Faster RCNN Pytorch 实现 代码级 详解

基本结构：

采用VGG提取特征的Faster RCNN.

self.backbone:提取出特征图->features

self.rpn:选出推荐框->proposals

self.roi heads:根据proposals在features上进行抠图->detections

        
features = self.backbone(images.tensors)
 
proposals, proposal_losses = self.rpn(images, features, targets)
detections, detector_losses = self.roi_heads(features, proposals, images.image_sizes, targets)
   

1.self.backbone

    def forward(self, x):
        identity = x

        out = self.conv1(x)
        out = self.bn1(out)
        out = self.relu(out)

        out = self.conv2(out)
        out = self.bn2(out)

        if self.downsample is not None:
            identity = self.downsample(x)

        out += identity
        out = self.relu(out)

        return out

这里pteaures就是提取的特征图、而台teaures是字思形式，由5张特征图组成，这里就是构成了不同的尺度的要求，特征图越小，所映射原图的范围越大。
注:这里的理解很重要，，其实这里能够完全理解，那对图像检测基本就入门了，

五种featureMap:

[1,256,11,21]1:是pytorch四要求的一般会用于batchsize的功效，多少张图片256:通道数
11:  height  高

21:  weight  宽

2.self.rpn

objectness, pred_bbox_deltas = self.head(features)
anchors = self.anchor_generator(images, features)

boxes, scores = self.filter_proposals(proposals, objectness, images.image_sizes, num_anchors_per_level)

self.head(features):

    def forward(self, x):
        # type: (List[Tensor])
        logits = []
        bbox_reg = []
        for feature in x:
            t = F.relu(self.conv(feature))
            logits.append(self.cls_logits(t))  # 对t分类
            bbox_reg.append(self.bbox_pred(t))  # 对t回归
        return logits, bbox_reg

x:就是输出的5张特征图features

objectness, pred_bbox_deltas = self.head(features)

锚框是由特征图上一个像素点在原图上得到的不同尺度的锚框，一般fasterrcnn论文里面是9个尺度
在这里是3

anchors = self.anchor_generator(images, features)

boxes, scores = self.filter_proposals(proposals, objectness, images.image_sizes, num_anchors_per_level)

这里的scores是的是前景的概率。(这里一般就是2分类，前景或背景)

top_n_idx = self._get_top_n_idx(objectness, num_anchors_per_level)

222603—》4693

 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
            
            # NMS的实现
            keep = box_ops.batched_nms(boxes, scores, lvl, self.nms_thresh)



            # keep only topk scoring predictions
            # keep就是最终保留的
            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

4693–》1324

1324–》1000
这里的1000是在faster_rcnn.py中设置的

为什么不是2000是因为训练的时候是2000，这里只是测试

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

 roi_heads()

        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)
        #class_logits： 分类概率 和 box_regression ：边界框回归

box_roi_pool 规整，为相同尺度的特征图，便于之后的分类与回归
box_roi_pool：两个FC层

        detections, detector_losses = self.roi_heads(features, proposals, images.image_sizes, targets)
        # 映射回原图
        detections = self.transform.postprocess(detections, images.image_sizes, original_image_sizes)  

detections = self.transform.postprocess(detections, images.image_sizes, original_image_sizes)  

补充：pytorch自带detection模块：

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 tools.my_dataset import PennFudanDataset
#from tools.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

#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, 30
    train_dir = os.path.join(BASE_DIR, "data", "PennFudanPed")
    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

欢迎点赞   收藏   关注

参考：Pytorch实现Faster-RCNN_pytorch faster rcnn-CSDN博客