零基础入门深度学习（十一）：目标检测之YOLOv3算法实现下篇
- 2020 年 2 月 19 日
- 筆記
01
导读
本课程是百度官方开设的零基础入门深度学习课程，主要面向没有深度学习技术基础或者基础薄弱的同学，帮助大家在深度学习领域实现从0到1+的跨越。从本系列课程中，你将学习到：
- numpy实现神经网络构建和梯度下降算法
- 深度学习基础知识
- 计算机视觉领域主要方向的原理、实践
- 自然语言处理领域主要方向的原理、实践
- 个性化推荐算法的原理、实践
百度深度学习技术平台部资深研发工程师孙老师，在上一讲中为大家讲解了YOLOv3算法中产生候选区域和卷积神经网络提取特征的部分，本讲将为大家介绍建立损失函数、多层级检测和预测输出的相关内容。
02
损失函数
上一讲中，我们已经从概念上将输出特征图上的像素点与预测框关联起来了，那么要对神经网络进行求解，还必须从数学上将网络输出和预测框关联起来，也就是要建立起损失函数跟网络输出之间的关系。下面讨论如何建立起YOLO-V3的损失函数。
对于每个预测框，YOLO-V3模型会建立三种类型的损失函数：
- 表征是否包含目标物体的损失函数，通过pred_objectness和label_objectness计算
loss_obj = fluid.layers.sigmoid_cross_entropy_with_logits(pred_objectness, label_objectness)
- 表征物体位置的损失函数，通过pred_location和label_location计算
pred_location_x = pred_location[:, :, 0, :, :] pred_location_y = pred_location[:, :, 1, :, :] pred_location_w = pred_location[:, :, 2, :, :] pred_location_h = pred_location[:, :, 3, :, :] loss_location_x = fluid.layers.sigmoid_cross_entropy_with_logits(pred_location_x, label_location_x) loss_location_y = fluid.layers.sigmoid_cross_entropy_with_logits(pred_location_y, label_location_y) loss_location_w = fluid.layers.abs(pred_location_w - label_location_w) loss_location_h = fluid.layers.abs(pred_location_h - label_location_h) loss_location = loss_location_x + loss_location_y + loss_location_w + loss_location_h
- 表征物体类别的损失函数，通过pred_classification和label_classification计算
loss_obj = fluid.layers.sigmoid_cross_entropy_with_logits(pred_classification, label_classification)
在前面几个小节中我们已经知道怎么计算这些预测值和标签了，但是遗留了一个小问题，就是没有标注出哪些锚框的objectness为-1。为了完成这一步，我们需要计算出所有预测框跟真实框之间的IoU，然后把那些IoU大于阈值的真实框挑选出来。实现代码如下：
# 挑选出跟真实框IoU大于阈值的预测框 def get_iou_above_thresh_inds(pred_box, gt_boxes, iou_threshold): batchsize = pred_box.shape[0] num_rows = pred_box.shape[1] num_cols = pred_box.shape[2] num_anchors = pred_box.shape[3] ret_inds = np.zeros([batchsize, num_rows, num_cols, num_anchors]) for i in range(batchsize): pred_box_i = pred_box[i] gt_boxes_i = gt_boxes[i] for k in range(len(gt_boxes_i)): #gt in gt_boxes_i: gt = gt_boxes_i[k] gtx_min = gt[0] - gt[2] / 2. gty_min = gt[1] - gt[3] / 2. gtx_max = gt[0] + gt[2] / 2. gty_max = gt[1] + gt[3] / 2. if (gtx_max - gtx_min < 1e-3) or (gty_max - gty_min < 1e-3): continue x1 = np.maximum(pred_box_i[:, :, :, 0], gtx_min) y1 = np.maximum(pred_box_i[:, :, :, 1], gty_min) x2 = np.minimum(pred_box_i[:, :, :, 2], gtx_max) y2 = np.minimum(pred_box_i[:, :, :, 3], gty_max) intersection = np.maximum(x2 - x1, 0.) * np.maximum(y2 - y1, 0.) s1 = (gty_max - gty_min) * (gtx_max - gtx_min) s2 = (pred_box_i[:, :, :, 2] - pred_box_i[:, :, :, 0]) * (pred_box_i[:, :, :, 3] - pred_box_i[:, :, :, 1]) union = s2 + s1 - intersection iou = intersection / union above_inds = np.where(iou > iou_threshold) ret_inds[i][above_inds] = 1 ret_inds = np.transpose(ret_inds, (0,3,1,2)) return ret_inds.astype('bool')
上面的函数可以得到哪些锚框的objectness需要被标注为-1，通过下面的程序，对label_objectness进行处理，将IoU大于阈值，但又不是正样本的那些锚框标注为-1。
def label_objectness_ignore(label_objectness, iou_above_thresh_indices): # 注意：这里不能简单的使用 label_objectness[iou_above_thresh_indices] = -1， # 这样可能会造成label_objectness为1的那些点被设置为-1了 # 只有将那些被标注为0，且与真实框IoU超过阈值的预测框才被标注为-1 negative_indices = (label_objectness < 0.5) ignore_indices = negative_indices * iou_above_thresh_indices label_objectness[ignore_indices] = -1 return label_objectness
下面通过调用这两个函数，实现如何将部分预测框的label_objectness设置为-1。
# 读取数据 reader = multithread_loader('/home/aistudio/work/insects/train', batch_size=2, mode='train') img, gt_boxes, gt_labels, im_shape = next(reader()) # 计算出锚框对应的标签 label_objectness, label_location, label_classification, scale_location = get_objectness_label(img, gt_boxes, gt_labels, iou_threshold = 0.7, anchors = [116, 90, 156, 198, 373, 326], num_classes=7, downsample=32) NUM_ANCHORS = 3 NUM_CLASSES = 7 num_filters=NUM_ANCHORS * (NUM_CLASSES + 5) with fluid.dygraph.guard(): backbone = DarkNet53_conv_body('yolov3_backbone', is_test=False) detection = YoloDetectionBlock('detection', channel=512, is_test=False) conv2d_pred = Conv2D('out_pred', num_filters=num_filters, filter_size=1) x = to_variable(img) C0, C1, C2 = backbone(x) route, tip = detection(C0) P0 = conv2d_pred(tip) # anchors包含了预先设定好的锚框尺寸 anchors = [116, 90, 156, 198, 373, 326] # downsample是特征图P0的步幅 pred_boxes = get_yolo_box_xxyy(P0.numpy(), anchors, num_classes=7, downsample=32) iou_above_thresh_indices = get_iou_above_thresh_inds(pred_boxes, gt_boxes, iou_threshold=0.7) label_objectness = label_objectness_ignore(label_objectness, iou_above_thresh_indices)
label_objectness.shape
