Merge pull request #7953 from qingqing01/multiclass_nms_op

 Add multi-class non-maximum suppression operator.

paddle/operators/bipartite_match_op.cc

-/* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
+/* Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserve.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -28,12 +28,18 @@ class BipartiteMatchOp : public framework::OperatorWithKernel {
   void InferShape(framework::InferShapeContext* ctx) const override {
     PADDLE_ENFORCE(ctx->HasInput("DistMat"),
                    "Input(DistMat) of BipartiteMatch should not be null.");
+    PADDLE_ENFORCE(
+        ctx->HasOutput("ColToRowMatchIndices"),
+        "Output(ColToRowMatchIndices) of BipartiteMatch should not be null.");
+    PADDLE_ENFORCE(
+        ctx->HasOutput("ColToRowMatchDist"),
+        "Output(ColToRowMatchDist) of BipartiteMatch should not be null.");
 
     auto dims = ctx->GetInputDim("DistMat");
     PADDLE_ENFORCE_EQ(dims.size(), 2, "The rank of Input(DistMat) must be 2.");
 
     ctx->SetOutputDim("ColToRowMatchIndices", dims);
-    ctx->SetOutputDim("ColToRowMatchDis", dims);
+    ctx->SetOutputDim("ColToRowMatchDist", dims);
   }
 };
 
@@ -91,7 +97,7 @@ class BipartiteMatchKernel : public framework::OpKernel<T> {
   void Compute(const framework::ExecutionContext& context) const override {
     auto* dist_mat = context.Input<LoDTensor>("DistMat");
     auto* match_indices = context.Output<Tensor>("ColToRowMatchIndices");
-    auto* match_dist = context.Output<Tensor>("ColToRowMatchDis");
+    auto* match_dist = context.Output<Tensor>("ColToRowMatchDist");
 
     auto& dev_ctx = context.device_context<platform::CPUDeviceContext>();
 
@@ -148,13 +154,13 @@ class BipartiteMatchOpMaker : public framework::OpProtoAndCheckerMaker {
               "Otherwise, it means B[j] is matched to row "
               "ColToRowMatchIndices[i][j] in i-th instance. The row number of "
               "i-th instance is saved in ColToRowMatchIndices[i][j].");
-    AddOutput("ColToRowMatchDis",
+    AddOutput("ColToRowMatchDist",
               "(Tensor) A 2-D Tensor with shape [N, M] in float type. "
               "N is batch size. If ColToRowMatchIndices[i][j] is -1, "
-              "ColToRowMatchDis[i][j] is also -1.0. Otherwise, assumed "
+              "ColToRowMatchDist[i][j] is also -1.0. Otherwise, assumed "
               "ColToRowMatchIndices[i][j] = d, and the row offsets of each "
               "instance are called LoD. Then "
-              "ColToRowMatchDis[i][j] = DistMat[d+LoD[i]][j]");
+              "ColToRowMatchDist[i][j] = DistMat[d+LoD[i]][j]");
     AddComment(R"DOC(
 This operator is a greedy bipartite matching algorithm, which is used to
 obtain the matching with the maximum distance based on the input

