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文中图片和部分内容、代码转自:
目标检测,也叫目标提取,是一种基于目标几何和统计特征的图像分割,它将目标的分割和识别合二为一,其准确性和实时性是整个系统的一项重要能力。尤其是在复杂场景中,需要对多个目标进行实时处理时,目标自动提取和识别就显得特别重要。
图像分类:只需要判断输入的图像中是否包含感兴趣物体。
目标检测:需要在识别出图片中目标类别的基础上,还要精确定位到目标的具体位置,并用外接矩形框标出。
即定义大量的候选框,计算每一个候选框中基于分类网络得到的得分(代表当前框中有某个物体的置信度),最终得分最高的就代表识别的最准确的框,其位置就是最终要检测的目标的位置。
**先确立众多候选框,再对候选框进行分类和微调。**(RCNN、YOLO、SSD等经典网络模型思路。)
目标框定义方式:
(x1,y1,x2,y2)
和(x_c,y_c,w,h)
。import torch# 两种不同的目标框信息表达格式互转def xy_to_cxcy(xy): """ Convert bounding boxes from boundary coordinates (x_min, y_min, x_max, y_max) to center-size coordinates (c_x, c_y, w, h). :param xy: bounding boxes in boundary coordinates, a tensor of size (n_boxes, 4) :return: bounding boxes in center-size coordinates, a tensor of size (n_boxes, 4) """ return torch.cat([(xy[:, 2:] + xy[:, :2]) / 2, # c_x, c_y xy[:, 2:] - xy[:, :2]], 1) # w, hdef cxcy_to_xy(cxcy): """ Convert bounding boxes from center-size coordinates (c_x, c_y, w, h) to boundary coordinates (x_min, y_min, x_max, y_max). :param cxcy: bounding boxes in center-size coordinates, a tensor of size (n_boxes, 4) :return: bounding boxes in boundary coordinates, a tensor of size (n_boxes, 4) """ return torch.cat([cxcy[:, :2] - (cxcy[:, 2:] / 2), # x_min, y_min cxcy[:, :2] + (cxcy[:, 2:] / 2)], 1) # x_max, y_max
IoU:Intersection over Union
计算流程:
1.首先获取两个框的坐标,红框坐标: 左上(red_x1, red_y1)
, 右下(red_x2, red_y2)
,绿框坐标: 左上(green_x1, green_y1)
,右下(green_x2, green_y2)
2.计算两个框左上点的坐标最大值:(max(red_x1, green_x1), max(red_y1, green_y1))
, 和右下点坐标最小值:(min(red_x2, green_x2), min(red_y2, green_y2))
3.利用2算出的信息计算黄框面积:yellow_area
4.计算红绿框的面积:red_area
和green_area
5.IoU = yellow_area / (red_area + green_area - yellow_area)
# 计算IoUdef find_intersection(set_1, set_2): """ Find the intersection of every box combination between two sets of boxes that are in boundary coordinates. :param set_1: set 1, a tensor of dimensions (n1, 4) [x_1,y_1,x_2,y_2] :param set_2: set 2, a tensor of dimensions (n2, 4) [x_1,y_1,x_2,y_2] :return: 返回交集的面积intersection of each of the boxes in set 1 with respect to each of the boxes in set 2, a tensor of dimensions (n1, n2) """ # PyTorch auto-broadcasts singleton dimensions # 计算出交集的左上角坐标max((red_x1, green_y1),(green_x1,red_y1)) lower_bounds = torch.max(set_1[:, :2].unsqueeze(1), set_2[:, :2].unsqueeze(0)) # (n1, n2, 2) # 计算出交集的右下角坐标min((red_x2, red_y2), (green_x2, green_y2)) upper_bounds = torch.min(set_1[:, 2:].unsqueeze(1), set_2[:, 2:].unsqueeze(0)) # (n1, n2, 2) # clamp:将输入input张量每个元素的夹紧到区间 [min,max][min,max],并返回结果到一个新张量。 # 这里这样的处理为了体现若无交集,则upper-lower为负,此时返回0. intersection_dims = torch.clamp(upper_bounds - lower_bounds, min=0) # (n1, n2, 2) # 返回交集的面积,有则返回实际计算的结果,无则是0 return intersection_dims[:, :, 0] * intersection_dims[:, :, 1] # (n1, n2)def find_jaccard_overlap(set_1, set_2): """ Find the Jaccard Overlap (IoU) of every box combination between two sets of boxes that are in boundary coordinates. :param set_1: set 1, a tensor of dimensions (n1, 4) :param set_2: set 2, a tensor of dimensions (n2, 4) :return: 返回IoU Jaccard Overlap of each of the boxes in set 1 with respect to each of the boxes in set 2, a tensor of dimensions (n1, n2) """ # Find intersections # 计算交集的面积 intersection = find_intersection(set_1, set_2) # (n1, n2) # Find areas of each box in both sets # 分别计算两个候选框的面积 areas_set_1 = (set_1[:, 2] - set_1[:, 0]) * (set_1[:, 3] - set_1[:, 1]) # (n1) areas_set_2 = (set_2[:, 2] - set_2[:, 0]) * (set_2[:, 3] - set_2[:, 1]) # (n2) # Find the union # PyTorch auto-broadcasts singleton dimensions # 计算并集的面积 union = areas_set_1.unsqueeze(1) + areas_set_2.unsqueeze(0) - intersection # (n1, n2) # 返回IoU return intersection / union # (n1, n2)
VOC数据及时目标检测领域最常用的标准数据集之一,在练习中主要使用
VOC2007
和VOC2012
这两个最流行的版本作为训练和测试的数据。
数据集类别:VOC数据集主要分为4大类,20个小类。
数据集量级:
数据集下载链接:
–解压码(7aek)数据集结构说明:
JPEGImages
:这个目录中的图片,包括了训练、验证和测试用到的所有图片。ImageSets
: Layout
:训练、验证、测试和训练+验证数据集的文件名;Segmentation
:分割所用的训练、验证、测试和训练+验证数据集的文件名。Main
:各个类别所有图片的文件名。Annotations
:存放了每张图片相关的标注信息,以xml
格式形式存储。某一张图片对应的文件如下:VOC2007 000001.jpg
dataloader
的构建xml
文件进行解析,将其转换为json
格式的文件,便于后面再训练时,能够更便捷的获取相应的标签信息。这样的处理取决于自己,相较于
练习中便于后面再训练时,能够更便捷的获取相应的标签信息。xml
格式而言,json
格式更便于解析和读取相应的字段信息。
练习中可以通过运行create_data_lists.py
脚本,使用utils.py
中的create_data_lists
方法实现:
"""python create_data_lists"""from utils import create_data_listsif __name__ == '__main__': # voc07_path,voc12_path为我们训练测试所需要用到的数据集,output_folder为我们生成构建dataloader所需文件的路径 # 参数中涉及的路径以个人实际路径为准,建议将数据集放到dataset目录下,和教程保持一致 create_data_lists(voc07_path='../../../dataset/VOCdevkit/VOC2007', voc12_path='../../../dataset/VOCdevkit/VOC2012', output_folder='../../../dataset/VOCdevkit')
dataset
目录中,并运行脚本,便生成了相应的json
文件,用于后面的训练中。xml
文件主要是通过parse_annotation
函数实现:""" xml文件解析"""import jsonimport osimport torchimport randomimport xml.etree.ElementTree as ET #解析xml文件所用工具import torchvision.transforms.functional as FT#GPU设置device = torch.device("cuda" if torch.cuda.is_available() else "cpu")# Label map#voc_labels为VOC数据集中20类目标的类别名称voc_labels = ('aeroplane', 'bicycle', 'bird', 'boat', 'bottle', 'bus', 'car', 'cat', 'chair', 'cow', 'diningtable', 'dog', 'horse', 'motorbike', 'person', 'pottedplant', 'sheep', 'sofa', 'train', 'tvmonitor')#创建label_map字典,用于存储类别和类别索引之间的映射关系。比如:{1:'aeroplane', 2:'bicycle',......}label_map = { k: v + 1 for v, k in enumerate(voc_labels)}#VOC数据集默认不含有20类目标中的其中一类的图片的类别为background,类别索引设置为0label_map['background'] = 0#将映射关系倒过来,{类别名称:类别索引}rev_label_map = { v: k for k, v in label_map.items()} # Inverse mapping#解析xml文件,最终返回这张图片中所有目标的标注框及其类别信息,以及这个目标是否是一个difficult目标def parse_annotation(annotation_path): #解析xml tree = ET.parse(annotation_path) root = tree.getroot() boxes = list() #存储bbox labels = list() #存储bbox对应的label difficulties = list() #存储bbox对应的difficult信息 #遍历xml文件中所有的object,前面说了,有多少个object就有多少个目标 for object in root.iter('object'): #提取每个object的difficult、label、bbox信息 difficult = int(object.find('difficult').text == '1') label = object.find('name').text.lower().strip() if label not in label_map: continue bbox = object.find('bndbox') xmin = int(bbox.find('xmin').text) - 1 ymin = int(bbox.find('ymin').text) - 1 xmax = int(bbox.find('xmax').text) - 1 ymax = int(bbox.find('ymax').text) - 1 #存储 boxes.append([xmin, ymin, xmax, ymax]) labels.append(label_map[label]) difficulties.append(difficult) #返回包含图片标注信息的字典 return { 'boxes': boxes, 'labels': labels, 'difficulties': difficulties}
json
文件部分内容如下。会发现仅将最重要的目标信息解析到出来,便于后面的训练测试使用:[{ "boxes": [ [262, 210, 323, 338], [164, 263, 252, 371], [4, 243, 66, 373], [240, 193, 294, 298], [276, 185, 311, 219] ], "labels": [9, 9, 9, 9, 9], "difficulties": [0, 0, 1, 0, 1]}, { "boxes": [ [140, 49, 499, 329] ], "labels": [7], "difficulties": [0]}, { "boxes": [ [68, 171, 269, 329], [149, 140, 228, 283], [284, 200, 326, 330], [257, 197, 296, 328] ], "labels": [13, 15, 15, 15], "difficulties": [0, 0, 0, 0]}]
dataloader
:#train_dataset和train_loader的实例化 # 实例化PascalVOCDataset类 train_dataset = PascalVOCDataset(data_folder, split='train', keep_difficult=keep_difficult) # 将train_dataset传入DataLoader得到train_loader train_loader = torch.utils.data.DataLoader(train_dataset, batch_size=batch_size, shuffle=True, collate_fn=train_dataset.collate_fn, num_workers=workers, pin_memory=True) # note that we're passing the collate function here
PascalVOCDataset
的定义如下,主要继承了torch.utils.data.Dataset,然后重写了__init__ , getitem, len 和 collate_fn 四个方法:"""python PascalVOCDataset具体实现过程"""import torchfrom torch.utils.data import Datasetimport jsonimport osfrom PIL import Imagefrom utils import transformclass PascalVOCDataset(Dataset): """ A PyTorch Dataset class to be used in a PyTorch DataLoader to create batches. """ #初始化相关变量 #读取images和objects标注信息 def __init__(self, data_folder, split, keep_difficult=False): """ :param data_folder: 数据目录。folder where data files are stored :param split: 数据分割为训练集和测试集。split, one of 'TRAIN' or 'TEST' :param keep_difficult: 保留或放弃难以检测的对象。keep or discard objects that are considered difficult to detect? """ self.split = split.upper() #保证输入为纯大写字母,便于匹配{'TRAIN', 'TEST'} assert self.split in { 'TRAIN', 'TEST'} self.data_folder = data_folder self.keep_difficult = keep_difficult # Read data files with open(os.path.join(data_folder, self.split + '_images.json'), 'r') as j: self.images = json.load(j) with open(os.path.join(data_folder, self.split + '_objects.json'), 'r') as j: self.objects = json.load(j) assert len(self.images) == len(self.objects) #循环读取image及对应objects #对读取的image及objects进行tranform操作(数据增广) #返回PIL格式图像,标注框,标注框对应的类别索引,对应的difficult标志(True or False) def __getitem__(self, i): # Read image #*需要注意,在pytorch中,图像的读取要使用Image.open()读取成PIL格式,不能使用opencv #*由于Image.open()读取的图片是四通道的(RGBA),因此需要.convert('RGB')转换为RGB通道 image = Image.open(self.images[i], mode='r') image = image.convert('RGB') # Read objects in this image (bounding boxes, labels, difficulties) # 读取图片对应的对象信息 objects = self.objects[i] boxes = torch.FloatTensor(objects['boxes']) # (n_objects, 4) labels = torch.LongTensor(objects['labels']) # (n_objects) difficulties = torch.ByteTensor(objects['difficulties']) # (n_objects) # Discard difficult objects, if desired #如果self.keep_difficult为False,即不保留difficult标志为True的目标 #那么这里将对应的目标删去 if not self.keep_difficult: boxes = boxes[1 - difficulties] labels = labels[1 - difficulties] difficulties = difficulties[1 - difficulties] # Apply transformations #对读取的图片应用transform image, boxes, labels, difficulties = transform(image, boxes, labels, difficulties, split=self.split) return image, boxes, labels, difficulties #获取图片的总数,用于计算batch数 def __len__(self): return len(self.images) #我们知道,我们输入到网络中训练的数据通常是一个batch一起输入,而通过__getitem__我们只读取了一张图片及其objects信息 #如何将读取的一张张图片及其object信息整合成batch的形式呢? #collate_fn就是做这个事情, #对于一个batch的images,collate_fn通过torch.stack()将其整合成4维tensor,对应的objects信息分别用一个list存储 def collate_fn(self, batch): """ Since each image may have a different number of objects, we need a collate function (to be passed to the DataLoader). This describes how to combine these tensors of different sizes. We use lists. Note: this need not be defined in this Class, can be standalone. :param batch: an iterable of N sets from __getitem__() :return: a tensor of images, lists of varying-size tensors of bounding boxes, labels, and difficulties """ images = list() boxes = list() labels = list() difficulties = list() for b in batch: images.append(b[0]) boxes.append(b[1]) labels.append(b[2]) difficulties.append(b[3]) #(3,224,224) -> (N,3,224,224) images = torch.stack(images, dim=0) return images, boxes, labels, difficulties # tensor (N, 3, 224, 224), 3 lists of N tensors each
通过数据增强,可以提升网络精度和泛化能力。
transform
数据增强实现代码如下:
"""python transform操作是训练模型中一项非常重要的工作, 其中不仅包含数据增强以提升模型性能的相关操作, 也包含如数据类型转换(PIL to Tensor)、归一化(Normalize)这些必要操作。"""import jsonimport osimport torchimport randomimport xml.etree.ElementTree as ETimport torchvision.transforms.functional as FT"""可以看到,transform分为TRAIN和TEST两种模式,以本实验为例:在TRAIN时进行的transform有:1.以随机顺序改变图片亮度,对比度,饱和度和色相,每种都有50%的概率被执行。photometric_distort2.扩大目标,expand3.随机裁剪图片,random_crop4.0.5的概率进行图片翻转,flip*注意:a. 第一种transform属于像素级别的图像增强,目标相对于图片的位置没有改变,因此bbox坐标不需要变化。 但是2,3,4,5都属于图片的几何变化,目标相对于图片的位置被改变,因此bbox坐标要进行相应变化。在TRAIN和TEST时都要进行的transform有:1.统一图像大小到(224,224),resize2.PIL to Tensor3.