【从零开始学Mask RCNN】三，Mask RCNN网络架构解析及TensorFlow和Keras的交互

矫情吗；* 2023-02-20 02:16 44阅读 0赞

# 0. 前言 #

上一节把握了一下Mask RCNN项目的整体逻辑，这一节主要从TensorFlow和Keras的交互以及Mask RCNN的网络结构入手来分析一下。

# 1. TensorFlow和Keras的交互说明 #

相信熟悉Keras的同学都经常看到这行代码：

`import keras.backend as K`

如果Keras的后端是基于TensorFlow的，那么这个K就是Tensorflow了，那么自然会想一个问题，为什么不直接`import tensorflow`呢，这样不是多此一举吗？这个问题就涉及到TensorFlow和Keras的交互方法了。在这个Mask RCNN项目的构建模型的文件（`mrcnn/model.py`）中就涉及到了很多TensorFlow和Keras的交互方法，这些交互方法基本上都是对Keras的函数式API进行操作，但是Keras的函数模型转化为Model对象也是很方便，只需要用`KM.Model(input_tensors, output_tensors)`操作一下即可，其中`KM`表示的是：

import keras.models as KM

接下啦我们结合`mrcnn/model.py`这个文件来展示一下TensorFlow和Keras交互的一些方法。

## 1.1 使用TensorFlow建立Keras新的Layer对象 ##

在`model.py`中可以看到大量的继承了`keras.engine.Layer`类的新类，例如`DetectionTargetLayer`，`PyramidROIAlign`，这是因为TensorFlow的函数可以操作Keras的Tensor，但是它返回的TensorFlow的Tensor不能被Keras继续处理，因此我们需要建立新的Keras层进行转换，将TensorFlow的Tensor作为Keras层的`__init__`函数进行构建层，然后在`__call__`方法中使用TensorFlow的函数进行细粒度的数据处理，最后返回Keras层对象。

接下来我们首先看一下Keras的Tensor是可以直接送入到TensorFlow中的，例如语义分割中计算Dice系数的代码如下：

def Dice_coeff(y_true, y_pred):
        smooth = 1.
        y_true_f = Flatten()(y_true)
        y_pred_f = Flatten()(y_pred)
        intersection = tf.reduce_sum(y_true_f * y_pred_f)
        score = (2. * intersection + smooth) / (tf.reduce_sum(y_true_f) + 
                        tf.reduce_sum(y_pred_f) + smooth)
        return score

接下来我们看一下`model.py`中`PyramidROIAlign`这个类的代码实现：

############################################################
    #  ROIAlign Layer
    ############################################################
    
    def log2_graph(x):
        """Log2函数的实现。TF没有现成的实现。"""
        return tf.log(x) / tf.log(2.0)
    
    # TensorFlow和Keras的交互
    class PyramidROIAlign(KE.Layer):
        """在特征金字塔的多个层上实现了ROI Pooling
    
        参数:
        - pool_shape: [pool_height, pool_width] 代表池化输出. 例如[7, 7]
    
        输入:
        - boxes: 归一化坐标系中的[batch, num_boxes, (y1, x1, y2, x2)]. 如果没有足够的边界框来填充数组就填0.
        - image_meta: [batch, (meta data)] Image details. See compose_image_meta()
        - feature_maps: 金字塔不同层级的特征图列表，每一个都是[batch, height, width, channels]
    
        输出:
        池化后的区域形状: [batch, num_boxes, pool_height, pool_width, channels].
        宽度和高度是Layer构造函数中池化层中的特定值。
        """
    
        def __init__(self, pool_shape, **kwargs):
            super(PyramidROIAlign, self).__init__(**kwargs)
            self.pool_shape = tuple(pool_shape)
    
        def call(self, inputs):
            # Crop boxes [batch, num_boxes, (y1, x1, y2, x2)] in normalized coords
            # num_boxes指的是proposal数目，它们均会作用于每张图片上，只是不同的proposal作用于图片
            # 的特征级别不同，这里通过循环特征层寻找符合的proposal，应用ROIAlign
            boxes = inputs[0]
    
            # Image meta
            # Holds details about the image. See compose_image_meta()
            image_meta = inputs[1]
    
            # Feature Maps. List of feature maps from different level of the
            # feature pyramid. Each is [batch, height, width, channels]
            feature_maps = inputs[2:]
    