(2, 3, 10, 10)
使用这种方式，就可以将那些没有被标注为正样本，但又与真实框IoU比较大的样本objectness标签设置为-1了，不计算其对任何一种损失函数的贡献。
计算总的损失函数的代码如下：
def get_loss(output, label_objectness, label_location, label_classification, scales, num_anchors=3, num_classes=7): # 将output从[N, C, H, W]变形为[N, NUM_ANCHORS, NUM_CLASSES + 5, H, W] reshaped_output = fluid.layers.reshape(output, [-1, num_anchors, num_classes + 5, output.shape[2], output.shape[3]]) # 从output中取出跟objectness相关的预测值 pred_objectness = reshaped_output[:, :, 4, :, :] loss_objectness = fluid.layers.sigmoid_cross_entropy_with_logits(pred_objectness, label_objectness, ignore_index=-1) ## 对第1，2，3维求和 #loss_objectness = fluid.layers.reduce_sum(loss_objectness, dim=[1,2,3], keep_dim=False) # pos_samples 只有在正样本的地方取值为1.，其它地方取值全为0. pos_objectness = label_objectness > 0 pos_samples = fluid.layers.cast(pos_objectness, 'float32') pos_samples.stop_gradient=True #从output中取出所有跟位置相关的预测值 tx = reshaped_output[:, :, 0, :, :] ty = reshaped_output[:, :, 1, :, :] tw = reshaped_output[:, :, 2, :, :] th = reshaped_output[:, :, 3, :, :] # 从label_location中取出各个位置坐标的标签 dx_label = label_location[:, :, 0, :, :] dy_label = label_location[:, :, 1, :, :] tw_label = label_location[:, :, 2, :, :] th_label = label_location[:, :, 3, :, :] # 构建损失函数 loss_location_x = fluid.layers.sigmoid_cross_entropy_with_logits(tx, dx_label) loss_location_y = fluid.layers.sigmoid_cross_entropy_with_logits(ty, dy_label) loss_location_w = fluid.layers.abs(tw - tw_label) loss_location_h = fluid.layers.abs(th - th_label) # 计算总的位置损失函数 loss_location = loss_location_x + loss_location_y + loss_location_h + loss_location_w # 乘以scales loss_location = loss_location * scales # 只计算正样本的位置损失函数 loss_location = loss_location * pos_samples #从ooutput取出所有跟物体类别相关的像素点 pred_classification = reshaped_output[:, :, 5:5+num_classes, :, :] # 计算分类相关的损失函数 loss_classification = fluid.layers.sigmoid_cross_entropy_with_logits(pred_classification, label_classification) # 将第2维求和 loss_classification = fluid.layers.reduce_sum(loss_classification, dim=2, keep_dim=False) # 只计算objectness为正的样本的分类损失函数 loss_classification = loss_classification * pos_samples total_loss = loss_objectness + loss_location + loss_classification # 对所有预测框的loss进行求和 total_loss = fluid.layers.reduce_sum(total_loss, dim=[1,2,3], keep_dim=False) # 对所有样本求平均 total_loss = fluid.layers.reduce_mean(total_loss) return total_loss
# 计算损失函数 # 读取数据 reader = multithread_loader('/home/aistudio/work/insects/train', batch_size=2, mode='train') img, gt_boxes, gt_labels, im_shape = next(reader()) # 计算出锚框对应的标签 label_objectness, label_location, label_classification, scale_location = get_objectness_label(img, gt_boxes, gt_labels, iou_threshold = 0.7, anchors = [116, 90, 156, 198, 373, 326], num_classes=7, downsample=32) NUM_ANCHORS = 3 NUM_CLASSES = 7 num_filters=NUM_ANCHORS * (NUM_CLASSES + 5) with fluid.dygraph.guard(): backbone = DarkNet53_conv_body('yolov3_backbone', is_test=False) detection = YoloDetectionBlock('detection', channel=512, is_test=False) conv2d_pred = Conv2D('out_pred', num_filters=num_filters, filter_size=1) x = to_variable(img) C0, C1, C2 = backbone(x) route, tip = detection(C0) P0 = conv2d_pred(tip) # anchors包含了预先设定好的锚框尺寸 anchors = [116, 90, 156, 198, 373, 326] # downsample是特征图P0的步幅 pred_boxes = get_yolo_box_xxyy(P0.numpy(), anchors, num_classes=7, downsample=32) iou_above_thresh_indices = get_iou_above_thresh_inds(pred_boxes, gt_boxes, iou_threshold=0.7) label_objectness = label_objectness_ignore(label_objectness, iou_above_thresh_indices) label_objectness = to_variable(label_objectness) label_location = to_variable(label_location) label_classification = to_variable(label_classification) scales = to_variable(scale_location) label_objectness.stop_gradient=True label_location.stop_gradient=True label_classification.stop_gradient=True scales.stop_gradient=True total_loss = get_loss(P0, label_objectness, label_location, label_classification, scales, num_anchors=NUM_ANCHORS, num_classes=NUM_CLASSES) total_loss_data = total_loss.numpy()
total_loss_data
