总结下来，用generate_anchors产生多种坐标变换，这种坐标变换由scale和ratio来，相当于提前计算好。anchor_target_layer先计算的是从feature map映射到原图的中点坐标，然后根据多种坐标变换生成不同的框。
anchor_target_layer层是产生在rpn训练阶段产生anchors的层
源代码：
1 # -------------------------------------------------------- 2 # Faster R-CNN 3 # Copyright (c) 2015 Microsoft 4 # Licensed under The MIT License [see LICENSE for details] 5 # Written by Ross Girshick and Sean Bell 6 # -------------------------------------------------------- 7 8 import os 9 import caffe 10 import yaml 11 from fast_rcnn.config import cfg 12 import numpy as np 13 import numpy.random as npr 14 from generate_anchors import generate_anchors 15 from utils.cython_bbox import bbox_overlaps 16 from fast_rcnn.bbox_transform import bbox_transform 17 18 DEBUG = False 19 20 class AnchorTargetLayer(caffe.Layer): 21 """ 22 Assign anchors to ground-truth targets. Produces anchor classification 23 labels and bounding-box regression targets. 24 """ 25 26 def setup(self, bottom, top): 27 layer_params = yaml.load(self.param_str_) 28 anchor_scales = layer_params.get('scales', (8, 16, 32)) 29 self._anchors = generate_anchors(scales=np.array(anchor_scales)) #generate_anchors函数根据ratio和scale产生坐标变换，这些坐标变换是让中心点产生不同的anchor 30 self._num_anchors = self._anchors.shape[0] 31 self._feat_stride = layer_params['feat_stride'] #feat_stride和roi_pooling中的spatial_scale是对应的，一个是16，一个是16分之 一，一个是把中心点坐标从feature map映射到原图，一个是把原图roi框坐标映射到feature map 32 33 if DEBUG: 34 print 'anchors:' 35 print self._anchors 36 print 'anchor shapes:' 37 print np.hstack(( 38 self._anchors[:, 2::4] - self._anchors[:, 0::4], 39 self._anchors[:, 3::4] - self._anchors[:, 1::4], 40 )) 41 self._counts = cfg.EPS 42 self._sums = np.zeros((1, 4)) 43 self._squared_sums = np.zeros((1, 4)) 44 self._fg_sum = 0 45 self._bg_sum = 0 46 self._count = 0 47 48 # allow boxes to sit over the edge by a small amount 49 self._allowed_border = layer_params.get('allowed_border', 0) 50 51 height, width = bottom[0].data.shape[-2:] 52 if DEBUG: 53 print 'AnchorTargetLayer: height', height, 'width', width 54 55 A = self._num_anchors 56 # labels 57 top[0].reshape(1, 1, A * height, width) 58 # bbox_targets 59 top[1].reshape(1, A * 4, height, width) #reshape输出的形状 60 # bbox_inside_weights 61 top[2].reshape(1, A * 4, height, width) 62 # bbox_outside_weights 63 top[3].reshape(1, A * 4, height, width) 64 65 def forward(self, bottom, top): 66 # Algorithm: 67 # 68 # for each (H, W) location i 69 # generate 9 anchor boxes centered on cell i 70 # apply predicted bbox deltas at cell i to each of the 9 anchors 71 # filter out-of-image anchors 72 # measure GT overlap 73 74 assert bottom[0].data.shape[0] == 1, \ 75 'Only single item batches are supported' 76 77 # map of shape (..., H, W) 78 height, width = bottom[0].data.shape[-2:] #得到特征提取层最后一层feature map的高度和宽度，具体原因讲解看代码框下面的分析 79 # GT boxes (x1, y1, x2, y2, label) 80 gt_boxes = bottom[1].data 81 # im_info 82 im_info = bottom[2].data[0, :] 83 84 if DEBUG: 85 print '' 86 print 'im_size: ({}, {})'.format(im_info[0], im_info[1]) 87 print 'scale: {}'.format(im_info[2]) 88 print 'height, width: ({}, {})'.format(height, width) 89 print 'rpn: gt_boxes.shape', gt_boxes.shape 90 print 'rpn: gt_boxes', gt_boxes 91 92 # 1. Generate proposals from bbox deltas and shifted anchors 93 shift_x = np.arange(0, width) * self._feat_stride 