            # Assign each ROI to a level in the pyramid based on the ROI area.
            y1, x1, y2, x2 = tf.split(boxes, 4, axis=2)
            h = y2 - y1
            w = x2 - x1
            # 使用第一张图像的形状，批处理中的图像必须具有相同的大小。
            image_shape = parse_image_meta_graph(image_meta)['image_shape'][0]
            # 特征金字塔网络论文中的方程式1。我们的坐标在这里是标准化的。
            # 例如 一个224x224 的ROI (像素) 和P4匹配
            image_area = tf.cast(image_shape[0] * image_shape[1], tf.float32)
            # h、w已经归一化
            roi_level = log2_graph(tf.sqrt(h * w) / (224.0 / tf.sqrt(image_area)))
            # 确保值位于2到5之间
            roi_level = tf.minimum(5, tf.maximum(
                2, 4 + tf.cast(tf.round(roi_level), tf.int32)))
            # 删掉维度为1的维也就是第二维，还剩下的Tensor形状为：[batch, num_boxes]
            roi_level = tf.squeeze(roi_level, 2)
    
            # Loop through levels and apply ROI pooling to each. P2 to P5.
            # 遍历每个维度并且对P2到P5应用ROI Pooling
            pooled = []
            box_to_level = []
            for i, level in enumerate(range(2, 6)):
                # tf.where 返回值格式 [坐标1, 坐标2……]
                # np.where 返回值格式 [[坐标1.x, 坐标2.x……], [坐标1.y, 坐标2.y……]]
                
                # 返回坐标表示：第n张图片的第i个proposal,shape为
                ix = tf.where(tf.equal(roi_level, level))
                
    
                # tf.gather_nd(params,indices,name=None)
                # 按照indices的格式从params中抽取切片（合并为一个Tensor）
                # indices是一个K维整数Tensor，
                # [本level的proposal数目, 4]
                level_boxes = tf.gather_nd(boxes, ix)
                # Box indices for crop_and_resize.
                box_indices = tf.cast(ix[:, 0], tf.int32)
    
                # Keep track of which box is mapped to which level
                # 对哪个box和哪个level匹配上的关系进行跟踪记录
                box_to_level.append(ix)
    
                # Stop gradient propogation to ROI proposals
                level_boxes = tf.stop_gradient(level_boxes)
                box_indices = tf.stop_gradient(box_indices)
    
                # Crop and Resize
                # 来自Mask RCNN论文: “我们对四个常规位置进行抽样，这样我们就可以评估
                # 最大池或平均池。事实上，在每个bin中心仅插入一个值（无池）几乎同样有效。”
                # 在这里，我们使用简单的方法，即每个bin一个值，使用tf.crop_and_resize() 
                # 结果: [batch * num_boxes, pool_height, pool_width, channels]
                pooled.append(tf.image.crop_and_resize(
                    feature_maps[i], level_boxes, box_indices, self.pool_shape,
                    method="bilinear"))
    
            # 将池化后的特征打包到一个Tensor中
            pooled = tf.concat(pooled, axis=0)
    
            # 将box_to_level打包映射到一个序列并且添加另外一列代表ROI Pool后的候选框的顺序
            box_to_level = tf.concat(box_to_level, axis=0)    # [batch*num_boxes, 2]
            box_range = tf.expand_dims(tf.range(tf.shape(box_to_level)[0]), 1) # [batch*num_boxes, 1]
            box_to_level = tf.concat([tf.cast(box_to_level, tf.int32), box_range],
                                     axis=1) # [batch*num_boxes, 3]
    
            # 截止到目前，我们获取了记录全部ROIAlign结果feat集合的张量pooled，和记录这些feat相关信息的张量box_to_level，
            # 由于提取方法的原因，此时的feat并不是按照原始顺序排序（先按batch然后按box index排序），下面我们设法将之恢复顺
            # 序（ROIAlign作用于对应图片的对应proposal生成feat）
            # Rearrange pooled features to match the order of the original boxes
            # Sort box_to_level by batch then box index
            # TF doesn't have a way to sort by two columns, so merge them and sort.
            # box_to_level[i, 0]表示的是当前feat隶属的图片索引，box_to_level[i, 1]表示的是其box序号
            
            sorting_tensor = box_to_level[:, 0] * 100000 + box_to_level[:, 1] # [batch*num_boxes]
            ix = tf.nn.top_k(sorting_tensor, k=tf.shape(
                box_to_level)[0]).indices[::-1]
            # tf.gather 切片操作
            ix = tf.gather(box_to_level[:, 2], ix)
            pooled = tf.gather(pooled, ix)
    