paddle/operators/multiclass_nms_op.cc

+/* Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserve.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
+
+#include "paddle/framework/op_registry.h"
+
+namespace paddle {
+namespace operators {
+
+using Tensor = framework::Tensor;
+using LoDTensor = framework::LoDTensor;
+
+constexpr int64_t kOutputDim = 6;
+constexpr int64_t kBBoxSize = 4;
+
+class MultiClassNMSOp : public framework::OperatorWithKernel {
+ public:
+  using framework::OperatorWithKernel::OperatorWithKernel;
+
+  void InferShape(framework::InferShapeContext* ctx) const override {
+    PADDLE_ENFORCE(ctx->HasInput("BBoxes"),
+                   "Input(BBoxes) of MultiClassNMS should not be null.");
+    PADDLE_ENFORCE(ctx->HasInput("Scores"),
+                   "Input(Scores) of MultiClassNMS should not be null.");
+    PADDLE_ENFORCE(ctx->HasOutput("Out"),
+                   "Output(Out) of MultiClassNMS should not be null.");
+
+    auto box_dims = ctx->GetInputDim("BBoxes");
+    auto score_dims = ctx->GetInputDim("Scores");
+
+    PADDLE_ENFORCE_EQ(box_dims.size(), 2,
+                      "The rank of Input(BBoxes) must be 2.");
+    PADDLE_ENFORCE_EQ(score_dims.size(), 3,
+                      "The rank of Input(Scores) must be 3.");
+    PADDLE_ENFORCE_EQ(box_dims[1], 4,
+                      "The 2nd dimension of Input(BBoxes) must be 4, "
+                      "represents the layout of coordinate "
+                      "[xmin, ymin, xmax, ymax]");
+    PADDLE_ENFORCE_EQ(box_dims[0], score_dims[2],
+                      "The 1st dimensiong of Input(BBoxes) must be equal to "
+                      "3rd dimension of Input(Scores), which represents the "
+                      "predicted bboxes.");
+
+    // Here the box_dims[0] is not the real dimension of output.
+    // It will be rewritten in the computing kernel.
+    ctx->SetOutputDim("Out", {box_dims[0], 6});
+  }
+
+ protected:
+  framework::OpKernelType GetExpectedKernelType(
+      const framework::ExecutionContext& ctx) const override {
+    return framework::OpKernelType(
+        framework::ToDataType(
+            ctx.Input<framework::LoDTensor>("Scores")->type()),
+        ctx.device_context());
+  }
+};
+
+template <class T>
+bool SortScorePairDescend(const std::pair<float, T>& pair1,
+                          const std::pair<float, T>& pair2) {
+  return pair1.first > pair2.first;
+}
+
+template <class T>
+static inline void GetMaxScoreIndex(
+    const std::vector<T>& scores, const T threshold, int top_k,
+    std::vector<std::pair<T, int>>* sorted_indices) {
+  for (size_t i = 0; i < scores.size(); ++i) {
+    if (scores[i] > threshold) {
+      sorted_indices->push_back(std::make_pair(scores[i], i));
+    }
+  }
+  // Sort the score pair according to the scores in descending order
+  std::stable_sort(sorted_indices->begin(), sorted_indices->end(),
+                   SortScorePairDescend<int>);
+  // Keep top_k scores if needed.
+  if (top_k > -1 && top_k < sorted_indices->size()) {
+    sorted_indices->resize(top_k);
+  }
+}
+
+template <class T>
+static inline T BBoxArea(const T* box, const bool normalized) {
+  if (box[2] < box[0] || box[3] < box[1]) {
+    // If coordinate values are is invalid
+    // (e.g. xmax < xmin or ymax < ymin), return 0.
+    return static_cast<T>(0.);
+  } else {
+    const T w = box[2] - box[0];
+    const T h = box[3] - box[1];
+    if (normalized) {
+      return w * h;
+    } else {
+      // If coordinate values are not within range [0, 1].
+      return (w + 1) * (h + 1);
+    }
+  }
+}
+
+template <class T>
+static inline T JaccardOverlap(const T* box1, const T* box2,
+                               const bool normalized) {
+  if (box2[0] > box1[2] || box2[2] < box1[0] || box2[1] > box1[3] ||
