[PyTorch 学习笔记] 8.2 目标检测简介

本章代码：

• https://github.com/zhangxiann/PyTorch_Practice/blob/master/lesson8/detection_demo.py
• https://github.com/zhangxiann/PyTorch_Practice/blob/master/lesson8/fasterrcnn_demo.py

这篇文章主要介绍了目标检测。

目标检测是判断目标在图像中的位置，有两个要素：

• 分类：分类向量 $P_0,P_1,P_2...$，shape 为 $[N,c+1]$
• 回归：回归边界框 $[x_1,x_2,y_1,y_2]$，shape 为 $[n,4]$

下面代码是加载预训练好的FasterRCNN_ResNet50_fpn，这个模型在是 COCO 模型上进行训练的，有 91 种类别。这里图片不再是BCHW的形状，而是一个list，每个元素是图片。输出也是一个 list，每个元素是一个 dict，每个 dict 包含三个元素：boxes、scores、labels，每个元素都是 list，因为一张图片中可能包含多个目标。接着是绘制框的代码，scores的的某个元素小于某个阈值，则不绘制这个框。

import os
import time
import torch.nn as nn
import torch
import numpy as np
import torchvision.transforms as transforms
import torchvision
from PIL import Image
from matplotlib import pyplot as plt

BASE_DIR = os.path.dirname(os.path.abspath(__file__))
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")


# classes_coco
COCO_INSTANCE_CATEGORY_NAMES = [
    '__background__', 'person', 'bicycle', 'car', 'motorcycle', 'airplane', 'bus',
    'train', 'truck', 'boat', 'traffic light', 'fire hydrant', 'N/A', 'stop sign',
    'parking meter', 'bench', 'bird', 'cat', 'dog', 'horse', 'sheep', 'cow',
    'elephant', 'bear', 'zebra', 'giraffe', 'N/A', 'backpack', 'umbrella', 'N/A', 'N/A',
    'handbag', 'tie', 'suitcase', 'frisbee', 'skis', 'snowboard', 'sports ball',
    'kite', 'baseball bat', 'baseball glove', 'skateboard', 'surfboard', 'tennis racket',
    'bottle', 'N/A', 'wine glass', 'cup', 'fork', 'knife', 'spoon', 'bowl',
    'banana', 'apple', 'sandwich', 'orange', 'broccoli', 'carrot', 'hot dog', 'pizza',
    'donut', 'cake', 'chair', 'couch', 'potted plant', 'bed', 'N/A', 'dining table',
    'N/A', 'N/A', 'toilet', 'N/A', 'tv', 'laptop', 'mouse', 'remote', 'keyboard', 'cell phone',
    'microwave', 'oven', 'toaster', 'sink', 'refrigerator', 'N/A', 'book',
    'clock', 'vase', 'scissors', 'teddy bear', 'hair drier', 'toothbrush'
]


if __name__ == "__main__":

    path_img = os.path.join(BASE_DIR, "demo_img1.png")
    # path_img = os.path.join(BASE_DIR, "demo_img2.png")

    # config
    preprocess = transforms.Compose([
        transforms.ToTensor(),
    ])

    # 1. load data & model
    input_image = Image.open(path_img).convert("RGB")
    model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)
    model.eval()

    # 2. preprocess
    img_chw = preprocess(input_image)

    # 3. to device
    if torch.cuda.is_available():
        img_chw = img_chw.to('cuda')
        model.to('cuda')

    # 4. forward
    # 这里图片不再是 BCHW 的形状，而是一个list，每个元素是图片
    input_list = [img_chw]
    with torch.no_grad():
        tic = time.time()
        print("input img tensor shape:{}".format(input_list[0].shape))
        output_list = model(input_list)
        # 输出也是一个 list，每个元素是一个 dict
        output_dict = output_list[0]
        print("pass: {:.3f}s".format(time.time() - tic))
        for k, v in output_dict.items():
            print("key:{}, value:{}".format(k, v))