            # Re-add the batch dimension
            shape = tf.concat([tf.shape(boxes)[:2], tf.shape(pooled)[1:]], axis=0)
            pooled = tf.reshape(pooled, shape)
            return pooled
    
        def compute_output_shape(self, input_shape):
            return input_shape[0][:2] + self.pool_shape + (input_shape[2][-1], )

## 1.2 利用Keras的Lambda函数将TensorFlow函数引入Keras ##

除了上面的方法外，我们还可以引入Keras的Lamda函数将TensorFlow的操作转化为Keras的数据流，举例如下：

rpn_bbox = KL.Lambda(lambda t: tf.reshape(t, [tf.shape(t)[0], -1, 4]))(x)

这样就可以将TensorFlow写好的函数输出直接转换为Keras的Module可以接收的类型。

## 1.3 继承Keras的层对象 ##

还有一种方法是直接继承某个`keras.layer`，这种方法和方法1相比同样需要实现`call`方法，不过一般会继承父类，以改写Keras已经实现的层方法。例子如下，重新定义了一个BatchNorm层，继承了Keras的BatchNormlization。

class BatchNorm(KL.BatchNormalization):
        """Extends the Keras BatchNormalization class to allow a central place
        to make changes if needed.
    
        Batch normalization has a negative effect on training if batches are small
        so this layer is often frozen (via setting in Config class) and functions
        as linear layer.
        """
        def call(self, inputs, training=None):
            """
            Note about training values:
                None: Train BN layers. This is the normal mode
                False: Freeze BN layers. Good when batch size is small
                True: (don't use). Set layer in training mode even when making inferences
            """
            return super(self.__class__, self).call(inputs, training=training)

# 2. 网络结构解析 #

首先我们来关注一下Mask RCNN整体结构图中的特征提取部分，也就是下图中的红色部分：

![Mask RCNN的特征提取部分][Mask RCNN]

上节已经讲到这里是主干网络为ResNet101的FPN网络作特征提取部分，接下来我们就来分析一下这部分的代码，首先我们定位到MaskRCNN这个最核心的类，我们先看一下`__init__`函数和`build`函数的准备部分，代码如下：

class MaskRCNN():
        """Encapsulates the Mask RCNN model functionality.
    
        The actual Keras model is in the keras_model property.
        """
    
        def __init__(self, mode, config, model_dir):
            """
            mode: Either "training" or "inference"
            config: A Sub-class of the Config class
            model_dir: Directory to save training logs and trained weights
            """
            assert mode in ['training', 'inference']
            self.mode = mode
            self.config = config
            self.model_dir = model_dir
            self.set_log_dir()
            self.keras_model = self.build(mode=mode, config=config)
    
        def build(self, mode, config):
            """Build Mask R-CNN architecture.
                input_shape: The shape of the input image.
                mode: Either "training" or "inference". The inputs and
                    outputs of the model differ accordingly.
            """
            assert mode in ['training', 'inference']
    
            # Image size must be dividable by 2 multiple times
            h, w = config.IMAGE_SHAPE[:2]
            if h / 2**6 != int(h / 2**6) or w / 2**6 != int(w / 2**6):
                raise Exception("Image size must be dividable by 2 at least 6 times "
                                "to avoid fractions when downscaling and upscaling."
                                "For example, use 256, 320, 384, 448, 512, ... etc. ")
    
            # Inputs
            input_image = KL.Input(
                shape=[None, None, config.IMAGE_SHAPE[2]], name="input_image")
            input_image_meta = KL.Input(shape=[config.IMAGE_META_SIZE],
                                        name="input_image_meta")
            if mode == "training":
                # RPN GT
                input_rpn_match = KL.Input(
                    shape=[None, 1], name="input_rpn_match", dtype=tf.int32)
                input_rpn_bbox = KL.Input(
                    shape=[None, 4], name="input_rpn_bbox", dtype=tf.float32)
    