+      box2[3] < box1[1]) {
+    return static_cast<T>(0.);
+  } else {
+    const T inter_xmin = std::max(box1[0], box2[0]);
+    const T inter_ymin = std::max(box1[1], box2[1]);
+    const T inter_xmax = std::min(box1[2], box2[2]);
+    const T inter_ymax = std::min(box1[3], box2[3]);
+    const T inter_w = inter_xmax - inter_xmin;
+    const T inter_h = inter_ymax - inter_ymin;
+    const T inter_area = inter_w * inter_h;
+    const T bbox1_area = BBoxArea<T>(box1, normalized);
+    const T bbox2_area = BBoxArea<T>(box2, normalized);
+    return inter_area / (bbox1_area + bbox2_area - inter_area);
+  }
+}
+
+template <typename T>
+class MultiClassNMSKernel : public framework::OpKernel<T> {
+ public:
+  void NMSFast(const Tensor& bbox, const Tensor& scores,
+               const T score_threshold, const T nms_threshold, const T eta,
+               const int64_t top_k, std::vector<int>* selected_indices) const {
+    // The total boxes for each instance.
+    int64_t num_boxes = bbox.dims()[0];
+    // 4: [xmin ymin xmax ymax]
+    int64_t box_size = bbox.dims()[1];
+
+    std::vector<T> scores_data(num_boxes);
+    std::copy_n(scores.data<T>(), num_boxes, scores_data.begin());
+    std::vector<std::pair<T, int>> sorted_indices;
+    GetMaxScoreIndex(scores_data, score_threshold, top_k, &sorted_indices);
+
+    selected_indices->clear();
+    T adaptive_threshold = nms_threshold;
+    const T* bbox_data = bbox.data<T>();
+
+    while (sorted_indices.size() != 0) {
+      const int idx = sorted_indices.front().second;
+      bool keep = true;
+      for (int k = 0; k < selected_indices->size(); ++k) {
+        if (keep) {
+          const int kept_idx = (*selected_indices)[k];
+          T overlap = JaccardOverlap<T>(bbox_data + idx * box_size,
+                                        bbox_data + kept_idx * box_size, true);
+          keep = overlap <= adaptive_threshold;
+        } else {
+          break;
+        }
+      }
+      if (keep) {
+        selected_indices->push_back(idx);
+      }
+      sorted_indices.erase(sorted_indices.begin());
+      if (keep && eta < 1 && adaptive_threshold > 0.5) {
+        adaptive_threshold *= eta;
+      }
+    }
+  }
+
+  void MultiClassNMS(const framework::ExecutionContext& ctx,
+                     const Tensor& scores, const Tensor& bboxes,
+                     std::map<int, std::vector<int>>& indices,
+                     int& num_nmsed_out) const {
+    int64_t background_label = ctx.Attr<int>("background_label");
+    int64_t nms_top_k = ctx.Attr<int>("nms_top_k");
+    int64_t keep_top_k = ctx.Attr<int>("keep_top_k");
+    T nms_threshold = static_cast<T>(ctx.Attr<float>("nms_threshold"));
+    T nms_eta = static_cast<T>(ctx.Attr<float>("nms_eta"));
+    T score_threshold = static_cast<T>(ctx.Attr<float>("score_threshold"));
+
+    int64_t class_num = scores.dims()[0];
+    int64_t predict_dim = scores.dims()[1];
+    int num_det = 0;
+    for (int64_t c = 0; c < class_num; ++c) {
+      if (c == background_label) continue;
+      Tensor score = scores.Slice(c, c + 1);
+      NMSFast(bboxes, score, score_threshold, nms_threshold, nms_eta, nms_top_k,
+              &(indices[c]));
+      num_det += indices[c].size();
+    }
+
+    num_nmsed_out = num_det;
+    const T* scores_data = scores.data<T>();
+    if (keep_top_k > -1 && num_det > keep_top_k) {
+      std::vector<std::pair<float, std::pair<int, int>>> score_index_pairs;
+      for (const auto& it : indices) {
+        int label = it.first;
+        const T* sdata = scores_data + label * predict_dim;
+        const std::vector<int>& label_indices = it.second;
+        for (int j = 0; j < label_indices.size(); ++j) {
+          int idx = label_indices[j];
+          PADDLE_ENFORCE_LT(idx, predict_dim);
