SSD: Single Shot MultiBox Detector, 是一个end to end 的目标检测识别模型。先小八卦下，它属于google派系，它的作者也是googlenet的作者。该模型旨在高精度的快速识别， 它不用额外计算bounding box而能达到相当的识别精度，而且速度有极大的提高，号称可以达到58的FPS 和 72.1%的mAP。
我们先来看下这个模型的全貌。它的最底几层是一个经典的VGG16的网络（也可以替换成ResNet），其中的卷积层conv4_3和全连接层fc7、 以及再往上的三个卷积层conv6、conv7、conv8，分别分支出mbox_conf, mbox_loc, priorbox三种节点（称X节点），然后通过对应的concat将来自不同层的X节点进行融合， 最后将concat结果输出一并进行分类决策。
更详细的，可以看下面的一次前向计算的代码输出。
[INFO 2016-08-30 21:58:39.619143 21429 net.cpp:540] Forwarding data
[INFO 2016-08-30 21:58:39.622481 21429 net.cpp:540] Forwarding data_data_0_split
[INFO 2016-08-30 21:58:39.622514 21429 net.cpp:540] Forwarding conv1_1
[INFO 2016-08-30 21:58:39.627096 21429 net.cpp:540] Forwarding relu1_1
[INFO 2016-08-30 21:58:39.627473 21429 net.cpp:540] Forwarding conv1_2
[INFO 2016-08-30 21:58:39.631721 21429 net.cpp:540] Forwarding relu1_2
[INFO 2016-08-30 21:58:39.631757 21429 net.cpp:540] Forwarding pool1
[INFO 2016-08-30 21:58:39.632096 21429 net.cpp:540] Forwarding conv2_1
[INFO 2016-08-30 21:58:39.634774 21429 net.cpp:540] Forwarding relu2_1
[INFO 2016-08-30 21:58:39.634809 21429 net.cpp:540] Forwarding conv2_2
[INFO 2016-08-30 21:58:39.639045 21429 net.cpp:540] Forwarding relu2_2
[INFO 2016-08-30 21:58:39.639080 21429 net.cpp:540] Forwarding pool2
[INFO 2016-08-30 21:58:39.639394 21429 net.cpp:540] Forwarding conv3_1
[INFO 2016-08-30 21:58:39.642501 21429 net.cpp:540] Forwarding relu3_1
[INFO 2016-08-30 21:58:39.642535 21429 net.cpp:540] Forwarding conv3_2
[INFO 2016-08-30 21:58:39.647202 21429 net.cpp:540] Forwarding relu3_2
[INFO 2016-08-30 21:58:39.647235 21429 net.cpp:540] Forwarding conv3_3
[INFO 2016-08-30 21:58:39.650738 21429 net.cpp:540] Forwarding relu3_3
[INFO 2016-08-30 21:58:39.650770 21429 net.cpp:540] Forwarding pool3
[INFO 2016-08-30 21:58:39.651074 21429 net.cpp:540] Forwarding conv4_1
[INFO 2016-08-30 21:58:39.655285 21429 net.cpp:540] Forwarding relu4_1
[INFO 2016-08-30 21:58:39.655323 21429 net.cpp:540] Forwarding conv4_2
[INFO 2016-08-30 21:58:39.660395 21429 net.cpp:540] Forwarding relu4_2
[INFO 2016-08-30 21:58:39.660429 21429 net.cpp:540] Forwarding conv4_3
[INFO 2016-08-30 21:58:39.665523 21429 net.cpp:540] Forwarding relu4_3
[INFO 2016-08-30 21:58:39.665555 21429 net.cpp:540] Forwarding conv4_3_relu4_3_0_split
[INFO 2016-08-30 21:58:39.665570 21429 net.cpp:540] Forwarding pool4
[INFO 2016-08-30 21:58:39.665881 21429 net.cpp:540] Forwarding conv5_1
[INFO 2016-08-30 21:58:39.668714 21429 net.cpp:540] Forwarding relu5_1
[INFO 2016-08-30 21:58:39.668748 21429 net.cpp:540] Forwarding conv5_2
[INFO 2016-08-30 21:58:39.671761 21429 net.cpp:540] Forwarding relu5_2
[INFO 2016-08-30 21:58:39.671807 21429 net.cpp:540] Forwarding conv5_3
[INFO 2016-08-30 21:58:39.675269 21429 net.cpp:540] Forwarding relu5_3
[INFO 2016-08-30 21:58:39.675302 21429 net.cpp:540] Forwarding pool5
[INFO 2016-08-30 21:58:39.675624 21429 net.cpp:540] Forwarding fc6
