add SoftNMS implementation

modules/dnn/include/opencv2/dnn/dnn.hpp

@@ -1130,6 +1130,39 @@ CV__DNN_INLINE_NS_BEGIN
                              CV_OUT std::vector<int>& indices,
                              const float eta = 1.f, const int top_k = 0);
 
+    /**
+     * @brief Enum of Soft NMS methods.
+     * @see softNMSBoxes
+     */
+    enum class SoftNMSMethod
+    {
+        SOFTNMS_LINEAR = 1,
+        SOFTNMS_GAUSSIAN = 2
+    };
+
+    /** @brief Performs soft non maximum suppression given boxes and corresponding scores.
+     * Reference: https://arxiv.org/abs/1704.04503
+     * @param bboxes a set of bounding boxes to apply Soft NMS.
+     * @param scores a set of corresponding confidences.
+     * @param updated_scores a set of corresponding updated confidences.
+     * @param score_threshold a threshold used to filter boxes by score.
+     * @param nms_threshold a threshold used in non maximum suppression.
+     * @param indices the kept indices of bboxes after NMS.
+     * @param top_k keep at most @p top_k picked indices.
+     * @param sigma parameter of Gaussian weighting.
+     * @param method Gaussian or linear.
+     * @see SoftNMSMethod
+     */
+    CV_EXPORTS_W void softNMSBoxes(const std::vector<Rect>& bboxes,
+                                   const std::vector<float>& scores,
+                                   CV_OUT std::vector<float>& updated_scores,
+                                   const float score_threshold,
+                                   const float nms_threshold,
+                                   CV_OUT std::vector<int>& indices,
+                                   size_t top_k = 0,
+                                   const float sigma = 0.5,
+                                   SoftNMSMethod method = SoftNMSMethod::SOFTNMS_GAUSSIAN);
+
 
      /** @brief This class is presented high-level API for neural networks.
       *

modules/dnn/src/nms.cpp

@@ -58,6 +58,83 @@ void NMSBoxes(const std::vector<RotatedRect>& bboxes, const std::vector<float>&
     NMSFast_(bboxes, scores, score_threshold, nms_threshold, eta, top_k, indices, rotatedRectIOU);
 }
 
+void softNMSBoxes(const std::vector<Rect>& bboxes,
+                  const std::vector<float>& scores,
+                  std::vector<float>& updated_scores,
+                  const float score_threshold,
+                  const float nms_threshold,
+                  std::vector<int>& indices,
+                  size_t top_k,
+                  const float sigma,
+                  SoftNMSMethod method)
+{
+    CV_Assert_N(bboxes.size() == scores.size(), score_threshold >= 0,
+                nms_threshold >= 0, sigma >= 0);
+
+    indices.clear();
+    updated_scores.clear();
+
+    std::vector<std::pair<float, size_t> > score_index_vec(scores.size());
+    for (size_t i = 0; i < scores.size(); i++)
+    {
+        score_index_vec[i].first = scores[i];
+        score_index_vec[i].second = i;
+    }
+
+    const auto score_cmp = [](const std::pair<float, size_t>& a, const std::pair<float, size_t>& b)
+    {
+        return a.first == b.first ? a.second > b.second : a.first < b.first;
+    };
+
+    top_k = top_k == 0 ? scores.size() : std::min(top_k, scores.size());
+    ptrdiff_t start = 0;
+    while (indices.size() < top_k)
+    {
+        auto it = std::max_element(score_index_vec.begin() + start, score_index_vec.end(), score_cmp);
+
+        float bscore = it->first;
+        size_t bidx = it->second;
+
+        if (bscore < score_threshold)
+        {
+            break;
+        }
+
+        indices.push_back(static_cast<int>(bidx));
+        updated_scores.push_back(bscore);
+        std::swap(score_index_vec[start], *it); // first start elements are chosen
+
+        for (size_t i = start + 1; i < scores.size(); ++i)
+        {
+            float& bscore_i = score_index_vec[i].first;
+            const size_t bidx_i = score_index_vec[i].second;
+
+            if (bscore_i < score_threshold)
+            {
+                continue;
+            }
+
+            float overlap = rectOverlap(bboxes[bidx], bboxes[bidx_i]);
+
+            switch (method)
+            {
+                case SoftNMSMethod::SOFTNMS_LINEAR:
+                    if (overlap > nms_threshold)
+                    {
+                        bscore_i *= 1.f - overlap;
+                    }
+                    break;
+                case SoftNMSMethod::SOFTNMS_GAUSSIAN:
+                    bscore_i *= exp(-(overlap * overlap) / sigma);
+                    break;
+                default:
+                    CV_Error(Error::StsBadArg, "Not supported SoftNMS method.");
+            }
+        }
+        ++start;
+    }
+}
+
 CV__DNN_INLINE_NS_END
 }// dnn
 }// cv

modules/dnn/test/test_nms.cpp

@@ -37,4 +37,41 @@ TEST(NMS, Accuracy)
         ASSERT_EQ(indices[i], ref_indices[i]);
 }
 
+TEST(SoftNMS, Accuracy)
+{
+    //reference results are obtained using TF v2.7 tf.image.non_max_suppression_with_scores
+    std::string dataPath = findDataFile("dnn/soft_nms_reference.yml");
+    FileStorage fs(dataPath, FileStorage::READ);
+
+    std::vector<Rect> bboxes;
+    std::vector<float> scores;
+    std::vector<int> ref_indices;
+    std::vector<float> ref_updated_scores;
+
+    fs["boxes"] >> bboxes;
+    fs["probs"] >> scores;
+    fs["indices"] >> ref_indices;
+    fs["updated_scores"] >> ref_updated_scores;
+
+    std::vector<float> updated_scores;
+    const float score_thresh = .01f;
+    const float nms_thresh = .5f;
+    std::vector<int> indices;
+    const size_t top_k = 0;
+    const float sigma = 1.; // sigma in TF is being multiplied by 2, so 0.5 should be passed there
+    cv::dnn::softNMSBoxes(bboxes, scores, updated_scores, score_thresh, nms_thresh, indices, top_k, sigma);
+
+    ASSERT_EQ(ref_indices.size(), indices.size());
+    for(size_t i = 0; i < indices.size(); i++)
+    {
+        ASSERT_EQ(indices[i], ref_indices[i]);
+    }
+
+    ASSERT_EQ(ref_updated_scores.size(), updated_scores.size());
+    for(size_t i = 0; i < updated_scores.size(); i++)
+    {
+        EXPECT_NEAR(updated_scores[i], ref_updated_scores[i], 1e-7);
+    }
+}
+
 }} // namespace