+          score_index_pairs.push_back(
+              std::make_pair(sdata[idx], std::make_pair(label, idx)));
+        }
+      }
+      // Keep top k results per image.
+      std::stable_sort(score_index_pairs.begin(), score_index_pairs.end(),
+                       SortScorePairDescend<std::pair<int, int>>);
+      score_index_pairs.resize(keep_top_k);
+
+      // Store the new indices.
+      std::map<int, std::vector<int>> new_indices;
+      for (int j = 0; j < score_index_pairs.size(); ++j) {
+        int label = score_index_pairs[j].second.first;
+        int idx = score_index_pairs[j].second.second;
+        new_indices[label].push_back(idx);
+      }
+      new_indices.swap(indices);
+      num_nmsed_out = keep_top_k;
+    }
+  }
+
+  void MultiClassOutput(const Tensor& scores, const Tensor& bboxes,
+                        std::map<int, std::vector<int>>& selected_indices,
+                        Tensor* outs) const {
+    int predict_dim = scores.dims()[1];
+    auto* scores_data = scores.data<T>();
+    auto* bboxes_data = bboxes.data<T>();
+    auto* odata = outs->data<T>();
+
+    int count = 0;
+    for (const auto& it : selected_indices) {
+      int label = it.first;
+      const T* sdata = scores_data + label * predict_dim;
+      const std::vector<int>& indices = it.second;
+      for (int j = 0; j < indices.size(); ++j) {
+        int idx = indices[j];
+        const T* bdata = bboxes_data + idx * kBBoxSize;
+        odata[count * kOutputDim] = label;           // label
+        odata[count * kOutputDim + 1] = sdata[idx];  // score
+        // xmin, ymin, xmax, ymax
+        std::memcpy(odata + count * kOutputDim + 2, bdata, 4 * sizeof(T));
+        count++;
+      }
+    }
+  }
+
+  void Compute(const framework::ExecutionContext& ctx) const override {
+    auto* boxes = ctx.Input<Tensor>("BBoxes");
+    auto* scores = ctx.Input<Tensor>("Scores");
+    auto* outs = ctx.Output<LoDTensor>("Out");
+
+    auto score_dims = scores->dims();
+
+    int64_t batch_size = score_dims[0];
+    int64_t class_num = score_dims[1];
+    int64_t predict_dim = score_dims[2];
+
+    std::vector<std::map<int, std::vector<int>>> all_indices;
+    std::vector<size_t> batch_starts = {0};
+    for (int64_t i = 0; i < batch_size; ++i) {
+      Tensor ins_score = scores->Slice(i, i + 1);
+      ins_score.Resize({class_num, predict_dim});
+      std::map<int, std::vector<int>> indices;
+      int num_nmsed_out = 0;
+      MultiClassNMS(ctx, ins_score, *boxes, indices, num_nmsed_out);
+      all_indices.push_back(indices);
+      batch_starts.push_back(batch_starts.back() + num_nmsed_out);
+    }
+
+    int num_kept = batch_starts.back();
+    if (num_kept == 0) {
+      T* od = outs->mutable_data<T>({1}, ctx.GetPlace());
+      od[0] = -1;
+    } else {
+      outs->mutable_data<T>({num_kept, kOutputDim}, ctx.GetPlace());
+      for (int64_t i = 0; i < batch_size; ++i) {
+        Tensor ins_score = scores->Slice(i, i + 1);
+        ins_score.Resize({class_num, predict_dim});
+        int64_t s = batch_starts[i];
+        int64_t e = batch_starts[i + 1];
+        if (e > s) {
+          Tensor out = outs->Slice(s, e);
+          MultiClassOutput(ins_score, *boxes, all_indices[i], &out);
+        }
+      }
+    }
+
+    framework::LoD lod;
+    lod.emplace_back(batch_starts);
+
+    outs->set_lod(lod);
+  }
+};
+
+class MultiClassNMSOpMaker : public framework::OpProtoAndCheckerMaker {
+ public:
+  MultiClassNMSOpMaker(OpProto* proto, OpAttrChecker* op_checker)
+      : OpProtoAndCheckerMaker(proto, op_checker) {
+    AddInput("BBoxes",
+             "(Tensor) A 2-D Tensor with shape [M, 4] represents the "
+             "predicted locations of M bounding bboxes. Each bounding box "
+             "has four coordinate values and the layout is "