[INFO 2016-08-30 21:58:39.685935 21429 net.cpp:540] Forwarding relu6
[INFO 2016-08-30 21:58:39.685971 21429 net.cpp:540] Forwarding fc7
[INFO 2016-08-30 21:58:39.688531 21429 net.cpp:540] Forwarding relu7
[INFO 2016-08-30 21:58:39.688565 21429 net.cpp:540] Forwarding fc7_relu7_0_split
[INFO 2016-08-30 21:58:39.688580 21429 net.cpp:540] Forwarding conv6_1
[INFO 2016-08-30 21:58:39.691439 21429 net.cpp:540] Forwarding conv6_1_relu
[INFO 2016-08-30 21:58:39.691473 21429 net.cpp:540] Forwarding conv6_2
[INFO 2016-08-30 21:58:39.695135 21429 net.cpp:540] Forwarding conv6_2_relu
[INFO 2016-08-30 21:58:39.695169 21429 net.cpp:540] Forwarding conv6_2_conv6_2_relu_0_split
[INFO 2016-08-30 21:58:39.695183 21429 net.cpp:540] Forwarding conv7_1
[INFO 2016-08-30 21:58:39.698765 21429 net.cpp:540] Forwarding conv7_1_relu
[INFO 2016-08-30 21:58:39.698796 21429 net.cpp:540] Forwarding conv7_2
[INFO 2016-08-30 21:58:39.701938 21429 net.cpp:540] Forwarding conv7_2_relu
[INFO 2016-08-30 21:58:39.702193 21429 net.cpp:540] Forwarding conv7_2_conv7_2_relu_0_split
[INFO 2016-08-30 21:58:39.702220 21429 net.cpp:540] Forwarding conv8_1
[INFO 2016-08-30 21:58:39.704677 21429 net.cpp:540] Forwarding conv8_1_relu
[INFO 2016-08-30 21:58:39.704716 21429 net.cpp:540] Forwarding conv8_2
[INFO 2016-08-30 21:58:39.707798 21429 net.cpp:540] Forwarding conv8_2_relu
[INFO 2016-08-30 21:58:39.707839 21429 net.cpp:540] Forwarding conv8_2_conv8_2_relu_0_split
[INFO 2016-08-30 21:58:39.707859 21429 net.cpp:540] Forwarding pool6
[INFO 2016-08-30 21:58:39.707926 21429 net.cpp:540] Forwarding pool6_pool6_0_split
[INFO 2016-08-30 21:58:39.707947 21429 net.cpp:540] Forwarding conv4_3_norm
[INFO 2016-08-30 21:58:39.711788 21429 net.cpp:540] Forwarding conv4_3_norm_conv4_3_norm_0_split
[INFO 2016-08-30 21:58:39.711818 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_loc
[INFO 2016-08-30 21:58:39.714972 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_loc_perm
[INFO 2016-08-30 21:58:39.717313 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_loc_flat
[INFO 2016-08-30 21:58:39.717339 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_conf
[INFO 2016-08-30 21:58:39.724395 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_conf_perm
[INFO 2016-08-30 21:58:39.731096 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_conf_flat
[INFO 2016-08-30 21:58:39.731127 21429 net.cpp:540] Forwarding conv4_3_norm_mbox_priorbox
[INFO 2016-08-30 21:58:39.731290 21429 net.cpp:540] Forwarding fc7_mbox_loc
[INFO 2016-08-30 21:58:39.733963 21429 net.cpp:540] Forwarding fc7_mbox_loc_perm
[INFO 2016-08-30 21:58:39.737503 21429 net.cpp:540] Forwarding fc7_mbox_loc_flat
[INFO 2016-08-30 21:58:39.737527 21429 net.cpp:540] Forwarding fc7_mbox_conf
[INFO 2016-08-30 21:58:39.746902 21429 net.cpp:540] Forwarding fc7_mbox_conf_perm
