从论文到代码实现 RoIPoolingRoI Pooling失去特色图和候选框，就会将候选框投影在特色图，而后进行一次缩放失去大小一致的特色图，在 Faster RCNN 中，区域候选框用来预测对象是前景还是背景，这是 class head 要做的工作，而 regression 是学习到基于 anchor 的差分，也就是核心的偏移量和宽高的缩放。
在投影过程中候选框的尺寸和地位是相干于输出图像，而不是相干于特色图，首先需要将其进行转换到候选框在特色图上具体地位，而后在对提取候选框进行尺寸的缩放。
给定一个特色图和一组提议，返回会合的特色示意。区域提议网络被用来预测对象性和回归盒的偏差（对锚点）。这些偏移量与 anchor 拆散起来生成候选框。这些倡导通常是输出图像的大小而不是特色层的大小。因此，这些倡导需要按比例缩小到特色图层，之所以这样做，以便上游的CNN层能够提取特色。
咱们在原图上有一个尺寸，也就是候选框中心点的坐标以及宽度，首先咱们投影在原图上坐标点除以下采样的倍数，也就是 32 倍下采样，如果坐标无奈整除则进行取整操作。
import numpy as npimport torchimport torch.nn as nnfloattype = torch.cuda.FloatTensorclass TorchROIPool(object): def __init__(self, output_size, scaling_factor): #输入特色图的尺寸 self.output_size = output_size #缩放比率 self.scaling_factor = scaling_factor def _roi_pool(self, features): """ 在给定的缩放提取特色图基础，返回固定大小的特色图 Args: features (np.Array): """ # 特色图的通道数、高 和 宽 num_channels, h, w = features.shape # 计算步长 w_stride = w/self.output_size h_stride = h/self.output_size # res = torch.zeros((num_channels, self.output_size, self.output_size)) res_idx = torch.zeros((num_channels, self.output_size, self.output_size)) for i in range(self.output_size): for j in range(self.output_size): # important to round the start and end, and then conver to int # w_start = int(np.floor(j*w_stride)) w_end = int(np.ceil((j+1)*w_stride)) h_start = int(np.floor(i*h_stride)) h_end = int(np.ceil((i+1)*h_stride)) # limiting start and end based on feature limits # w_start = min(max(w_start, 0), w) w_end = min(max(w_end, 0), w) h_start = min(max(h_start, 0), h) h_end = min(max(h_end, 0), h) patch = features[:, h_start: h_end, w_start: w_end] max_val, max_idx = torch.max(patch.reshape(num_channels, -1), dim=1) res[:, i, j] = max_val res_idx[:, i, j] = max_idx return res, res_idx def __call__(self, feature_layer, proposals): """Given feature layers and a list of proposals, it returns pooled respresentations of the proposals. Proposals are scaled by scaling factor before pooling. Args: feature_layer (np.Array): 特色层尺寸 proposals (list of np.Array): 列表中每一个元素 Each element of the list represents a bounding box as (w,y,w,h) Returns: np.Array: proposal 数量，通道数，输入特色图高度, self.output_size """ batch_size, num_channels, _, _ = feature_layer.shape # first scale proposals based on self.scaling factor scaled_proposals = torch.zeros_like(proposals) # the rounding by torch.ceil is important for ROI pool scaled_proposals[:, 0] = torch.ceil(proposals[:, 0] * self.scaling_factor) scaled_proposals[:, 1] = torch.ceil(proposals[:, 1] * self.scaling_factor) scaled_proposals[:, 2] = torch.ceil(proposals[:, 2] * self.scaling_factor) scaled_proposals[:, 3] = torch.ceil(proposals[:, 3] * self.scaling_factor) res = torch.zeros((len(proposals), num_channels, self.output_size, self.output_size)) res_idx = torch.zeros((len(proposals), num_channels, self.output_size, self.output_size)) # 遍历候选框 for idx in range(len(proposals)): # proposal = scaled_proposals[idx] # adding 1 to include the end indices from proposal extracted_feat = feature_layer[0, :, proposal[1].to(dtype=torch.int8):proposal[3].to(dtype=torch.int8)+1, proposal[0].to(dtype=torch.int8):proposal[2].to(dtype=torch.int8)+1] res[idx], res_idx[idx] = self._roi_pool(extracted_feat) return res