array([623.6282], dtype=float32)
上面的程序计算出了总的损失函数，看到这里，读者已经了解到了YOLO-V3算法的大部分内容，包括如何生成锚框、给锚框打上标签、通过卷积神经网络提取特征、将输出特征图跟预测框相关联、建立起损失函数。
03
多尺度检测
目前我们计算损失函数是在特征图P0的基础上进行的，它的步幅stride=32。特征图的尺寸比较小，像素点数目比较少，每个像素点的感受野很大，具有非常丰富的高层级语义信息，可能比较容易检测到较大的目标。为了能够检测到尺寸较小的那些目标，需要在尺寸较大的特征图上面建立预测输出。如果我们在C2或者C1这种层级的特征图上直接产生预测输出，可能面临新的问题，它们没有经过充分的特征提取，像素点包含的语义信息不够丰富，有可能难以提取到有效的特征模式。在目标检测中，解决这一问题的方式是，将高层级的特征图尺寸放大之后跟低层级的特征图进行融合，得到的新特征图既能包含丰富的语义信息，又具有较多的像素点，能够描述更加精细的结构。
具体的网络实现方式如 图19 所示：
图19：生成多层级的输出特征图P0、P1、P2
YOLO-V3在每个区域的中心位置产生3个锚框，在3个层级的特征图上产生锚框的大小分别为P2 [(10×13),(16×30),(33×23)]，P1 [(30×61),(62×45),(59× 119)]，P0[(116 × 90), (156 × 198), (373 × 326]。越往后的特征图上用到的锚框尺寸也越大，能捕捉到大尺寸目标的信息；越往前的特征图上锚框尺寸越小，能捕捉到小尺寸目标的信息。
因为有多尺度的检测，所以需要对上面的代码进行较大的修改，而且实现过程也略显繁琐，所以推荐大家直接使用Paddle提供的API fluid.layers.yolov3_loss，其具体说明如下：
- fluid.layers.yolov3_loss(x, gt_box, gt_label, anchors, anchor_mask, class_num, ignore_thresh, downsample_ratio, gt_score=None, use_label_smooth=True, name=None))
- x: 输入的图像数据
- gt_box: 真实框
- gt_label: 真实框标签
- anchors: 使用到的anchor的尺寸，如[10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326]
- anchor_mask: 每个层级上使用的anchor的掩码，[[6, 7, 8], [3, 4, 5], [0, 1, 2]]
- class_num，物体类别数，AI识虫数据集为7
- ignore_thresh，预测框与真实框IoU阈值超过ignore_thresh时，不作为负样本，YOLO-V3模型里设置为0.7
- downsample_ratio，特征图P0的下采样比例，使用Darknet53骨干网络时为32
- gt_score，真实框的置信度，在使用了mixup技巧时会用到
- use_label_smooth，一种训练技巧，不使用就设置为False
- name，该层的名字，比如'yolov3_loss'，可以不设置
对于使用了多层级特征图产生预测框的方法，其具体实现代码如下：
# 定义上采样模块 class Upsample(fluid.dygraph.Layer): def __init__(self, name_scope, scale=2): super(Upsample,self).__init__(name_scope) self.scale = scale def forward(self, inputs): # get dynamic upsample output shape shape_nchw = fluid.layers.shape(inputs) shape_hw = fluid.layers.slice(shape_nchw, axes=[0], starts=[2], ends=[4]) shape_hw.stop_gradient = True in_shape = fluid.layers.cast(shape_hw, dtype='int32') out_shape = in_shape * self.scale out_shape.stop_gradient = True # reisze by actual_shape out = fluid.layers.resize_nearest( input=inputs, scale=self.scale, actual_shape=out_shape) return out # 定义YOLO-V3模型 class YOLOv3(fluid.dygraph.Layer): def __init__(self,name_scope, num_classes=7, is_train=True): super(YOLOv3,self).__init__(name_scope) self.is_train = is_train self.num_classes = num_classes # 提取图像特征的骨干代码 self.block = DarkNet53_conv_body(self.full_name(), is_test = not self.is_train) self.block_outputs = [] self.yolo_blocks = [] self.route_blocks_2 = [] # 生成3个层级的特征图P0, P1, P2 for i in range(3): # 添加从ci生成ri和ti的模块 yolo_block = self.add_sublayer( "yolo_detecton_block_%d" % (i), YoloDetectionBlock(self.full_name(), channel = 512//(2**i), is_test = not self.is_train)) self.yolo_blocks.append(yolo_block) num_filters = 3 * (self.num_classes + 5) # 添加从ti生成pi的模块，这是一个Conv2D操作，输出通道数为3 * (num_classes + 5) block_out = self.add_sublayer( "block_out_%d" % (i), Conv2D(self.full_name(), num_filters=num_filters, filter_size=1, stride=1, padding=0, act=None, param_attr=ParamAttr( initializer=fluid.initializer.Normal(0., 0.02)), bias_attr=ParamAttr( initializer=fluid.initializer.Constant(0.0), regularizer=L2Decay(0.)))) self.block_outputs.append(block_out) if i < 2: # 对ri进行卷积 route = self.add_sublayer("route2_%d"%i, ConvBNLayer(self.full_name(), ch_out=256//(2**i), filter_size=1, stride=1, padding=0, is_test=(not self.is_train))) self.route_blocks_2.append(route) # 将ri放大以便跟c_{i+1}保持同样的尺寸 self.upsample = Upsample(self.full_name()) def forward(self, inputs): outputs = [] blocks = self.block(inputs) for i, block in enumerate(blocks): if i > 0: # 将r_{i-1}经过卷积和上采样之后得到特征图，与这一级的ci进行拼接 block = fluid.layers.concat(input=[route, block], axis=1) # 从ci生成ti和ri route, tip = self.yolo_blocks[i](block) # 从ti生成pi block_out = self.block_outputs[i](tip) # 将pi放入列表 outputs.append(block_out) if i < 2: # 对ri进行卷积调整通道数 route = self.route_blocks_2[i](route) # 对ri进行放大，使其尺寸和c_{i+1}保持一致 route = self.upsample(route) return outputs def get_loss(self, outputs, gtbox, gtlabel, gtscore=None, anchors = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326], anchor_masks = [[6, 7, 8], [3, 4, 5], [0, 1, 2]], ignore_thresh=0.7, use_label_smooth=False): """ 使用fluid.layers.yolov3_loss，直接计算损失函数，过程更简洁，速度也更快 """ self.losses = [] downsample = 32 for i, out in enumerate(outputs): # 对三个层级分别求损失函数 anchor_mask_i = anchor_masks[i] loss = fluid.layers.yolov3_loss( x=out, # out是P0, P1, P2中的一个 gt_box=gtbox, # 真实框坐标 gt_label=gtlabel, # 真实框类别 gt_score=gtscore, # 真实框得分，使用mixup训练技巧时需要，不使用该技巧时直接设置为1，形状与gtlabel相同 anchors=anchors, # 锚框尺寸，包含[w0, h0, w1, h1, ..., w8, h8]共9个锚框的尺寸 anchor_mask=anchor_mask_i, # 筛选锚框的mask，例如anchor_mask_i=[3, 4, 5]，将anchors中第3、4、5个锚框挑选出来给该层级使用 class_num=self.num_classes, # 分类类别数 ignore_thresh=ignore_thresh, # 当预测框与真实框IoU > ignore_thresh，标注objectness = -1 downsample_ratio=downsample, # 特征图相对于原图缩小的倍数，例如P0是32， P1是16，P2是8 use_label_smooth=False) # 使用label_smooth训练技巧时会用到，这里没用此技巧，直接设置为False self.losses.append(fluid.layers.reduce_mean(loss)) #reduce_mean对每张图片求和 downsample = downsample // 2 # 下一级特征图的缩放倍数会减半 return sum(self.losses) # 对每个层级求和
开启端到端训练
训练过程的流程如下图所示，输入图片经过特征提取得到三个层级的输出特征图P0(stride=32)、P1(stride=16)和P2(stride=8)，相应的分别使用不同大小的小方块区域去生成对应的锚框和预测框，并对这些锚框进行标注。
- P0层级特征图，对应着使用大小的小方块，在每个区域中心生成大小分别为, , 的三种锚框。
- P1层级特征图，对应着使用大小的小方块，在每个区域中心生成大小分别为, , 的三种锚框。
- P2层级特征图，对应着使用大小的小方块，在每个区域中心生成大小分别为, , 的三种锚框。
将三个层级的特征图与对应锚框之间的标签关联起来，并建立损失函数，总的损失函数等于三个层级的损失函数相加。通过极小化损失函数，可以开启端到端的训练过程。
图20：端到端训练流程
训练过程的具体实现代码如下：
############# 这段代码在本地机器上运行请慎重，容易造成死机####################### import time import os import paddle import paddle.fluid as fluid ANCHORS = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326] ANCHOR_MASKS = [[6, 7, 8], [3, 4, 5], [0, 1, 2]] IGNORE_THRESH = .7 NUM_CLASSES = 7 def get_lr(base_lr = 0.0001, lr_decay = 0.1): bd = [10000, 20000] lr = [base_lr, base_lr * lr_decay, base_lr * lr_decay * lr_decay] learning_rate = fluid.layers.piecewise_decay(boundaries=bd, values=lr) return learning_rate if __name__ == '__main__': TRAINDIR = '/home/aistudio/work/insects/train' TESTDIR = '/home/aistudio/work/insects/test' VALIDDIR = '/home/aistudio/work/insects/val' with fluid.dygraph.guard(): model = YOLOv3('yolov3', num_classes = NUM_CLASSES, is_train=True) #创建模型 learning_rate = get_lr() opt = fluid.optimizer.Momentum( learning_rate=learning_rate, momentum=0.9, regularization=fluid.regularizer.L2Decay(0.0005)) #创建优化器 train_loader = multithread_loader(TRAINDIR, batch_size= 10, mode='train') #创建训练数据读取器 valid_loader = multithread_loader(VALIDDIR, batch_size= 10, mode='valid') #创建验证数据读取器 MAX_EPOCH = 200 for epoch in range(MAX_EPOCH): for i, data in enumerate(train_loader()): img, gt_boxes, gt_labels, img_scale = data gt_scores = np.ones(gt_labels.shape).astype('float32') gt_scores = to_variable(gt_scores) img = to_variable(img) gt_boxes = to_variable(gt_boxes) gt_labels = to_variable(gt_labels) outputs = model(img) #前向传播，输出[P0, P1, P2] loss = model.get_loss(outputs, gt_boxes, gt_labels, gtscore=gt_scores, anchors = ANCHORS, anchor_masks = ANCHOR_MASKS, ignore_thresh=IGNORE_THRESH, use_label_smooth=False) # 计算损失函数 loss.backward() # 反向传播计算梯度 opt.minimize(loss) # 更新参数 model.clear_gradients() if i % 1 == 0: timestring = time.strftime("%Y-%m-%d %H:%M:%S",time.localtime(time.time())) print('{}[TRAIN]epoch {}, iter {}, output loss: {}'.format(timestring, epoch, i, loss.numpy())) # save params of model if (epoch % 5 == 0) or (epoch == MAX_EPOCH -1): fluid.save_dygraph(model.state_dict(), 'yolo_epoch{}'.format(epoch)) # 每个epoch结束之后在验证集上进行测试 model.eval() for i, data in enumerate(valid_loader()): img, gt_boxes, gt_labels, img_scale = data gt_scores = np.ones(gt_labels.shape).astype('float32') gt_scores = to_variable(gt_scores) img = to_variable(img) gt_boxes = to_variable(gt_boxes) gt_labels = to_variable(gt_labels) outputs = model(img) loss = model.get_loss(outputs, gt_boxes, gt_labels, gtscore=gt_scores, anchors = ANCHORS, anchor_masks = ANCHOR_MASKS, ignore_thresh=IGNORE_THRESH, use_label_smooth=False) if i % 1 == 0: timestring = time.strftime("%Y-%m-%d %H:%M:%S",time.localtime(time.time())) print('{}[VALID]epoch {}, iter {}, output loss: {}'.format(timestring, epoch, i, loss.numpy())) model.train()
04
预测
预测过程流程 图21 如下所示：
图21：端到端训练流程
预测过程可以分为两步：
- 通过网络输出计算出预测框位置和所属类别的得分。
- 使用非极大值抑制来消除重叠较大的预测框。
对于第1步，前面我们已经讲过如何通过网络输出值计算pred_objectness_probability, pred_boxes以及pred_classification_probability，这里推荐大家直接使用fluid.layers.yolo_box，其使用方法是：