+             "[xmin, ymin, xmax, ymax].");
+    AddInput("Scores",
+             "(Tensor) A 3-D Tensor with shape [N, C, M] represents the "
+             "predicted confidence predictions. N is the batch size, C is the "
+             "class number, M is number of bounding boxes. For each category "
+             "there are total M scores which corresponding M bounding boxes. "
+             " Please note, M is equal to the 1st dimension of BBoxes. ");
+    AddAttr<int>(
+        "background_label",
+        "(int64_t, defalut: 0) "
+        "The index of background label, the background label will be ignored. "
+        "If set to -1, then all categories will be considered.")
+        .SetDefault(0);
+    AddAttr<float>("score_threshold",
+                   "(float) "
+                   "Threshold to filter out bounding boxes with low "
+                   "confidence score. If not provided, consider all boxes.");
+    AddAttr<int>("nms_top_k",
+                 "(int64_t) "
+                 "Maximum number of detections to be kept according to the "
+                 "confidences aftern the filtering detections based on "
+                 "score_threshold");
+    AddAttr<float>("nms_threshold",
+                   "(float, defalut: 0.3) "
+                   "The threshold to be used in NMS.")
+        .SetDefault(0.3);
+    AddAttr<float>("nms_eta",
+                   "(float) "
+                   "The parameter for adaptive NMS.")
+        .SetDefault(1.0);
+    AddAttr<int>("keep_top_k",
+                 "(int64_t) "
+                 "Number of total bboxes to be kept per image after NMS "
+                 "step. -1 means keeping all bboxes after NMS step.");
+    AddOutput("Out",
+              "(LoDTensor) A 2-D LoDTensor with shape [No, 6] represents the "
+              "detections. Each row has 6 values: "
+              "[label, confidence, xmin, ymin, xmax, ymax], No is the total "
+              "number of detections in this mini-batch. For each instance, "
+              "the offsets in first dimension are called LoD, the number of "
+              "offset is N + 1, if LoD[i + 1] - LoD[i] == 0, means there is "
+              "no detected bbox.");
+    AddComment(R"DOC(
+This operator is to do multi-class non maximum suppression (NMS) on a batched
+of boxes and scores.
+
+In the NMS step, this operator greedily selects a subset of detection bounding
+boxes that have high scores larger than score_threshold, if providing this
+threshold, then selects the largest nms_top_k confidences scores if nms_top_k
+is larger than -1. Then this operator pruns away boxes that have high IOU
+(intersection over union) overlap with already selected boxes by adaptive
+threshold NMS based on parameters of nms_threshold and nms_eta.
+
+Aftern NMS step, at most keep_top_k number of total bboxes are to be kept
+per image if keep_top_k is larger than -1.
+
+This operator support multi-class and batched inputs. It applying NMS
+independently for each class. The outputs is a 2-D LoDTenosr, for each
+image, the offsets in first dimension of LoDTensor are called LoD, the number
+of offset is N + 1, where N is the batch size. If LoD[i + 1] - LoD[i] == 0,
+means there is no detected bbox for this image. If there is no detected boxes
+for all images, all the elements in LoD are 0, and the Out only contains one
+value which is -1.
+)DOC");
+  }
+};
+
+}  // namespace operators
+}  // namespace paddle
+
+namespace ops = paddle::operators;
+REGISTER_OPERATOR(multiclass_nms, ops::MultiClassNMSOp,
+                  ops::MultiClassNMSOpMaker,
+                  paddle::framework::EmptyGradOpMaker);
+REGISTER_OP_CPU_KERNEL(multiclass_nms, ops::MultiClassNMSKernel<float>,
+                       ops::MultiClassNMSKernel<double>);