                # Detection GT (class IDs, bounding boxes, and masks)
                # 1. GT Class IDs (zero padded)
                input_gt_class_ids = KL.Input(
                    shape=[None], name="input_gt_class_ids", dtype=tf.int32)
                # 2. GT Boxes in pixels (zero padded)
                # [batch, MAX_GT_INSTANCES, (y1, x1, y2, x2)] in image coordinates
                input_gt_boxes = KL.Input(
                    shape=[None, 4], name="input_gt_boxes", dtype=tf.float32)
                # Normalize coordinates
                gt_boxes = KL.Lambda(lambda x: norm_boxes_graph(
                    x, K.shape(input_image)[1:3]))(input_gt_boxes)
                # 3. GT Masks (zero padded)
                # [batch, height, width, MAX_GT_INSTANCES]
                if config.USE_MINI_MASK:
                    input_gt_masks = KL.Input(
                        shape=[config.MINI_MASK_SHAPE[0],
                               config.MINI_MASK_SHAPE[1], None],
                        name="input_gt_masks", dtype=bool)
                else:
                    input_gt_masks = KL.Input(
                        shape=[config.IMAGE_SHAPE[0], config.IMAGE_SHAPE[1], None],
                        name="input_gt_masks", dtype=bool)
            elif mode == "inference":
                # Anchors in normalized coordinates
                input_anchors = KL.Input(shape=[None, 4], name="input_anchors")

这里强制要求了图像的输入尺寸为 2 6 2^6 26的倍数，以保证下采样后不产生小数。可以看到初始化函数中需要外面输入的变量只有三个，另外注意到在`build()`函数里面有大量的变量用`KL.Input`来定义，我们看一下`Keras.Input`层的API：

tf.keras.Input(
        shape=None,
        batch_size=None,
        name=None,
        dtype=None,
        sparse=False,
        tensor=None,
        **kwargs
    )

其中，

参数：
    shape：形状元组（整数），不包括批量大小。例如，shape=(32,)表示预期输入将是32维向量的批次。
    batch_size：可选的静态批处理大小（整数）。
    name：图层的可选名称字符串。在模型中应该是唯一的（不要重复使用相同的名称两次）。如果没有提供，它将自动生成。
    dtype：数据类型由输入预期的，作为字符串（float32，float64，int32...）
    sparse：一个布尔值，指定要创建的占位符是否稀疏。
    tensor：可选的现有张量以包装到Input图层中。如果设置，该图层将不会创建占位符张量。
    **kwargs：不推荐的参数支持。

由于在上面的代码中没有指定`batch_size`参数，所以它们的实际`shape`还需要加上`batch`，实际`shape`如下：

input_image：输入图片，[batch, None, None, config.IMAGE_SHAPE[2]]
    input_image_meta：图片的信息（包含形状、预处理信息等，后面会介绍），[batch, config.IMAGE_META_SIZE]
    input_anchors：锚框，[batch, None, 4]

接下来就是BackBone(ResNet101)部分的代码了：

## ResNet101部分
            # Build the shared convolutional layers.
            # Bottom-up Layers
            # Returns a list of the last layers of each stage, 5 in total.
            # Don't create the thead (stage 5), so we pick the 4th item in the list.
            if callable(config.BACKBONE):
                _, C2, C3, C4, C5 = config.BACKBONE(input_image, stage5=True,
                                                    train_bn=config.TRAIN_BN)
            else:
                _, C2, C3, C4, C5 = resnet_graph(input_image, config.BACKBONE,
                                                 stage5=True, train_bn=config.TRAIN_BN)

可以看到代码设置了2种可选的方式，一种是在`config`中自定义的BackBone网络，一种是默认实现即ResNet101 BackBone，我们来分析一下`resnet_graph`函数：

############################################################
    #  Resnet Graph
    ############################################################
    