- fluid.layers.yolo_box(x, img_size, anchors, class_num, conf_thresh, downsample_ratio, name=None)
- x，网络输出特征图，例如上面提到的P0或者P1、P2
- img_size，输入图片尺寸
- anchors，使用到的anchor的尺寸，如[10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326]
- anchor_mask: 每个层级上使用的anchor的掩码，[[6, 7, 8], [3, 4, 5], [0, 1, 2]]
- class_num，物体类别数目
- conf_thresh, 置信度阈值，得分低于该阈值的预测框位置数值不用计算直接设置为0.0
- downsample_ratio, 特征图的下采样比例，例如P0是32，P1是16，P2是8
- name=None，名字，例如'yolo_box'
- 返回值包括两项，boxes和scores，其中boxes是所有预测框的坐标值，scores是所有预测框的得分。
预测框得分的定义是所属类别的概率乘以其预测框是否包含目标物体的objectness概率，即
在上面定义的类YOLO-V3下面添加函数，get_pred，通过调用fluid.layers.yolo_box获得P0、P1、P2三个层级的特征图对应的预测框和得分，并将他们拼接在一块，即可得到所有的预测框及其属于各个类别的得分。
class YOLOv3(fluid.dygraph.Layer): def __init__(self,name_scope, num_classes=7, is_train=True): super(YOLOv3,self).__init__(name_scope) self.is_train = is_train self.num_classes = num_classes # 提取图像特征的骨干代码 self.block = DarkNet53_conv_body(self.full_name(), is_test = not self.is_train) self.block_outputs = [] self.yolo_blocks = [] self.route_blocks_2 = [] for i in range(3): # 添加从ci生成ri和ti的模块 yolo_block = self.add_sublayer( "yolo_detecton_block_%d" % (i), YoloDetectionBlock(self.full_name(), channel = 512//(2**i), is_test = not self.is_train)) self.yolo_blocks.append(yolo_block) num_filters = 3 * (self.num_classes + 5) # 添加从ti生成pi的模块，这是一个Conv2D操作，输出通道数为3 * (num_classes + 5) block_out = self.add_sublayer( "block_out_%d" % (i), Conv2D(self.full_name(), num_filters=num_filters, filter_size=1, stride=1, padding=0, act=None, param_attr=ParamAttr( initializer=fluid.initializer.Normal(0., 0.02)), bias_attr=ParamAttr( initializer=fluid.initializer.Constant(0.0), regularizer=L2Decay(0.)))) self.block_outputs.append(block_out) if i < 2: # 对ri进行卷积 route = self.add_sublayer("route2_%d"%i, ConvBNLayer(self.full_name(), ch_out=256//(2**i), filter_size=1, stride=1, padding=0, is_test=(not self.is_train))) self.route_blocks_2.append(route) # 将ri放大以便跟c_{i+1}保持同样的尺寸 self.upsample = Upsample(self.full_name()) def forward(self, inputs): outputs = [] blocks = self.block(inputs) for i, block in enumerate(blocks): if i > 0: # 将r_{i-1}经过卷积和上采样之后得到特征图，与这一级的ci进行拼接 block = fluid.layers.concat(input=[route, block], axis=1) # 从ci生成ti和ri route, tip = self.yolo_blocks[i](block) # 从ti生成pi block_out = self.block_outputs[i](tip) # 将pi放入列表 outputs.append(block_out) if i < 2: # 对ri进行卷积调整通道数 route = self.route_blocks_2[i](route) # 对ri进行放大，使其尺寸和c_{i+1}保持一致 route = self.upsample(route) return outputs def get_loss(self, outputs, gtbox, gtlabel, gtscore=None, anchors = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326], anchor_masks = [[6, 7, 8], [3, 4, 5], [0, 1, 2]], ignore_thresh=0.7, use_label_smooth=False): self.losses = [] downsample = 32 for i, out in enumerate(outputs): anchor_mask_i = anchor_masks[i] loss = fluid.layers.yolov3_loss( x=out, gt_box=gtbox, gt_label=gtlabel, gt_score=gtscore, anchors=anchors, anchor_mask=anchor_mask_i, class_num=self.num_classes, ignore_thresh=ignore_thresh, downsample_ratio=downsample, use_label_smooth=False) self.losses.append(fluid.layers.reduce_mean(loss)) downsample = downsample // 2 return sum(self.losses) def get_pred(self, outputs, im_shape=None, anchors = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326], anchor_masks = [[6, 7, 8], [3, 4, 5], [0, 1, 2]], valid_thresh = 0.01): downsample = 32 total_boxes = [] total_scores = [] for i, out in enumerate(outputs): anchor_mask = anchor_masks[i] anchors_this_level = [] for m in anchor_mask: anchors_this_level.append(anchors[2 * m]) anchors_this_level.append(anchors[2 * m + 1]) boxes, scores = fluid.layers.yolo_box( x=out, img_size=im_shape, anchors=anchors_this_level, class_num=self.num_classes, conf_thresh=valid_thresh, downsample_ratio=downsample, name="yolo_box" + str(i)) total_boxes.append(boxes) total_scores.append( fluid.layers.transpose( scores, perm=[0, 2, 1])) downsample = downsample // 2 yolo_boxes = fluid.layers.concat(total_boxes, axis=1) yolo_scores = fluid.layers.concat(total_scores, axis=2) return yolo_boxes, yolo_scores
第1步的计算结果会在每个小方块区域都会产生多个预测框，输出预测框中会有很多重合度比较大，需要消除重叠较大的冗余预测框。
下面示例代码中的预测框是使用模型对图片预测之后输出的，这里一共选出了11个预测框，在图上画出预测框如下所示。在每个人像周围，都出现了多个预测框，需要消除冗余的预测框以得到最终的预测结果。