94 shift_y = np.arange(0, height) * self._feat_stride 95 shift_x, shift_y = np.meshgrid(shift_x, shift_y) 96 shifts = np.vstack((shift_x.ravel(), shift_y.ravel(), 97 shift_x.ravel(), shift_y.ravel())).transpose() 98 # add A anchors (1, A, 4) to 99 # cell K shifts (K, 1, 4) to get100 # shift anchors (K, A, 4)101 # reshape to (K*A, 4) shifted anchors102 A = self._num_anchors103 K = shifts.shape[0]104 all_anchors = (self._anchors.reshape((1, A, 4)) +105 shifts.reshape((1, K, 4)).transpose((1, 0, 2)))106 all_anchors = all_anchors.reshape((K * A, 4))107 total_anchors = int(K * A)108 109 # only keep anchors inside the image110 inds_inside = np.where(111 (all_anchors[:, 0] >= -self._allowed_border) &112 (all_anchors[:, 1] >= -self._allowed_border) &113 (all_anchors[:, 2] < im_info[1] + self._allowed_border) & # width114 (all_anchors[:, 3] < im_info[0] + self._allowed_border) # height115 )[0]116 117 if DEBUG:118 print 'total_anchors', total_anchors119 print 'inds_inside', len(inds_inside)120 121 # keep only inside anchors122 anchors = all_anchors[inds_inside, :]123 if DEBUG:124 print 'anchors.shape', anchors.shape125 126 # label: 1 is positive, 0 is negative, -1 is dont care127 labels = np.empty((len(inds_inside), ), dtype=np.float32)128 labels.fill(-1)129 130 # overlaps between the anchors and the gt boxes131 # overlaps (ex, gt)132 overlaps = bbox_overlaps(133 np.ascontiguousarray(anchors, dtype=np.float),134 np.ascontiguousarray(gt_boxes, dtype=np.float))135 argmax_overlaps = overlaps.argmax(axis=1) #argmax_overlaps是每个anchor对应最大overlap的gt_boxes的下标136 max_overlaps = overlaps[np.arange(len(inds_inside)), argmax_overlaps]137 gt_argmax_overlaps = overlaps.argmax(axis=0) #gt_argmax_overlaps是每个gt_boxes对应最大overlap的anchor的下标138 gt_max_overlaps = overlaps[gt_argmax_overlaps,139 np.arange(overlaps.shape[1])]140 gt_argmax_overlaps = np.where(overlaps == gt_max_overlaps)[0]141 142 if not cfg.TRAIN.RPN_CLOBBER_POSITIVES:143 # assign bg labels first so that positive labels can clobber them144 labels[max_overlaps < cfg.TRAIN.RPN_NEGATIVE_OVERLAP] = 0145 146 # fg label: for each gt, anchor with highest overlap147 labels[gt_argmax_overlaps] = 1148 149 # fg label: above threshold IOU150 labels[max_overlaps >= cfg.TRAIN.RPN_POSITIVE_OVERLAP] = 1151 152 if cfg.TRAIN.RPN_CLOBBER_POSITIVES:153 # assign bg labels last so that negative labels can clobber positives154 labels[max_overlaps < cfg.TRAIN.RPN_NEGATIVE_OVERLAP] = 0155 156 # subsample positive labels if we have too many157 num_fg = int(cfg.TRAIN.RPN_FG_FRACTION * cfg.TRAIN.RPN_BATCHSIZE)158 fg_inds = np.where(labels == 1)[0]159 if len(fg_inds) > num_fg:160 disable_inds = npr.choice(161 fg_inds, size=(len(fg_inds) - num_fg), replace=False)162 labels[disable_inds] = -1163 164 # subsample negative labels if we have too many165 num_bg = cfg.TRAIN.RPN_BATCHSIZE - np.sum(labels == 1)166 bg_inds = np.where(labels == 0)[0]167 if len(bg_inds) > num_bg:168 disable_inds = npr.choice(169 bg_inds, size=(len(bg_inds) - num_bg), replace=False)170 labels[disable_inds] = -1171 #print "was %s inds, disabling %s, now %s inds" % (172 #len(bg_inds), len(disable_inds), np.sum(labels == 0))173 174 bbox_targets = np.zeros((len(inds_inside), 4), dtype=np.float32)175 bbox_targets = _compute_targets(anchors, gt_boxes[argmax_overlaps, :])176 