    # 5. visualization
    out_boxes = output_dict["boxes"].cpu()
    out_scores = output_dict["scores"].cpu()
    out_labels = output_dict["labels"].cpu()

    fig, ax = plt.subplots(figsize=(12, 12))
    ax.imshow(input_image, aspect='equal')

    num_boxes = out_boxes.shape[0]
    # 这里最多绘制 40 个框
    max_vis = 40
    thres = 0.5

    for idx in range(0, min(num_boxes, max_vis)):

        score = out_scores[idx].numpy()
        bbox = out_boxes[idx].numpy()
        class_name = COCO_INSTANCE_CATEGORY_NAMES[out_labels[idx]]
        # 如果分数小于这个阈值，则不绘制
        if score < thres:
            continue

        ax.add_patch(plt.Rectangle((bbox[0], bbox[1]), bbox[2] - bbox[0], bbox[3] - bbox[1], fill=False,
                                   edgecolor='red', linewidth=3.5))
        ax.text(bbox[0], bbox[1] - 2, '{:s} {:.3f}'.format(class_name, score), bbox=dict(facecolor='blue', alpha=0.5),
                fontsize=14, color='white')
    plt.show()
    plt.close()



    # appendix
    classes_pascal_voc = ['__background__',
                       'aeroplane', 'bicycle', 'bird', 'boat',
                       'bottle', 'bus', 'car', 'cat', 'chair',
                       'cow', 'diningtable', 'dog', 'horse',
                       'motorbike', 'person', 'pottedplant',
                       'sheep', 'sofa', 'train', 'tvmonitor']

    # classes_coco
    COCO_INSTANCE_CATEGORY_NAMES = [
        '__background__', 'person', 'bicycle', 'car', 'motorcycle', 'airplane', 'bus',
        'train', 'truck', 'boat', 'traffic light', 'fire hydrant', 'N/A', 'stop sign',
        'parking meter', 'bench', 'bird', 'cat', 'dog', 'horse', 'sheep', 'cow',
        'elephant', 'bear', 'zebra', 'giraffe', 'N/A', 'backpack', 'umbrella', 'N/A', 'N/A',
        'handbag', 'tie', 'suitcase', 'frisbee', 'skis', 'snowboard', 'sports ball',
        'kite', 'baseball bat', 'baseball glove', 'skateboard', 'surfboard', 'tennis racket',
        'bottle', 'N/A', 'wine glass', 'cup', 'fork', 'knife', 'spoon', 'bowl',
        'banana', 'apple', 'sandwich', 'orange', 'broccoli', 'carrot', 'hot dog', 'pizza',
        'donut', 'cake', 'chair', 'couch', 'potted plant', 'bed', 'N/A', 'dining table',
        'N/A', 'N/A', 'toilet', 'N/A', 'tv', 'laptop', 'mouse', 'remote', 'keyboard', 'cell phone',
        'microwave', 'oven', 'toaster', 'sink', 'refrigerator', 'N/A', 'book',
        'clock', 'vase', 'scissors', 'teddy bear', 'hair drier', 'toothbrush'
    ]

输出如下：

目标检测中难题之一是边界框的数量 $N$ 的确定。

传统的方法是滑动窗口策略，缺点是重复计算量大，窗口大小难以确定。

将全连接层改为卷积层，最后一层特征图的一个像素就对应着原图的一个区域，就可以使用利用卷积操作实现滑动窗口。

目标检测模型可以划分为 one-stage 和 two-stage。

one-stage 包括：

• YOLO
• SSD
• Retina-Net

two-stage 包括：

• RCNN
• SPPNet
• Fast RCNN
• Faster RCNN
• Pyramid Network

one-stage 的模型是直接把得到的特征图划分为多个网格，每个网格分别做分类和回归。

two-stage 的模型多了 proposal generation，输出 多个候选框，通常默认 2000 个候选框

在 Faster RCNN 中，proposal generation 是 RPN(Region Proposal Network)，会根据 feature map 生成数十万个候选框，通过 NMS 选出前景概率最高的 2000 个框。由于候选框的大小各异，通过 ROI pooling，得到固定大小的输出，channel 数量就是框的数量。ROI pooling 的特点是输入特征图尺寸不固定，但是输出特征图尺寸固定。最后经过全连接层得到回归和分类的输出。