# 画图展示目标物体边界框 import numpy as np import matplotlib.pyplot as plt import matplotlib.patches as patches from matplotlib.image import imread import math # 定义画矩形框的程序 def draw_rectangle(currentAxis, bbox, edgecolor = 'k', facecolor = 'y', fill=False, linestyle='-'): # currentAxis，坐标轴，通过plt.gca()获取 # bbox，边界框，包含四个数值的list， [x1, y1, x2, y2] # edgecolor，边框线条颜色 # facecolor，填充颜色 # fill, 是否填充 # linestype，边框线型 # patches.Rectangle需要传入左上角坐标、矩形区域的宽度、高度等参数 rect=patches.Rectangle((bbox[0], bbox[1]), bbox[2]-bbox[0]+1, bbox[3]-bbox[1]+1, linewidth=1, edgecolor=edgecolor,facecolor=facecolor,fill=fill, linestyle=linestyle) currentAxis.add_patch(rect) plt.figure(figsize=(10, 10)) filename = '/home/aistudio/work/images/section3/000000086956.jpg' im = imread(filename) plt.imshow(im) currentAxis=plt.gca() # 预测框位置 boxes = np.array([[4.21716537e+01, 1.28230896e+02, 2.26547668e+02, 6.00434631e+02], [3.18562988e+02, 1.23168472e+02, 4.79000000e+02, 6.05688416e+02], [2.62704697e+01, 1.39430557e+02, 2.20587097e+02, 6.38959656e+02], [4.24965363e+01, 1.42706665e+02, 2.25955185e+02, 6.35671204e+02], [2.37462646e+02, 1.35731537e+02, 4.79000000e+02, 6.31451294e+02], [3.19390472e+02, 1.29295090e+02, 4.79000000e+02, 6.33003845e+02], [3.28933838e+02, 1.22736115e+02, 4.79000000e+02, 6.39000000e+02], [4.44292603e+01, 1.70438187e+02, 2.26841858e+02, 6.39000000e+02], [2.17988785e+02, 3.02472412e+02, 4.06062927e+02, 6.29106628e+02], [2.00241089e+02, 3.23755096e+02, 3.96929321e+02, 6.36386108e+02], [2.14310303e+02, 3.23443665e+02, 4.06732849e+02, 6.35775269e+02]]) # 预测框得分 scores = np.array([0.5247661 , 0.51759845, 0.86075854, 0.9910175 , 0.39170712, 0.9297706 , 0.5115228 , 0.270992 , 0.19087596, 0.64201415, 0.879036]) # 画出所有预测框 for box in boxes: draw_rectangle(currentAxis, box)
这里使用非极大值抑制（non-maximum suppression, nms）来消除冗余框，其基本思想是，如果有多个预测框都对应同一个物体，则只选出得分最高的那个预测框，剩下的预测框被丢弃掉。那么如何判断两个预测框对应的是同一个物体呢，标准该怎么设置？如果两个预测框的类别一样，而且他们的位置重合度比较大，则可以认为他们是在预测同一个目标。非极大值抑制的做法是，选出某个类别得分最高的预测框，然后看哪些预测框跟它的IoU大于阈值，就把这些预测框给丢弃掉。这里IoU的阈值是超参数，需要提前设置，YOLO-V3模型里面设置的是0.5。
比如在上面的程序中，boxes里面一共对应11个预测框，scores给出了它们预测"人"这一类别的得分。
- Step0 创建选中列表，keep_list = []
- Step1 对得分进行排序，remain_list = [ 3, 5, 10, 2, 9, 0, 1, 6, 4, 7, 8]，
- Step2 选出boxes[3]，此时keep_list为空，不需要计算IoU，直接将其放入keep_list，keep_list = [3]， remain_list=[5, 10, 2, 9, 0, 1, 6, 4, 7, 8]
- Step3 选出boxes[5]，此时keep_list中已经存在boxes[3]，计算出IoU(boxes[3], boxes[5]) = 0.0，显然小于阈值，则keep_list=[3, 5], remain_list = [10, 2, 9, 0, 1, 6, 4, 7, 8]
- Step4 选出boxes[10]，此时keep_list=[3, 5]，计算IoU(boxes[3], boxes[10])=0.0268，IoU(boxes[5], boxes[10])=0.0268 = 0.24，都小于阈值，则keep_list = [3, 5, 10]，remain_list=[2, 9, 0, 1, 6, 4, 7, 8]
- Step5 选出boxes[2]，此时keep_list = [3, 5, 10]，计算IoU(boxes[3], boxes[2]) = 0.88，超过了阈值，直接将boxes[2]丢弃，keep_list=[3, 5, 10]，remain_list=[9, 0, 1, 6, 4, 7, 8]
- Step6 选出boxes[9]，此时keep_list = [3, 5, 10]，计算IoU(boxes[3], boxes[9]) = 0.0577，IoU(boxes[5], boxes[9]) = 0.205，IoU(boxes[10], boxes[9]) = 0.88，超过了阈值，将boxes[9]丢弃掉。keep_list=[3, 5, 10]，remain_list=[0, 1, 6, 4, 7, 8]
- Step7 重复上述Step6直到remain_list为空
最终得到keep_list=[3, 5, 10]，也就是预测框3、5、10被最终挑选出来了，如下图所示
# 画图展示目标物体边界框 import numpy as np import matplotlib.pyplot as plt import matplotlib.patches as patches from matplotlib.image import imread import math # 定义画矩形框的程序 def draw_rectangle(currentAxis, bbox, edgecolor = 'k', facecolor = 'y', fill=False, linestyle='-'): # currentAxis，坐标轴，通过plt.gca()获取 # bbox，边界框，包含四个数值的list， [x1, y1, x2, y2] # edgecolor，边框线条颜色 # facecolor，填充颜色 # fill, 是否填充 # linestype，边框线型 # patches.Rectangle需要传入左上角坐标、矩形区域的宽度、高度等参数 rect=patches.Rectangle((bbox[0], bbox[1]), bbox[2]-bbox[0]+1, bbox[3]-bbox[1]+1, linewidth=1, edgecolor=edgecolor,facecolor=facecolor,fill=fill, linestyle=linestyle) currentAxis.add_patch(rect) plt.figure(figsize=(10, 10)) filename = '/home/aistudio/work/images/section3/000000086956.jpg' im = imread(filename) plt.imshow(im) currentAxis=plt.gca() boxes = np.array([[4.21716537e+01, 1.28230896e+02, 2.26547668e+02, 6.00434631e+02], [3.18562988e+02, 1.23168472e+02, 4.79000000e+02, 6.05688416e+02], [2.62704697e+01, 1.39430557e+02, 2.20587097e+02, 6.38959656e+02], [4.24965363e+01, 1.42706665e+02, 2.25955185e+02, 6.35671204e+02], [2.37462646e+02, 1.35731537e+02, 4.79000000e+02, 6.31451294e+02], [3.19390472e+02, 1.29295090e+02, 4.79000000e+02, 6.33003845e+02], [3.28933838e+02, 