pytorch，numpy两种方法实现nms类间+类内

类间：也就是不同类之间也进行nms

类内：就是只把同类的bboxes进行nms

numpy实现 nms类间+类内：

import numpy as np

# 类间nms
def nms(bboxes, scores, thresh):
    x1, y1, x2, y2 = bboxes[:, 0], bboxes[:, 1], bboxes[:, 2], bboxes[:, 3]
    areas = (x2 - x1 + 1) * (y2 - y1 + 1)
    # 按照score降序排序（保存的是索引）
    indices = scores.argsort()[::-1]

    indice_res = []
    while indices.size > 0:
        i = indices[0]
        indice_res.append(i)
        inter_x1 = np.maximum(x1[i], x1[indices[1:]])
        inter_y1 = np.maximum(y1[i], y1[indices[1:]])
        inter_x2 = np.minimum(x2[i], x2[indices[1:]])
        inter_y2 = np.minimum(y2[i], y2[indices[1:]])
        inter_w = np.maximum(0.0, inter_x2 - inter_x1 + 1)
        inter_h = np.maximum(0.0, inter_y2 - inter_y1 + 1)
        inter_area = inter_w * inter_h
        union_area = areas[i] + areas[indices[1:]] - inter_area + 1e-6
        ious = inter_area / union_area

        idxs = np.where(ious < thresh)[0]  # np.where(ious < thresh)返回的是一个tuple，第一个元素是一个满足条件的array
        indices = indices[idxs + 1]
    return indice_res

# 类内nms，把不同类别的乘以一个偏移量，把不同类别的bboxes给偏移到不同位置。
def class_nms(bboxes, scores, cat_ids, iou_threshold):
    '''
    :param bboxes: np.array, shape of (N, 4), N is the number of bboxes, np.float32
    :param scores: np.array, shape of (N, 1), np.float32
    :param cat_ids: np.array, shape of (N, 1),np.int32
    :param iou_threshold: float
    '''
    max_coordinate = bboxes.max()

    # 为每一个类别/每一层生成一个足够大的偏移量，使不同类别的bboxes不会相交
    offsets = cat_ids * (max_coordinate + 1)
    # bboxes加上对应类别的偏移量后，保证不同类别之间bboxes不会有重合的现象
    bboxes_for_nms = bboxes + offsets[:, None]
    indice_res = nms(bboxes_for_nms, scores, iou_threshold)
    return indice_res

torch实现 nms类间+类内:

import torch

# 类间nms
def nms(bboxes, scores, thresh):
    x1, y1, x2, y2 = bboxes[:, 0], bboxes[:, 1], bboxes[:, 2], bboxes[:, 3]
    areas = (x2 - x1 + 1) * (y2 - y1 + 1)
    # 按照score降序排序（保存的是索引）
    # values, indices = torch.sort(scores, descending=True)
    indices = scores.sort(descending=True)[1]  # torch

    indice_res = torch.randn([1, 4]).to(bboxes)
    while indices.size()[0] > 0:  # indices.size()是一个Size对象，我们要取第一个元素是int，才能比较
        save_idx, other_idx = indices[0], indices[1:]
        indice_res = torch.cat((indice_res, bboxes[save_idx].unsqueeze(0)),
                               dim=0)  # unsqueeze是添加一个维度，让bboxes.shape从[4]-->[1,4]

        inter_x1 = torch.max(x1[save_idx], x1[other_idx])
        inter_y1 = torch.max(y1[save_idx], y1[other_idx])
        inter_x2 = torch.min(x2[save_idx], x2[other_idx])
        inter_y2 = torch.min(y2[save_idx], y2[other_idx])
        inter_w = torch.max(inter_x2 - inter_x1 + 1, torch.tensor(0).to(bboxes))
        inter_h = torch.max(inter_y2 - inter_y1 + 1, torch.tensor(0).to(bboxes))

        inter_area = inter_w * inter_h
        union_area = areas[save_idx] + areas[other_idx] - inter_area + 1e-6
        iou = inter_area / union_area

        indices = other_idx[iou < thresh]
    return indice_res[1:]

# 类内nms，把不同类别的乘以一个偏移量，把不同类别的bboxes给偏移到不同位置。
def class_nms(bboxes, scores, cat_ids, iou_threshold):
    '''
    :param bboxes: torch.tensor([n, 4], dtype=torch.float32)
    :param scores: torch.tensor([n], dtype=torch.float32)
    :param cat_ids: torch.tensor([n], dtype=torch.int32)
    :param iou_threshold: float
    '''
    max_coordinate = bboxes.max()

    # 为每一个类别/每一层生成一个很大的偏移量
    offsets = cat_ids * (max_coordinate + 1)
    # bboxes加上对应类别的偏移量后，保证不同类别之间bboxes不会有重合的现象
    bboxes_for_nms = bboxes + offsets[:, None]
    indice_res = nms(bboxes_for_nms, scores, iou_threshold)
    return indice_res