177 bbox_inside_weights = np.zeros((len(inds_inside), 4), dtype=np.float32)178 bbox_inside_weights[labels == 1, :] = np.array(cfg.TRAIN.RPN_BBOX_INSIDE_WEIGHTS)179 180 bbox_outside_weights = np.zeros((len(inds_inside), 4), dtype=np.float32)181 if cfg.TRAIN.RPN_POSITIVE_WEIGHT < 0:182 # uniform weighting of examples (given non-uniform sampling)183 num_examples = np.sum(labels >= 0)184 positive_weights = np.ones((1, 4)) * 1.0 / num_examples185 negative_weights = np.ones((1, 4)) * 1.0 / num_examples186 else:187 assert ((cfg.TRAIN.RPN_POSITIVE_WEIGHT > 0) &188 (cfg.TRAIN.RPN_POSITIVE_WEIGHT < 1))189 positive_weights = (cfg.TRAIN.RPN_POSITIVE_WEIGHT /190 np.sum(labels == 1))191 negative_weights = ((1.0 - cfg.TRAIN.RPN_POSITIVE_WEIGHT) /192 np.sum(labels == 0))193 bbox_outside_weights[labels == 1, :] = positive_weights194 bbox_outside_weights[labels == 0, :] = negative_weights195 196 if DEBUG:197 self._sums += bbox_targets[labels == 1, :].sum(axis=0)198 self._squared_sums += (bbox_targets[labels == 1, :] ** 2).sum(axis=0)199 self._counts += np.sum(labels == 1)200 means = self._sums / self._counts201 stds = np.sqrt(self._squared_sums / self._counts - means ** 2)202 print 'means:'203 print means204 print 'stdevs:'205 print stds206 207 # map up to original set of anchors208 labels = _unmap(labels, total_anchors, inds_inside, fill=-1)209 bbox_targets = _unmap(bbox_targets, total_anchors, inds_inside, fill=0)210 bbox_inside_weights = _unmap(bbox_inside_weights, total_anchors, inds_inside, fill=0)211 bbox_outside_weights = _unmap(bbox_outside_weights, total_anchors, inds_inside, fill=0)212 213 if DEBUG:214 print 'rpn: max max_overlap', np.max(max_overlaps)215 print 'rpn: num_positive', np.sum(labels == 1)216 print 'rpn: num_negative', np.sum(labels == 0)217 self._fg_sum += np.sum(labels == 1)218 self._bg_sum += np.sum(labels == 0)219 self._count += 1220 print 'rpn: num_positive avg', self._fg_sum / self._count221 print 'rpn: num_negative avg', self._bg_sum / self._count222 223 # labels224 labels = labels.reshape((1, height, width, A)).transpose(0, 3, 1, 2)225 labels = labels.reshape((1, 1, A * height, width))226 top[0].reshape(*labels.shape)227 top[0].data[...] = labels228 229 # bbox_targets230 bbox_targets = bbox_targets \231 .reshape((1, height, width, A * 4)).transpose(0, 3, 1, 2)232 top[1].reshape(*bbox_targets.shape)233 top[1].data[...] = bbox_targets234 235 # bbox_inside_weights236 bbox_inside_weights = bbox_inside_weights \237 .reshape((1, height, width, A * 4)).transpose(0, 3, 1, 2)238 assert bbox_inside_weights.shape[2] == height239 assert bbox_inside_weights.shape[3] == width240 top[2].reshape(*bbox_inside_weights.shape)241 top[2].data[...] = bbox_inside_weights242 243 # bbox_outside_weights244 bbox_outside_weights = bbox_outside_weights \245 .reshape((1, height, width, A * 4)).transpose(0, 3, 1, 2)246 assert bbox_outside_weights.shape[2] == height247 assert bbox_outside_weights.shape[3] == width248 top[3].reshape(*bbox_outside_weights.shape)249 top[3].data[...] = bbox_outside_weights250 251 def backward(self, top, propagate_down, bottom):252 """This layer does not propagate gradients."""253 pass254 255 def reshape(self, bottom, top):256 """Reshaping happens during the call to forward."""257 pass258 259 260 def _unmap(data, count, inds, fill=0):261 """ Unmap a subset of item (data) back to the original set of items (of262 size count) """263 if len(data.shape) == 1:264 ret = np.empty((count, ), dtype=np.float32)265 ret.fill(fill)266 ret[inds] = data267 else:268 ret = np.empty((count, ) + data.shape[1:], dtype=np.float32)269 ret.fill(fill)270 ret[inds, :] = data271 return ret272 273 274 def _compute_targets(ex_rois, gt_rois):275 """Compute bounding-box regression targets for an image."""276 277 assert ex_rois.shape[0] == gt_rois.shape[0]278 assert ex_rois.shape[1] == 4279 assert gt_rois.shape[1] == 5280 281 return bbox_transform(ex_rois, gt_rois[:, :4]).astype(np.float32, copy=False) self._anchors：
我采用的是模型默认的3x3的anchor设置，np array的shape是(9,4)。第一个坐标是中心点的x坐标需要变化的值，生成的是框的最小x；第二个坐标是中心点的y坐标需要变化的值，生成的是框的最小y，这两个组成了框的左上坐标。第三个坐标是中心点的x坐标需要变化的值，生成的是框的最大x；第四个坐标是中心点的y坐标需要变化的值，生成的是框的最大y，这两个组成了框的右下坐标。
shift_x：
shift_x的长度是61，实际上这就是最后一层feature map的宽度大小。从0到60依次取整数，然后乘以16构成了shift_x。0到60是feature map上的每一个坐标点，也是anchor的中心点，乘以16之后就映射到了原图的坐标，这些就成了anchor在原图的中心点。
shift_y：
shift_y的长度是39，是最后一层feature map的长度大小，其他和shift_x类似。
shift_x, shift_y = np.meshgrid(shift_x, shift_y)，以下是这段代码生成的shift_x和shift_y：
shift_x和shift_y都变成了39x91的array，不同的是shift_x是按照行重复了39行，shift_y是按照列重复了61列
shifts = np.vstack((shift_x.ravel(), shift_y.ravel(), shift_x.ravel(), shift_y.ravel())).transpose()，以下是这段代码生成的shifts： shifts变成了2379x4的形状，shift_x.ravel()是把之前的61x39的shift_x reshape成2379x1的形状然后做第一列和第三列，shift_y.ravel()是把之前的61x39的shift_y reshape成2379x1的形状然后做 第二列和第四列
之前一直没搞懂为什么要弄两个shift_x。原因是，你要进行anchor的坐标变换是基于中心点进行加减，这一步生成的就是2379个anchor的中心点坐标。中心点坐标是二维的，只有x和y，但是因为之后需要进行坐标变换，即从anchor坐标中心点生成anchor框，anchor框是左上右下4个点，所以变成了4维。第一列生成的是框的最小x，第三列生成的是框的最大x，这两个都需要在中心点 的x坐标下进行加减变化。同理，第二列和第四列是在中心点的y坐标上进行操作的。
self._anchors.reshape((1, A, 4))进行的变化如下图，实际上增加了一维：
all_anchors = (self._anchors.reshape((1, A, 4)) + shifts.reshape((1, K, 4)).transpose((1, 0, 2)))生成的all_anchors如下图：
self._anchors的shape是（1,9,4），shifts经过变换后变成（2379，1,4），得到的all_anchors的shape是（2379,9,4），相当于把2379个中点坐标分别和9个anchor变换坐标相加（我在numpy里面写了一个+的计算总结，类似的）
比如第一维的第一个就是（0,0，0,0）和9个anchor坐标变换分别相加：
all_anchors = all_anchors.reshape((K * A, 4))就会生成2379*9个roi框坐标
layer { name: 'rpn-data' type: 'Python' bottom: 'rpn_cls_score' bottom: 'gt_boxes' bottom: 'im_info' bottom: 'data' top: 'rpn_labels' top: 'rpn_bbox_targets' top: 'rpn_bbox_inside_weights' top: 'rpn_bbox_outside_weights' python_param { module: 'rpn.anchor_target_layer' layer: 'AnchorTargetLayer' param_str: "'feat_stride': 16" }} 这是rpn-data层的prototxt，可以看到输入4个，输出4个
height, width = bottom[0].data.shape[-2:]
这一段代码得到的是特征提取层最后一层的高度和宽度。为什么呢？bottom[0]是rpn_cls_score，这是经过rpn3x3卷积和1x1卷积得到的某一类的框为前景、背景的预测概率值，可以发现这一个feature map的shape是(2*k，height，width)。这里的height，width实际上和特征层提取层最后一层的height，width一样大，因为这个3x3卷积stride和pad为1,1x1卷积本身不改变feature map的尺寸。论文中是在特征提取层最后一层进行rpn的滑动，在实际代码中，用rpn_cls_score的shape替代了特征提取层最后一层卷积。所以rpn_cls_score并不是要给rpn-data层输入概率值，而只是传rpn滑动所需的shape。
.data表示提取具体的data，.shape就是这个具体data的形状,[-2:]就是提取shape的倒数第二位到最后一位。
gt_boxes是输入的标准框，im_info包含了图片的尺寸（注意不是feature map尺寸，而是原图），data是这个图片本身的所有像素组成的array。
if DEBUG: 85 print '' 86 print 'im_size: ({}, {})'.format(im_info[0], im_info[1]) 87 print 'scale: {}'.format(im_info[2]) 88 print 'height, width: ({}, {})'.format(height, width) 89 print 'rpn: gt_boxes.shape', gt_boxes.shape 90 print 'rpn: gt_boxes', gt_boxes 从这个debug部分可以轻松看出这些信息。