fasterrcnn_resnet50_fpn会返回一个FasterRCNN，FasterRCNN继承于GeneralizedRCNN，GeneralizedRCNN的forward()函数中包括下面 3 个模块：

• backbone：features = self.backbone(images.tensors)

  在构建 backbone 时，会根据backbone_name选择对应的 backbone，这里使用 resnet50。

• rpn：proposals, proposal_losses = self.rpn(images, features, targets)

• roi_heads：detections, detector_losses = self.roi_heads(features, proposals, images.image_sizes, targets)

GeneralizedRCNN的forward()函数如下：

    def forward(self, images, targets=None):
    ...
    ...
    ...
        features = self.backbone(images.tensors)
        if isinstance(features, torch.Tensor):
            features = OrderedDict([('0', features)])
        proposals, proposal_losses = self.rpn(images, features, targets)
        detections, detector_losses = self.roi_heads(features, proposals, images.image_sizes, targets)
        detections = self.transform.postprocess(detections, images.image_sizes, original_image_sizes)

        losses = {}
        losses.update(detector_losses)
        losses.update(proposal_losses)

        if torch.jit.is_scripting():
            if not self._has_warned:
                warnings.warn("RCNN always returns a (Losses, Detections) tuple in scripting")
                self._has_warned = True
            return (losses, detections)
        else:
            return self.eager_outputs(losses, detections)

self.backbone(images.tensors)返回的features是一个 dict，每个元素是一个 feature map，每个特征图的宽高是上一个特征图宽高的一半。

这 5 个 feature map 分别对应 ResNet 中的 5 个特征图

接下来进入 rpn 网络，rpn 网络代码如下。

    def forward(self, images, features, targets=None):
	...
	...
	...
        features = list(features.values())
        objectness, pred_bbox_deltas = self.head(features)
        anchors = self.anchor_generator(images, features)

        num_images = len(anchors)
        num_anchors_per_level = [o[0].numel() for o in objectness]
        objectness, pred_bbox_deltas = \
            concat_box_prediction_layers(objectness, pred_bbox_deltas)
        # apply pred_bbox_deltas to anchors to obtain the decoded proposals
        # note that we detach the deltas because Faster R-CNN do not backprop through
        # the proposals
        proposals = self.box_coder.decode(pred_bbox_deltas.detach(), anchors)
        proposals = proposals.view(num_images, -1, 4)
        boxes, scores = self.filter_proposals(proposals, objectness, images.image_sizes, num_anchors_per_level)

self.head(features)会调用RPNHead，返回的objectness和pred_bbox_deltas都是和features大小一样的 dict，只是 channel 数量不一样。objectness的 channel 数量是 3，表示特征图的一个像素点输出 3 个可能的框；pred_bbox_deltas的 channel 数量是 12，表示每个特征图的 3 个框的坐标的偏移量。

self.anchor_generator(images, features)的输出是anchors，形状是 $242991\times 4$，这是真正的框。

self.filter_proposals()对应的是 NMS，用于挑选出一部分框。

    def filter_proposals(self, proposals, objectness, image_shapes, num_anchors_per_level):
		...
		...
		...
        # select top_n boxes independently per level before applying nms
        top_n_idx = self._get_top_n_idx(objectness, num_anchors_per_level)

        image_range = torch.arange(num_images, device=device)
        batch_idx = image_range[:, None]