归一化,FT.normalize()注1: resize也是一种几何变化,要知道应用数据增强策略时,哪些属于几何变化,哪些属于像素变化注2: PIL to Tensor操作,normalize操作必须执行"""def transform(image, boxes, labels, difficulties, split): """ Apply the transformations above. :param image: image, a PIL Image :param boxes: bounding boxes in boundary coordinates, a tensor of dimensions (n_objects, 4) :param labels: labels of objects, a tensor of dimensions (n_objects) :param difficulties: difficulties of detection of these objects, a tensor of dimensions (n_objects) :param split: one of 'TRAIN' or 'TEST', since different sets of transformations are applied :return: transformed image, transformed bounding box coordinates, transformed labels, transformed difficulties """ #在训练和测试时使用的transform策略往往不完全相同,所以需要split变量指明是TRAIN还是TEST时的transform方法 assert split in { 'TRAIN', 'TEST'} # Mean and standard deviation of ImageNet data that our base VGG from torchvision was trained on # see: https://pytorch.org/docs/stable/torchvision/models.html #为了防止由于图片之间像素差异过大而导致的训练不稳定问题,图片在送入网络训练之间需要进行归一化 #对所有图片各通道求mean和std来获得 mean = [0.485, 0.456, 0.406] std = [0.229, 0.224, 0.225] new_image = image new_boxes = boxes new_labels = labels new_difficulties = difficulties # Skip the following operations for evaluation/testing if split == 'TRAIN': # A series of photometric distortions in random order, each with 50% chance of occurrence, as in Caffe repo # 以随机的顺序改变图片的亮度、对比度、饱和度和色相(都有50%的概率) new_image = photometric_distort(new_image) # Convert PIL image to Torch tensor # 将图片转换为tensor new_image = FT.to_tensor(new_image) # Expand image (zoom out) with a 50% chance - helpful for training detection of small objects # Fill surrounding space with the mean of ImageNet data that our base VGG was trained on if random.random() < 0.5: # 扩大目标 new_image, new_boxes = expand(new_image, boxes, filler=mean) # Randomly crop image (zoom in) # 随机裁剪图片 new_image, new_boxes, new_labels, new_difficulties = random_crop(new_image, new_boxes, new_labels, new_difficulties) # Convert Torch tensor to PIL image # 将tensor转换为图片格式PIL new_image = FT.to_pil_image(new_image) # Flip image with a 50% chance if random.random() < 0.5: # 以0.5的概率进行图片翻转 new_image, new_boxes = flip(new_image, new_boxes) # Resize image to (224, 224) - this also converts absolute boundary coordinates to their fractional form # 统一图片的大小到(224,224) new_image, new_boxes = resize(new_image, new_boxes, dims=(224, 224)) # Convert PIL image to Torch tensor # PIL转换为tensor new_image = FT.to_tensor(new_image) # Normalize by mean and standard deviation of ImageNet data that our base VGG was trained on # 归一化 new_image = FT.normalize(new_image, mean=mean, std=std) return new_image, new_boxes, new_labels, new_difficulties
"""python DataLoader"""#参数说明:#在train时一般设置shufle=True打乱数据顺序,增强模型的鲁棒性#num_worker表示读取数据时的线程数,一般根据自己设备配置确定(如果是windows系统,建议设默认值0,防止出错)#pin_memory,在计算机内存充足的时候设置为True可以加快内存中的tensor转换到GPU的速度train_loader = torch.utils.data.DataLoader(train_dataset, batch_size=batch_size, shuffle=True, collate_fn=train_dataset.collate_fn, num_workers=workers, pin_memory=True) # note that we're passing the collate function here
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