python/paddle/v2/fluid/tests/test_bipartite_match_op.py

@@ -62,7 +62,7 @@ def batch_bipartite_match(distance, lod):
     return match_indices, match_dist
 
 
-class TestBipartiteMatchOpForWithLoD(OpTest):
+class TestBipartiteMatchOpWithLoD(OpTest):
     def setUp(self):
         self.op_type = 'bipartite_match'
         lod = [[0, 5, 11, 23]]
@@ -72,7 +72,7 @@ class TestBipartiteMatchOpForWithLoD(OpTest):
         self.inputs = {'DistMat': (dist, lod)}
         self.outputs = {
             'ColToRowMatchIndices': (match_indices),
-            'ColToRowMatchDis': (match_dist),
+            'ColToRowMatchDist': (match_dist),
         }
 
     def test_check_output(self):
@@ -89,7 +89,7 @@ class TestBipartiteMatchOpWithoutLoD(OpTest):
         self.inputs = {'DistMat': dist}
         self.outputs = {
             'ColToRowMatchIndices': match_indices,
-            'ColToRowMatchDis': match_dist,
+            'ColToRowMatchDist': match_dist,
         }
 
     def test_check_output(self):

python/paddle/v2/fluid/tests/test_multiclass_nms_op.py

+#  Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserve.
+#
+#Licensed under the Apache License, Version 2.0 (the "License");
+#you may not use this file except in compliance with the License.
+#You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+#Unless required by applicable law or agreed to in writing, software
+#distributed under the License is distributed on an "AS IS" BASIS,
+#WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#See the License for the specific language governing permissions and
+#limitations under the License.
+import unittest
+import numpy as np
+import copy
+from op_test import OpTest
+
+
+def iou(box_a, box_b):
+    """Apply intersection-over-union overlap between box_a and box_b
+    """
+    xmin_a = min(box_a[0], box_a[2])
+    ymin_a = min(box_a[1], box_a[3])
+    xmax_a = max(box_a[0], box_a[2])
+    ymax_a = max(box_a[1], box_a[3])
+
+    xmin_b = min(box_b[0], box_b[2])
+    ymin_b = min(box_b[1], box_b[3])
+    xmax_b = max(box_b[0], box_b[2])
+    ymax_b = max(box_b[1], box_b[3])
+
+    area_a = (ymax_a - ymin_a) * (xmax_a - xmin_a)
+    area_b = (ymax_b - ymin_b) * (xmax_b - xmin_b)
+    if area_a <= 0 and area_b <= 0:
+        return 0.0
+
+    xa = max(xmin_a, xmin_b)
+    ya = max(ymin_a, ymin_b)
+    xb = min(xmax_a, xmax_b)
+    yb = min(ymax_a, ymax_b)
+
+    inter_area = max(xb - xa, 0.0) * max(yb - ya, 0.0)
+
+    box_a_area = (box_a[2] - box_a[0]) * (box_a[3] - box_a[1])
+    box_b_area = (box_b[2] - box_b[0]) * (box_b[3] - box_b[1])
+
+    iou_ratio = inter_area / (area_a + area_b - inter_area)
+
+    return iou_ratio
+
+
+def nms(boxes, scores, score_threshold, nms_threshold, top_k=200, eta=1.0):
+    """Apply non-maximum suppression at test time to avoid detecting too many
+    overlapping bounding boxes for a given object.
+    Args:
+        boxes: (tensor) The location preds for the img, Shape: [num_priors,4].
+        scores: (tensor) The class predscores for the img, Shape:[num_priors].
+        score_threshold: (float) The confidence thresh for filtering low
+            confidence boxes.
+        nms_threshold: (float) The overlap thresh for suppressing unnecessary
+            boxes.
+        top_k: (int) The maximum number of box preds to consider.
+        eta: (float) The parameter for adaptive NMS.
+    Return:
+        The indices of the kept boxes with respect to num_priors.
+    """
+    all_scores = copy.deepcopy(scores)
+    all_scores = all_scores.flatten()
+    selected_indices = np.argwhere(all_scores > score_threshold)
+    selected_indices = selected_indices.flatten()
+    all_scores = all_scores[selected_indices]
+
+    sorted_indices = np.argsort(-all_scores, axis=0, kind='mergesort')
+    sorted_scores = all_scores[sorted_indices]
+    if top_k > -1 and top_k < sorted_indices.shape[0]:
+        sorted_indices = sorted_indices[:top_k]
+        sorted_scores = sorted_scores[:top_k]
+
+    selected_indices = []
+    adaptive_threshold = nms_threshold
+    for i in range(sorted_scores.shape[0]):
+        idx = sorted_indices[i]
+        keep = True
+        for k in range(len(selected_indices)):
+            if keep:
+                kept_idx = selected_indices[k]
+                overlap = iou(boxes[idx], boxes[kept_idx])
+                keep = True if overlap <= adaptive_threshold else False
+            else:
+                break
+        if keep:
+            selected_indices.append(idx)