    # Code adopted from:
    # https://github.com/fchollet/deep-learning-models/blob/master/resnet50.py
    
    def identity_block(input_tensor, kernel_size, filters, stage, block,
                       use_bias=True, train_bn=True):
        """The identity_block is the block that has no conv layer at shortcut
        # Arguments
            input_tensor: input tensor
            kernel_size: default 3, the kernel size of middle conv layer at main path
            filters: list of integers, the nb_filters of 3 conv layer at main path
            stage: integer, current stage label, used for generating layer names
            block: 'a','b'..., current block label, used for generating layer names
            use_bias: Boolean. To use or not use a bias in conv layers.
            train_bn: Boolean. Train or freeze Batch Norm layers
        """
        nb_filter1, nb_filter2, nb_filter3 = filters
        conv_name_base = 'res' + str(stage) + block + '_branch'
        bn_name_base = 'bn' + str(stage) + block + '_branch'
    
        x = KL.Conv2D(nb_filter1, (1, 1), name=conv_name_base + '2a',
                      use_bias=use_bias)(input_tensor)
        x = BatchNorm(name=bn_name_base + '2a')(x, training=train_bn)
        x = KL.Activation('relu')(x)
    
        x = KL.Conv2D(nb_filter2, (kernel_size, kernel_size), padding='same',
                      name=conv_name_base + '2b', use_bias=use_bias)(x)
        x = BatchNorm(name=bn_name_base + '2b')(x, training=train_bn)
        x = KL.Activation('relu')(x)
    
        x = KL.Conv2D(nb_filter3, (1, 1), name=conv_name_base + '2c',
                      use_bias=use_bias)(x)
        x = BatchNorm(name=bn_name_base + '2c')(x, training=train_bn)
    
        x = KL.Add()([x, input_tensor])
        x = KL.Activation('relu', name='res' + str(stage) + block + '_out')(x)
        return x
    
    
    def conv_block(input_tensor, kernel_size, filters, stage, block,
                   strides=(2, 2), use_bias=True, train_bn=True):
        """conv_block is the block that has a conv layer at shortcut
        # Arguments
            input_tensor: input tensor
            kernel_size: default 3, the kernel size of middle conv layer at main path
            filters: list of integers, the nb_filters of 3 conv layer at main path
            stage: integer, current stage label, used for generating layer names
            block: 'a','b'..., current block label, used for generating layer names
            use_bias: Boolean. To use or not use a bias in conv layers.
            train_bn: Boolean. Train or freeze Batch Norm layers
        Note that from stage 3, the first conv layer at main path is with subsample=(2,2)
        And the shortcut should have subsample=(2,2) as well
        """
        nb_filter1, nb_filter2, nb_filter3 = filters
        conv_name_base = 'res' + str(stage) + block + '_branch'
        bn_name_base = 'bn' + str(stage) + block + '_branch'
    
        x = KL.Conv2D(nb_filter1, (1, 1), strides=strides,
                      name=conv_name_base + '2a', use_bias=use_bias)(input_tensor)
        x = BatchNorm(name=bn_name_base + '2a')(x, training=train_bn)
        x = KL.Activation('relu')(x)
    
        x = KL.Conv2D(nb_filter2, (kernel_size, kernel_size), padding='same',
                      name=conv_name_base + '2b', use_bias=use_bias)(x)
        x = BatchNorm(name=bn_name_base + '2b')(x, training=train_bn)
        x = KL.Activation('relu')(x)
    
        x = KL.Conv2D(nb_filter3, (1, 1), name=conv_name_base +
                      '2c', use_bias=use_bias)(x)
        x = BatchNorm(name=bn_name_base + '2c')(x, training=train_bn)
    
        shortcut = KL.Conv2D(nb_filter3, (1, 1), strides=strides,
                             name=conv_name_base + '1', use_bias=use_bias)(input_tensor)
        shortcut = BatchNorm(name=bn_name_base + '1')(shortcut, training=train_bn)
    
        x = KL.Add()([x, shortcut])
        x = KL.Activation('relu', name='res' + str(stage) + block + '_out')(x)
        return x
    
