anchor-cluster.py

#coding=utf-8
import xml.etree.ElementTree as ET
import numpy as np
import glob
 
def iou(box, clusters):
    """
    计算一个ground truth边界盒和k个先验框(Anchor)的交并比(IOU)值。
    参数box: 元组或者数据，代表ground truth的长宽。
    参数clusters: 形如(k,2)的numpy数组，其中k是聚类Anchor框的个数
    返回：ground truth和每个Anchor框的交并比。
    """
    x = np.minimum(clusters[:, 0], box[0])
    y = np.minimum(clusters[:, 1], box[1])
    if np.count_nonzero(x == 0) > 0 or np.count_nonzero(y == 0) > 0:
        raise ValueError("Box has no area")
    intersection = x * y
    box_area = box[0] * box[1]
    cluster_area = clusters[:, 0] * clusters[:, 1]
    iou_ = intersection / (box_area + cluster_area - intersection)
    return iou_


def avg_iou(boxes, clusters):
    """
    计算一个ground truth和k个Anchor的交并比的均值。
    """
    return np.mean([np.max(iou(boxes[i], clusters)) for i in range(boxes.shape[0])])

def kmeans(boxes, k, dist=np.median):
    """
    利用IOU值进行K-means聚类
    参数boxes: 形状为(r, 2)的ground truth框，其中r是ground truth的个数
    参数k: Anchor的个数
    参数dist: 距离函数
    返回值：形状为(k, 2)的k个Anchor框
    """
    # 即是上面提到的r
    rows = boxes.shape[0]
    # 距离数组，计算每个ground truth和k个Anchor的距离
    distances = np.empty((rows, k))
    # 上一次每个ground truth"距离"最近的Anchor索引
    last_clusters = np.zeros((rows,))
    # 设置随机数种子
    np.random.seed()

    # 初始化聚类中心，k个簇，从r个ground truth随机选k个
    clusters = boxes[np.random.choice(rows, k, replace=False)]
    # 开始聚类
    while True:
        # 计算每个ground truth和k个Anchor的距离，用1-IOU(box,anchor)来计算
        for row in range(rows):
            distances[row] = 1 - iou(boxes[row], clusters)
        # 对每个ground truth，选取距离最小的那个Anchor，并存下索引
        nearest_clusters = np.argmin(distances, axis=1)
        # 如果当前每个ground truth"距离"最近的Anchor索引和上一次一样，聚类结束
        if (last_clusters == nearest_clusters).all():
            break
        # 更新簇中心为簇里面所有的ground truth框的均值
        for cluster in range(k):
            clusters[cluster] = dist(boxes[nearest_clusters == cluster], axis=0)
        # 更新每个ground truth"距离"最近的Anchor索引
        last_clusters = nearest_clusters

    return clusters

# 加载自己的数据集，只需要所有labelimg标注出来的xml文件即可
def load_dataset(path):
    dataset = []
    for xml_file in glob.glob("{}/*xml".format(path)):
        tree = ET.parse(xml_file)
        # 图片高度
        height = int(tree.findtext("./size/height"))
        # 图片宽度
        width = int(tree.findtext("./size/width"))
        
        for obj in tree.iter("object"):
            # 偏移量
            xmin = int(obj.findtext("bndbox/xmin")) / width
            ymin = int(obj.findtext("bndbox/ymin")) / height
            xmax = int(obj.findtext("bndbox/xmax")) / width
            ymax = int(obj.findtext("bndbox/ymax")) / height
            xmin = np.float64(xmin)
            ymin = np.float64(ymin)
            xmax = np.float64(xmax)
            ymax = np.float64(ymax)
            if xmax == xmin or ymax == ymin:
                print(xml_file)
            # 将Anchor的长宽放入dateset，运行kmeans获得Anchor
            dataset.append([xmax - xmin, ymax - ymin])
    return np.array(dataset)
 
if __name__ == '__main__':
    import argparse
    import os
    parser = argparse.ArgumentParser()
    parser.add_argument('--voc-root', help="VOC格式数据集路径", type=str)
    parser.add_argument('--clusters', help="anchor数量", type=int, default=9)
    parser.add_argument('--input-size', help="输入网络大小", type=str, default=416)

    args = parser.parse_args()

    ANNOTATIONS_PATH = os.path.join(args.voc_root,'Annotations')  # xml文件所在文件夹
    CLUSTERS = args.clusters    #聚类数量，anchor数量
    INPUTDIM = args.input_size  #输入网络大小
 
    data = load_dataset(ANNOTATIONS_PATH)
    out = kmeans(data, k=CLUSTERS)
    print('Boxes:')
    print(np.array(out)*INPUTDIM)    
    print("Accuracy: {:.2f}%".format(avg_iou(data, out) * 100))       
    final_anchors = np.around(out[:, 0] / out[:, 1], decimals=2).tolist()
    print("Before Sort Ratios:\n {}".format(final_anchors))
    print("After Sort Ratios:\n {}".format(sorted(final_anchors)))