector<std::vector<cv::Range> > sliceRanges_rw = finalizeSliceRange(inpShape, axis_rw, sliceRanges);
+        std::vector<std::vector<cv::Range> > sliceRanges_rw = finalizeSliceRange(inpShape, axis_rw, sliceRanges_);
 
         if (!sliceRanges_rw.empty())
         {
@@ -224,8 +262,8 @@ public:
                     if (shapesInitialized || inpShape[j] > 0)
                         outputs[i][j] = normalizeRange(sliceRanges_rw[i][j], inpShape[j]).size();
 
-                    if (!sliceSteps.empty() && (i < sliceSteps.size()) && (j < sliceSteps[i].size()) && (sliceSteps[i][j] > 1))
+                    if (!sliceSteps_.empty() && (i < sliceSteps_.size()) && (j < sliceSteps_[i].size()) && (sliceSteps_[i][j] > 1))
-                        outputs[i][j] = (outputs[i][j] + sliceSteps[i][j] - 1) / sliceSteps[i][j];
+                        outputs[i][j] = (outputs[i][j] + sliceSteps_[i][j] - 1) / sliceSteps_[i][j];
                 }
             }
         }
@@ -257,7 +295,10 @@ public:
         outputs_arr.getMatVector(outputs);
 
         CV_Assert(inputs.size() == 1);
-        const MatSize& inpShape = inputs[0].size;
+        MatShape inpShape = shape(inputs[0]);
+
+        if (hasSteps && !neg_step_dims.empty())
+            tranformForNegSteps(inpShape, sliceRanges, sliceSteps);
 
         finalSliceRanges = finalizeSliceRange(shape(inputs[0]), axis, sliceRanges);
 
@@ -280,9 +321,9 @@ public:
 
         for (int i = 0; i < outputs.size(); ++i)
         {
-            CV_Assert(finalSliceRanges[i].size() <= inpShape.dims());
+            CV_Assert(finalSliceRanges[i].size() <= inpShape.size());
             // Fill the rest of ranges.
-            for (int j = finalSliceRanges[i].size(); j < inpShape.dims(); ++j)
+            for (int j = finalSliceRanges[i].size(); j < inpShape.size(); ++j)
             {
                 finalSliceRanges[i].push_back(Range::all());
             }
@@ -586,6 +627,8 @@ public:
                     getSliceRecursive<int8_t>(inpMat, inpIdx, finalSliceRanges[i], sliceSteps[i], 0, dimsNum, outputs[i], outIdx);
                 else
                     getSliceRecursive<float>(inpMat, inpIdx, finalSliceRanges[i], sliceSteps[i], 0, dimsNum, outputs[i], outIdx);
+                // flip for negative steps
+                flip(outputs[i]);
             }
         }
     }
@@ -650,7 +693,6 @@ public:
     }
 #endif
 
-
 #ifdef HAVE_DNN_NGRAPH
     virtual Ptr<BackendNode> initNgraph(const std::vector<Ptr<BackendWrapper> >& inputs,
                                         const std::vector<Ptr<BackendNode> >& nodes) CV_OVERRIDE
@@ -739,9 +781,15 @@ private:
         }
     }
 
+    void flip(Mat& output) // break if 1d tensor?
+    {
+        for (int i = 0; i < neg_step_dims.size(); ++i)
+                cv::flipND(output, output, neg_step_dims[i]);
+    }
 protected:
     // The actual non-negative values determined from @p sliceRanges depends on input size.
     std::vector<std::vector<Range> > finalSliceRanges;
+    std::vector<int> neg_step_dims;
     bool hasDynamicShapes;
     bool shapesInitialized;
     bool hasSteps;

modules/dnn/test/test_onnx_importer.cpp

@@ -1145,6 +1145,7 @@ TEST_P(Test_ONNX_layers, Slice)
     testONNXModels("slice");
     testONNXModels("slice_neg_starts");
     testONNXModels("slice_opset_11");
+    testONNXModels("slice_neg_steps", pb);
 #endif
 }