        objectness = objectness[batch_idx, top_n_idx]
        levels = levels[batch_idx, top_n_idx]
        proposals = proposals[batch_idx, top_n_idx]

        final_boxes = []
        final_scores = []
        for boxes, scores, lvl, img_shape in zip(proposals, objectness, levels, image_shapes):
            boxes = box_ops.clip_boxes_to_image(boxes, img_shape)
            keep = box_ops.remove_small_boxes(boxes, self.min_size)
            boxes, scores, lvl = boxes[keep], scores[keep], lvl[keep]
            # non-maximum suppression, independently done per level
            keep = box_ops.batched_nms(boxes, scores, lvl, self.nms_thresh)
            # keep only topk scoring predictions
            keep = keep[:self.post_nms_top_n()]
            boxes, scores = boxes[keep], scores[keep]
            final_boxes.append(boxes)
            final_scores.append(scores)
        return final_boxes, final_scores

其中self._get_top_n_idx()函数去取出概率最高的前 4000 个框的索引。最后的for循环是根据特征图的框还原为原图的框，并选出最前面的 1000 个框（训练时是 2000 个，测试时是 1000 个）。

然后回到GeneralizedRCNN的forward()函数里的roi_heads()，实际上是调用RoIHeads，forward()函数如下：

    def forward(self, features, proposals, image_shapes, targets=None):
		...
		...
		...
        box_features = self.box_roi_pool(features, proposals, image_shapes)
        box_features = self.box_head(box_features)
        class_logits, box_regression = self.box_predictor(box_features)

其中box_roi_pool()是调用MultiScaleRoIAlign把不同尺度的特征图池化到相同尺度，返回给box_features，形状是 $[1000,256,7,7]$，1000 表示有 1000 个框（在训练时会从 2000 个选出 512 个，测试时则全部选，所以是 1000）。box_head()是两个全连接层，返回的数形状是 $[1000,1024]$，一个候选框使用一个 1024 的向量来表示。box_predictor()输出最终的分类和边界框，class_logits的形状是 $[1000,91]$，box_regression的形状是 $[1000,364]$，$364=91\times 4$。

然后回到GeneralizedRCNN的forward()函数中，transform.postprocess()对输出进行后处理，将输出转换到原始图像的维度上。

下面总结一下 Faster RCNN 的主要组件：

1. backbone
2. rpn
3. filter_proposals(NMS)
4. rio_heads

下面的例子是使用 Faster RCNN 进行行人检测的 Finetune。数据集下载地址是https://www.cis.upenn.edu/~jshi/ped_html/，包括 70 张行人照片，345 个行人标签。

数据存放结构如下：

• PennFudanPed
  • Annotation：标注文件，为txt
  • PedMasks：不清楚，没用到
  • PNGImages：图片数据

在Dataset中，首先在构造函数中保存所有图片的文件名，后面用于查找对应的 txt 标签文件；在__getitem__()函数中根据 index 获得图片和 txt 文件，查找 txt 文件的每一行是否有数字，有数字的则是带有标签的行，处理得到 boxes 和 labels，最后构造 target，target 是一个 dict，包括 boxes 和 labels。

在构造 DataLoader 时，还要传入一个collate_fn()函数。这是因为在目标检测中，图片的宽高可能不一样，无法以 4D 张量的形式拼接一个 batch 的图片，因此这里使用 tuple 来拼接数据。

    # 收集batch data的函数
    def collate_fn(batch):
        return tuple(zip(*batch))

collate_fn 的输入是 list，每个元素是 tuple；每个 tuple 是 Dataset 中的 __getitem__()返回的数据，包括(image, target)

举个例子：

image=[1,2,3]
target=[4,5,6]
batch=list(zip(image,target))
print("batch:")
print(batch)
collate_result = tuple(zip(*batch))
print("collate_result:")
print(collate_result)

输出为：

batch:
[(1, 4), (2, 5), (3, 6)]
collate_result:
((1, 2, 3), (4, 5, 6))