1.22736115e+02, 4.79000000e+02, 6.39000000e+02], [4.44292603e+01, 1.70438187e+02, 2.26841858e+02, 6.39000000e+02], [2.17988785e+02, 3.02472412e+02, 4.06062927e+02, 6.29106628e+02], [2.00241089e+02, 3.23755096e+02, 3.96929321e+02, 6.36386108e+02], [2.14310303e+02, 3.23443665e+02, 4.06732849e+02, 6.35775269e+02]]) scores = np.array([0.5247661 , 0.51759845, 0.86075854, 0.9910175 , 0.39170712, 0.9297706 , 0.5115228 , 0.270992 , 0.19087596, 0.64201415, 0.879036]) left_ind = np.where((boxes[:, 0]<60) * (boxes[:, 0]>20)) left_boxes = boxes[left_ind] left_scores = scores[left_ind] colors = ['r', 'g', 'b', 'k'] # 画出最终保留的预测框 inds = [3, 5, 10] for i in range(3): box = boxes[inds[i]] draw_rectangle(currentAxis, box, edgecolor=colors[i])
非极大值抑制的具体实现代码如下面nms函数的定义，需要说明的是数据集中含有多个类别的物体，所以这里需要做多分类非极大值抑制，其实现原理与非极大值抑制相同，区别在于需要对每个类别都做非极大值抑制，实现代码如下面的multiclass_nms所示。
# 非极大值抑制 def nms(bboxes, scores, score_thresh, nms_thresh, pre_nms_topk, i=0, c=0): """ nms """ inds = np.argsort(scores) inds = inds[::-1] keep_inds = [] while(len(inds) > 0): cur_ind = inds[0] cur_score = scores[cur_ind] # if score of the box is less than score_thresh, just drop it if cur_score < score_thresh: break keep = True for ind in keep_inds: current_box = bboxes[cur_ind] remain_box = bboxes[ind] iou = box_iou_xyxy(current_box, remain_box) if iou > nms_thresh: keep = False break if i == 0 and c == 4 and cur_ind == 951: print('suppressed, ', keep, i, c, cur_ind, ind, iou) if keep: keep_inds.append(cur_ind) inds = inds[1:] return np.array(keep_inds) # 多分类非极大值抑制 def multiclass_nms(bboxes, scores, score_thresh=0.01, nms_thresh=0.45, pre_nms_topk=1000, pos_nms_topk=100): """ This is for multiclass_nms """ batch_size = bboxes.shape[0] class_num = scores.shape[1] rets = [] for i in range(batch_size): bboxes_i = bboxes[i] scores_i = scores[i] ret = [] for c in range(class_num): scores_i_c = scores_i[c] keep_inds = nms(bboxes_i, scores_i_c, score_thresh, nms_thresh, pre_nms_topk, i=i, c=c) if len(keep_inds) < 1: continue keep_bboxes = bboxes_i[keep_inds] keep_scores = scores_i_c[keep_inds] keep_results = np.zeros([keep_scores.shape[0], 6]) keep_results[:, 0] = c keep_results[:, 1] = keep_scores[:] keep_results[:, 2:6] = keep_bboxes[:, :] ret.append(keep_results) if len(ret) < 1: rets.append(ret) continue ret_i = np.concatenate(ret, axis=0) scores_i = ret_i[:, 1] if len(scores_i) > pos_nms_topk: inds = np.argsort(scores_i)[::-1] inds = inds[:pos_nms_topk] ret_i = ret_i[inds] rets.append(ret_i) return rets
下面是完整的测试程序，在测试数据集上的输出结果将会被保存在pred_results.json文件中。
import json ANCHORS = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326] ANCHOR_MASKS = [[6, 7, 8], [3, 4, 5], [0, 1, 2]] VALID_THRESH = 0.01 NMS_TOPK = 400 NMS_POSK = 100 NMS_THRESH = 0.45 NUM_CLASSES = 7 if __name__ == '__main__': TRAINDIR = '/home/aistudio/work/insects/train/images' TESTDIR = '/home/aistudio/work/insects/test/images' VALIDDIR = '/home/aistudio/work/insects/val' with fluid.dygraph.guard(): model = YOLOv3('yolov3', num_classes=NUM_CLASSES, is_train=False) params_file_path = '/home/aistudio/work/yolo_epoch50' model_state_dict, _ = fluid.load_dygraph(params_file_path) model.load_dict(model_state_dict) model.eval() total_results = [] test_loader = test_data_loader(TESTDIR, batch_size= 1, mode='test') for i, data in enumerate(test_loader()): img_name, img_data, img_scale_data = data img = to_variable(img_data) img_scale = to_variable(img_scale_data) outputs = model.forward(img) bboxes, scores = model.get_pred(outputs, im_shape=img_scale, anchors=ANCHORS, anchor_masks=ANCHOR_MASKS, valid_thresh = VALID_THRESH) bboxes_data = bboxes.numpy() scores_data = scores.numpy() result = multiclass_nms(bboxes_data, scores_data, score_thresh=VALID_THRESH, nms_thresh=NMS_THRESH, pre_nms_topk=NMS_TOPK, pos_nms_topk=NMS_POSK) for j in range(len(result)): result_j = result[j] img_name_j = img_name[j] total_results.append([img_name_j, result_j.tolist()]) print('processed {} pictures'.format(len(total_results))) print('') json.dump(total_results, open('pred_results.json', 'w'))
json文件中保存着测试结果，是包含所有图片预测结果的list，其构成如下：