+        if keep and eta < 1 and adaptive_threshold > 0.5:
+            adaptive_threshold *= eta
+    return selected_indices
+
+
+def multiclass_nms(boxes, scores, background, score_threshold, nms_threshold,
+                   nms_top_k, keep_top_k):
+    class_num = scores.shape[0]
+    priorbox_num = scores.shape[1]
+
+    selected_indices = {}
+    num_det = 0
+    for c in range(class_num):
+        if c == background: continue
+        indices = nms(boxes, scores[c], score_threshold, nms_threshold,
+                      nms_top_k)
+        selected_indices[c] = indices
+        num_det += len(indices)
+
+    if keep_top_k > -1 and num_det > keep_top_k:
+        score_index = []
+        for c, indices in selected_indices.iteritems():
+            for idx in indices:
+                score_index.append((scores[c][idx], c, idx))
+
+        sorted_score_index = sorted(
+            score_index, key=lambda tup: tup[0], reverse=True)
+        sorted_score_index = sorted_score_index[:keep_top_k]
+        selected_indices = {}
+
+        for _, c, _ in sorted_score_index:
+            selected_indices[c] = []
+        for s, c, idx in sorted_score_index:
+            selected_indices[c].append(idx)
+        num_det = keep_top_k
+
+    return selected_indices, num_det
+
+
+def batched_multiclass_nms(boxes, scores, background, score_threshold,
+                           nms_threshold, nms_top_k, keep_top_k):
+    batch_size = scores.shape[0]
+
+    det_outs = []
+    lod = [0]
+    for n in range(batch_size):
+        nmsed_outs, nmsed_num = multiclass_nms(boxes, scores[n], background,
+                                               score_threshold, nms_threshold,
+                                               nms_top_k, keep_top_k)
+        lod.append(lod[-1] + nmsed_num)
+        if nmsed_num == 0: continue
+
+        for c, indices in nmsed_outs.iteritems():
+            for idx in indices:
+                xmin, ymin, xmax, ymax = boxes[idx][:]
+                det_outs.append([c, scores[n][c][idx], xmin, ymin, xmax, ymax])
+
+    return det_outs, lod
+
+
+class TestMulticlassNMSOp(OpTest):
+    def set_argument(self):
+        self.score_threshold = 0.01
+
+    def setUp(self):
+        self.set_argument()
+        N = 7
+        M = 1200
+        C = 21
+        BOX_SIZE = 4
+
+        background = 0
+        nms_threshold = 0.3
+        nms_top_k = 400
+        keep_top_k = 200
+        score_threshold = self.score_threshold
+
+        scores = np.random.random((N * M, C)).astype('float32')
+
+        def softmax(x):
+            shiftx = x - np.max(x).clip(-64.)
+            exps = np.exp(shiftx)
+            return exps / np.sum(exps)
+
+        scores = np.apply_along_axis(softmax, 1, scores)
+        scores = np.reshape(scores, (N, M, C))
+        scores = np.transpose(scores, (0, 2, 1))
+
+        boxes = np.random.random((M, BOX_SIZE)).astype('float32')
+        boxes[:, 0:2] = boxes[:, 0:2] * 0.5
+        boxes[:, 2:4] = boxes[:, 2:4] * 0.5 + 0.5
+
+        nmsed_outs, lod = batched_multiclass_nms(boxes, scores, background,
+                                                 score_threshold, nms_threshold,
+                                                 nms_top_k, keep_top_k)
+        nmsed_outs = [-1] if not nmsed_outs else nmsed_outs
+        nmsed_outs = np.array(nmsed_outs).astype('float32')
+
+        self.op_type = 'multiclass_nms'
+        self.inputs = {'BBoxes': boxes, 'Scores': scores}
+        self.outputs = {'Out': (nmsed_outs, [lod])}
+        self.attrs = {
+            'background_label': 0,
+            'nms_threshold': nms_threshold,
+            'nms_top_k': nms_top_k,
+            'keep_top_k': keep_top_k,
+            'score_threshold': score_threshold,
+            'nms_eta': 1.0,
+        }
+
+    def test_check_output(self):
+        self.check_output()
+
+
+class TestMulticlassNMSOpNoOutput(TestMulticlassNMSOp):
+    def set_argument(self):
+        # Here set 2.0 to test the case there is no outputs.
+        # In practical use, 0.0 < score_threshold < 1.0 
+        self.score_threshold = 2.0
+
+
+class TestIOU(unittest.TestCase):
+    def test_iou(self):
+        box1 = np.array([4.0, 3.0, 7.0, 5.0]).astype('float32')
+        box2 = np.array([3.0, 4.0, 6.0, 8.0]).astype('float32')
+
+        expt_output = np.array([2.0 / 16.0]).astype('float32')
+        calc_output = np.array([iou(box1, box2)]).astype('float32')
+        self.assertTrue(np.allclose(calc_output, expt_output))
+
+
+if __name__ == '__main__':
+    unittest.main()