在代码中首先对数据和标签同时进行数据增强，因为对图片进行改变，框的位置也会变化，这里主要做了翻转图像和边界框的数据增强。

构建模型时，需要修改输出的类别为 2，一类是背景，一类是行人。

model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)
in_features = model.roi_heads.box_predictor.cls_score.in_features
model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes)

这里不用构造 Loss，因为在 Faster RCNN 中已经构建了 Loss。在训练时，需要把 image 和 target 的 tuple 转换为 list，再输入模型。模型返回的不是真正的标签，而是直接返回 Loss，所以我们可以直接利用这个 Loss 进行反向传播。

代码如下：

import os
import time
import torch.nn as nn
import torch
import random
import numpy as np
import torchvision.transforms as transforms
import torchvision
from PIL import Image
import torch.nn.functional as F
from my_dataset import PennFudanDataset
from common_tools import set_seed
from torch.utils.data import DataLoader
from matplotlib import pyplot as plt
from torchvision.models.detection.faster_rcnn import FastRCNNPredictor
from torchvision.transforms import functional as F
import enviroments

set_seed(1)  # 设置随机种子

BASE_DIR = os.path.dirname(os.path.abspath(__file__))
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

# classes_coco
COCO_INSTANCE_CATEGORY_NAMES = [
    '__background__', 'person', 'bicycle', 'car', 'motorcycle', 'airplane', 'bus',
    'train', 'truck', 'boat', 'traffic light', 'fire hydrant', 'N/A', 'stop sign',
    'parking meter', 'bench', 'bird', 'cat', 'dog', 'horse', 'sheep', 'cow',
    'elephant', 'bear', 'zebra', 'giraffe', 'N/A', 'backpack', 'umbrella', 'N/A', 'N/A',
    'handbag', 'tie', 'suitcase', 'frisbee', 'skis', 'snowboard', 'sports ball',
    'kite', 'baseball bat', 'baseball glove', 'skateboard', 'surfboard', 'tennis racket',
    'bottle', 'N/A', 'wine glass', 'cup', 'fork', 'knife', 'spoon', 'bowl',
    'banana', 'apple', 'sandwich', 'orange', 'broccoli', 'carrot', 'hot dog', 'pizza',
    'donut', 'cake', 'chair', 'couch', 'potted plant', 'bed', 'N/A', 'dining table',
    'N/A', 'N/A', 'toilet', 'N/A', 'tv', 'laptop', 'mouse', 'remote', 'keyboard', 'cell phone',
    'microwave', 'oven', 'toaster', 'sink', 'refrigerator', 'N/A', 'book',
    'clock', 'vase', 'scissors', 'teddy bear', 'hair drier', 'toothbrush'
]


def vis_bbox(img, output, classes, max_vis=40, prob_thres=0.4):
    fig, ax = plt.subplots(figsize=(12, 12))
    ax.imshow(img, aspect='equal')

    out_boxes = output_dict["boxes"].cpu()
    out_scores = output_dict["scores"].cpu()
    out_labels = output_dict["labels"].cpu()

    num_boxes = out_boxes.shape[0]
    for idx in range(0, min(num_boxes, max_vis)):

        score = out_scores[idx].numpy()
        bbox = out_boxes[idx].numpy()
        class_name = classes[out_labels[idx]]

        if score < prob_thres:
            continue

        ax.add_patch(plt.Rectangle((bbox[0], bbox[1]), bbox[2] - bbox[0], bbox[3] - bbox[1], fill=False,
                                   edgecolor='red', linewidth=3.5))
        ax.text(bbox[0], bbox[1] - 2, '{:s} {:.3f}'.format(class_name, score), bbox=dict(facecolor='blue', alpha=0.5),
                fontsize=14, color='white')
    plt.show()
    plt.close()


class Compose(object):
    def __init__(self, transforms):
        self.transforms = transforms