[[img_name, [[label, score, x1, x2, y1, y2], ..., [label, score, x1, x2, y1, y2]]], [img_name, [[label, score, x1, x2, y1, y2], ..., [label, score, x1, x2, y1, y2]]], ... [img_name, [[label, score, x1, x2, y1, y2],..., [label, score, x1, x2, y1, y2]]]]
list中的每一个元素是一张图片的预测结果，list的总长度等于图片的数目，每张图片预测结果的格式是：
[img_name, [[label, score, x1, x2, y1, y2],..., [label, score, x1, x2, y1, y2]]]
其中第一个元素是图片名称image_name，第二个元素是包含该图片所有预测框的list， 预测框列表：
[[label, score, x1, x2, y1, y2],..., [label, score, x1, x2, y1, y2]]
预测框列表中每个元素[label, score, x1, x2, y1, y2]描述了一个预测框，label是预测框所属类别标签，score是预测框的得分；x1, x2, y1, y2对应预测框左上角坐标(x1, y1)，右下角坐标(x2, y2)。每张图片可能有很多个预测框，则将其全部放在预测框列表中。
在AI识虫比赛的基础版本中，老师提供了MAP指标计算代码，使用此pred_results.json文件即可计算出最终的评估指标。
模型效果及可视化展示
上面的程序展示了如何读取测试数据集的读片，并将最终结果保存在json格式的文件中。为了更直观的给读者展示模型效果，下面的程序添加了如何读取单张图片，并画出其产生的预测框。
- 创建数据读取器以读取单张图片的数据
# 读取单张测试图片 def single_image_data_loader(filename, test_image_size=608, mode='test'): """ 加载测试用的图片，测试数据没有groundtruth标签 """ batch_size= 1 def reader(): batch_data = [] img_size = test_image_size file_path = os.path.join(filename) img = cv2.imread(file_path) img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB) H = img.shape[0] W = img.shape[1] img = cv2.resize(img, (img_size, img_size)) mean = [0.485, 0.456, 0.406] std = [0.229, 0.224, 0.225] mean = np.array(mean).reshape((1, 1, -1)) std = np.array(std).reshape((1, 1, -1)) out_img = (img / 255.0 - mean) / std out_img = out_img.astype('float32').transpose((2, 0, 1)) img = out_img #np.transpose(out_img, (2,0,1)) im_shape = [H, W] batch_data.append((image_name.split('.')[0], img, im_shape)) if len(batch_data) == batch_size: yield make_test_array(batch_data) batch_data = [] return reader
- 定义绘制预测框的画图函数，代码如下。
# 定义画图函数 INSECT_NAMES = ['Boerner', 'Leconte', 'Linnaeus', 'acuminatus', 'armandi', 'coleoptera', 'linnaeus'] # 定义画矩形框的函数 def draw_rectangle(currentAxis, bbox, edgecolor = 'k', facecolor = 'y', fill=False, linestyle='-'): # currentAxis，坐标轴，通过plt.gca()获取 # bbox，边界框，包含四个数值的list， [x1, y1, x2, y2] # edgecolor，边框线条颜色 # facecolor，填充颜色 # fill, 是否填充 # linestype，边框线型 # patches.Rectangle需要传入左上角坐标、矩形区域的宽度、高度等参数 rect=patches.Rectangle((bbox[0], bbox[1]), bbox[2]-bbox[0]+1, bbox[3]-bbox[1]+1, linewidth=1, edgecolor=edgecolor,facecolor=facecolor,fill=fill, linestyle=linestyle) currentAxis.add_patch(rect) # 定义绘制预测结果的函数 def draw_results(result, filename, draw_thresh=0.5): plt.figure(figsize=(10, 10)) im = imread(filename) plt.imshow(im) currentAxis=plt.gca() colors = ['r', 'g', 'b', 'k', 'y', 'c', 'purple'] for item in result: box = item[2:6] label = int(item[0]) name = INSECT_NAMES[label] if item[1] > draw_thresh: draw_rectangle(currentAxis, box, edgecolor = colors[label]) plt.text(box[0], box[1], name, fontsize=12, color=colors[label])
- 使用上面定义的single_image_data_loader函数读取指定的图片，输入网络并计算出预测框和得分，然后使用多分类非极大值抑制消除冗余的框。将最终结果画图展示出来。
import json import paddle import paddle.fluid as fluid ANCHORS = [10, 13, 16, 30, 33, 23, 30, 61, 62, 45, 59, 119, 116, 90, 156, 198, 373, 326] ANCHOR_MASKS = [[6, 7, 8], [3, 4, 5], [0, 1, 2]] VALID_THRESH = 0.01 NMS_TOPK = 400 NMS_POSK = 100 NMS_THRESH = 0.45 NUM_CLASSES = 7 if __name__ == '__main__': image_name = '/home/aistudio/work/insects/test/images/2599.jpeg' params_file_path = '/home/aistudio/work/yolo_epoch50' with fluid.dygraph.guard(): model = YOLOv3('yolov3', num_classes=NUM_CLASSES, is_train=False) model_state_dict, _ = fluid.load_dygraph(params_file_path) model.load_dict(model_state_dict) model.eval() total_results = [] test_loader = single_image_data_loader(image_name, mode='test') for i, data in enumerate(test_loader()): img_name, img_data, img_scale_data = data img = to_variable(img_data) img_scale = to_variable(img_scale_data) outputs = model.forward(img) bboxes, scores = model.get_pred(outputs, im_shape=img_scale, anchors=ANCHORS, anchor_masks=ANCHOR_MASKS, valid_thresh = VALID_THRESH) bboxes_data = bboxes.numpy() scores_data = scores.numpy() results = multiclass_nms(bboxes_data, scores_data, score_thresh=VALID_THRESH, nms_thresh=NMS_THRESH, pre_nms_topk=NMS_TOPK, pos_nms_topk=NMS_POSK) result = results[0] draw_results(result, image_name, draw_thresh=0.5)
通过上面的程序，清晰的给读者展示了如何使用训练好的权重，对图片进行预测并将结果可视化。最终输出的图片上，检测出了每个昆虫，标出了它们的边界框和具体类别。
05
总结
在过去的四讲中，孙老师为读者详细讲解了YOLOv3的设计思想以及具体算法实现，并且以业病虫害数据集为例完成了一个具体的AI识虫的任务。在后期课程中，将继续为大家带来内容更丰富的课程，帮助学员快速掌握深度学习方法。