    
    def resnet_graph(input_image, architecture, stage5=False, train_bn=True):
        """Build a ResNet graph.
            architecture: Can be resnet50 or resnet101
            stage5: Boolean. If False, stage5 of the network is not created
            train_bn: Boolean. Train or freeze Batch Norm layers
        """
        assert architecture in ["resnet50", "resnet101"]
        # Stage 1
        x = KL.ZeroPadding2D((3, 3))(input_image)
        x = KL.Conv2D(64, (7, 7), strides=(2, 2), name='conv1', use_bias=True)(x)
        x = BatchNorm(name='bn_conv1')(x, training=train_bn)
        x = KL.Activation('relu')(x)
        C1 = x = KL.MaxPooling2D((3, 3), strides=(2, 2), padding="same")(x)
        # Stage 2
        x = conv_block(x, 3, [64, 64, 256], stage=2, block='a', strides=(1, 1), train_bn=train_bn)
        x = identity_block(x, 3, [64, 64, 256], stage=2, block='b', train_bn=train_bn)
        C2 = x = identity_block(x, 3, [64, 64, 256], stage=2, block='c', train_bn=train_bn)
        # Stage 3
        x = conv_block(x, 3, [128, 128, 512], stage=3, block='a', train_bn=train_bn)
        x = identity_block(x, 3, [128, 128, 512], stage=3, block='b', train_bn=train_bn)
        x = identity_block(x, 3, [128, 128, 512], stage=3, block='c', train_bn=train_bn)
        C3 = x = identity_block(x, 3, [128, 128, 512], stage=3, block='d', train_bn=train_bn)
        # Stage 4
        x = conv_block(x, 3, [256, 256, 1024], stage=4, block='a', train_bn=train_bn)
        block_count = {
        "resnet50": 5, "resnet101": 22}[architecture]
        for i in range(block_count):
            x = identity_block(x, 3, [256, 256, 1024], stage=4, block=chr(98 + i), train_bn=train_bn)
        C4 = x
        # Stage 5
        if stage5:
            x = conv_block(x, 3, [512, 512, 2048], stage=5, block='a', train_bn=train_bn)
            x = identity_block(x, 3, [512, 512, 2048], stage=5, block='b', train_bn=train_bn)
            C5 = x = identity_block(x, 3, [512, 512, 2048], stage=5, block='c', train_bn=train_bn)
        else:
            C5 = None
        return [C1, C2, C3, C4, C5]

从这个ResNet101的实现我们总结出这个网络中主要包含了应用了ShortCut和没有ShortCut的两种子结构，如下图所示：

![ResNet 101中两种典型卷积模块][ResNet 101]  
接下来就是Mask RCNN结构图里面的FPN部分了，代码如下：

# FPN部分
            # Top-down Layers
            # TODO: add assert to varify feature map sizes match what's in config
            P5 = KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (1, 1), name='fpn_c5p5')(C5)
            P4 = KL.Add(name="fpn_p4add")([
                KL.UpSampling2D(size=(2, 2), name="fpn_p5upsampled")(P5),
                KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (1, 1), name='fpn_c4p4')(C4)])
            P3 = KL.Add(name="fpn_p3add")([
                KL.UpSampling2D(size=(2, 2), name="fpn_p4upsampled")(P4),
                KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (1, 1), name='fpn_c3p3')(C3)])
            P2 = KL.Add(name="fpn_p2add")([
                KL.UpSampling2D(size=(2, 2), name="fpn_p3upsampled")(P3),
                KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (1, 1), name='fpn_c2p2')(C2)])
            # Attach 3x3 conv to all P layers to get the final feature maps.
            P2 = KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (3, 3), padding="SAME", name="fpn_p2")(P2)
            P3 = KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (3, 3), padding="SAME", name="fpn_p3")(P3)
            P4 = KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (3, 3), padding="SAME", name="fpn_p4")(P4)
            P5 = KL.Conv2D(config.TOP_DOWN_PYRAMID_SIZE, (3, 3), padding="SAME", name="fpn_p5")(P5)
            # P6 is used for the 5th anchor scale in RPN. Generated by
            # subsampling from P5 with stride of 2.
            P6 = KL.MaxPooling2D(pool_size=(1, 1), strides=2, name="fpn_p6")(P5)

最后网络提取到的特征集合为：

# 汇总提取到的特征集合
            # Note that P6 is used in RPN, but not in the classifier heads.
            rpn_feature_maps = [P2, P3, P4, P5, P6]
            mrcnn_feature_maps = [P2, P3, P4, P5]

# 3. 小结 #

本节就暂时讲到这里，现在我们获得了汇总提取到的特征图集合，接下来在这个特征图上我们会完成Anchor的生成以及Proposal生成，下一节我们就来讲解一下Anchor生成和Proposal候选框生成。

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[Mask RCNN]: https://img-blog.csdnimg.cn/20200623225158470.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA==,size_16,color_FFFFFF,t_70
[ResNet 101]: https://img-blog.csdnimg.cn/20200623234512715.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA==,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200110234905879.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA==,size_16,color_FFFFFF,t_70
[QQ]: https://img-blog.csdnimg.cn/20200517190745584.png#pic_center