    def __call__(self, image, target):
        for t in self.transforms:
            image, target = t(image, target)
        return image, target


class RandomHorizontalFlip(object):
    def __init__(self, prob):
        self.prob = prob

    def __call__(self, image, target):
        if random.random() < self.prob:
            height, width = image.shape[-2:]
            image = image.flip(-1)
            bbox = target["boxes"]
            bbox[:, [0, 2]] = width - bbox[:, [2, 0]]
            target["boxes"] = bbox
        return image, target


class ToTensor(object):
    def __call__(self, image, target):
        image = F.to_tensor(image)
        return image, target


if __name__ == "__main__":

    # config
    LR = 0.001
    num_classes = 2
    batch_size = 1
    start_epoch, max_epoch = 0, 5
    train_dir = enviroments.pennFudanPed_data_dir
    train_transform = Compose([ToTensor(), RandomHorizontalFlip(0.5)])

    # step 1: data
    train_set = PennFudanDataset(data_dir=train_dir, transforms=train_transform)

    # 收集batch data的函数
    def collate_fn(batch):
        return tuple(zip(*batch))

    train_loader = DataLoader(train_set, batch_size=batch_size, collate_fn=collate_fn)

    # step 2: model
    model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)
    in_features = model.roi_heads.box_predictor.cls_score.in_features
    model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes) # replace the pre-trained head with a new one

    model.to(device)

    # step 3: loss
    # in lib/python3.6/site-packages/torchvision/models/detection/roi_heads.py
    # def fastrcnn_loss(class_logits, box_regression, labels, regression_targets)

    # step 4: optimizer scheduler
    params = [p for p in model.parameters() if p.requires_grad]
    optimizer = torch.optim.SGD(params, lr=LR, momentum=0.9, weight_decay=0.0005)
    lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=10, gamma=0.1)

    # step 5: Iteration

    for epoch in range(start_epoch, max_epoch):

        model.train()
        for iter, (images, targets) in enumerate(train_loader):

            images = list(image.to(device) for image in images)
            targets = [{k: v.to(device) for k, v in t.items()} for t in targets]

            # if torch.cuda.is_available():
            #     images, targets = images.to(device), targets.to(device)

            loss_dict = model(images, targets)  # images is list; targets is [ dict["boxes":**, "labels":**], dict[] ]

            losses = sum(loss for loss in loss_dict.values())

            print("Training:Epoch[{:0>3}/{:0>3}] Iteration[{:0>3}/{:0>3}] Loss: {:.4f} ".format(
                epoch, max_epoch, iter + 1, len(train_loader), losses.item()))

            optimizer.zero_grad()
            losses.backward()
            optimizer.step()

        lr_scheduler.step()

    # test
    model.eval()

    # config
    vis_num = 5
    vis_dir = os.path.join(BASE_DIR, "..", "..", "data", "PennFudanPed", "PNGImages")
    img_names = list(filter(lambda x: x.endswith(".png"), os.listdir(vis_dir)))
    random.shuffle(img_names)
    preprocess = transforms.Compose([transforms.ToTensor(), ])

    for i in range(0, vis_num):

        path_img = os.path.join(vis_dir, img_names[i])
        # preprocess
        input_image = Image.open(path_img).convert("RGB")
        img_chw = preprocess(input_image)

        # to device
        if torch.cuda.is_available():
            img_chw = img_chw.to('cuda')
            model.to('cuda')

        # forward
        input_list = [img_chw]
        with torch.no_grad():
            tic = time.time()
            print("input img tensor shape:{}".format(input_list[0].shape))
            output_list = model(input_list)
            output_dict = output_list[0]
            print("pass: {:.3f}s".format(time.time() - tic))

        # visualization
        vis_bbox(input_image, output_dict, COCO_INSTANCE_CATEGORY_NAMES, max_vis=20, prob_thres=0.5)  # for 2 epoch for nms

参考资料

• 深度之眼 PyTorch 框架班

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