[INFO 2016-08-30 21:58:39.750918 21429 net.cpp:540] Forwarding fc7_mbox_conf_flat
[INFO 2016-08-30 21:58:39.750946 21429 net.cpp:540] Forwarding fc7_mbox_priorbox
[INFO 2016-08-30 21:58:39.751056 21429 net.cpp:540] Forwarding conv6_2_mbox_loc
[INFO 2016-08-30 21:58:39.753976 21429 net.cpp:540] Forwarding conv6_2_mbox_loc_perm
[INFO 2016-08-30 21:58:39.756206 21429 net.cpp:540] Forwarding conv6_2_mbox_loc_flat
[INFO 2016-08-30 21:58:39.756239 21429 net.cpp:540] Forwarding conv6_2_mbox_conf
[INFO 2016-08-30 21:58:39.763130 21429 net.cpp:540] Forwarding conv6_2_mbox_conf_perm
[INFO 2016-08-30 21:58:39.764664 21429 net.cpp:540] Forwarding conv6_2_mbox_conf_flat
[INFO 2016-08-30 21:58:39.764689 21429 net.cpp:540] Forwarding conv6_2_mbox_priorbox
[INFO 2016-08-30 21:58:39.764760 21429 net.cpp:540] Forwarding conv7_2_mbox_loc
[INFO 2016-08-30 21:58:39.768630 21429 net.cpp:540] Forwarding conv7_2_mbox_loc_perm
[INFO 2016-08-30 21:58:39.772903 21429 net.cpp:540] Forwarding conv7_2_mbox_loc_flat
[INFO 2016-08-30 21:58:39.772927 21429 net.cpp:540] Forwarding conv7_2_mbox_conf
[INFO 2016-08-30 21:58:39.777669 21429 net.cpp:540] Forwarding conv7_2_mbox_conf_perm
[INFO 2016-08-30 21:58:39.781180 21429 net.cpp:540] Forwarding conv7_2_mbox_conf_flat
[INFO 2016-08-30 21:58:39.781205 21429 net.cpp:540] Forwarding conv7_2_mbox_priorbox
[INFO 2016-08-30 21:58:39.781263 21429 net.cpp:540] Forwarding conv8_2_mbox_loc
[INFO 2016-08-30 21:58:39.783634 21429 net.cpp:540] Forwarding conv8_2_mbox_loc_perm
[INFO 2016-08-30 21:58:39.788920 21429 net.cpp:540] Forwarding conv8_2_mbox_loc_flat
[INFO 2016-08-30 21:58:39.788944 21429 net.cpp:540] Forwarding conv8_2_mbox_conf
[INFO 2016-08-30 21:58:39.793294 21429 net.cpp:540] Forwarding conv8_2_mbox_conf_perm
[INFO 2016-08-30 21:58:39.797371 21429 net.cpp:540] Forwarding conv8_2_mbox_conf_flat
[INFO 2016-08-30 21:58:39.797397 21429 net.cpp:540] Forwarding conv8_2_mbox_priorbox
[INFO 2016-08-30 21:58:39.797449 21429 net.cpp:540] Forwarding pool6_mbox_loc
[INFO 2016-08-30 21:58:39.800542 21429 net.cpp:540] Forwarding pool6_mbox_loc_perm
[INFO 2016-08-30 21:58:39.804468 21429 net.cpp:540] Forwarding pool6_mbox_loc_flat
[INFO 2016-08-30 21:58:39.804493 21429 net.cpp:540] Forwarding pool6_mbox_conf
[INFO 2016-08-30 21:58:39.808717 21429 net.cpp:540] Forwarding pool6_mbox_conf_perm
[INFO 2016-08-30 21:58:39.812292 21429 net.cpp:540] Forwarding pool6_mbox_conf_flat
[INFO 2016-08-30 21:58:39.812317 21429 net.cpp:540] Forwarding pool6_mbox_priorbox
[INFO 2016-08-30 21:58:39.812382 21429 net.cpp:540] Forwarding mbox_loc
[INFO 2016-08-30 21:58:39.812604 21429 net.cpp:540] Forwarding mbox_conf
[INFO 2016-08-30 21:58:39.812834 21429 net.cpp:540] Forwarding mbox_priorbox
[INFO 2016-08-30 21:58:39.819844 21429 net.cpp:540] Forwarding mbox_conf_reshape
[INFO 2016-08-30 21:58:39.819871 21429 net.cpp:540] Forwarding mbox_conf_softmax
[INFO 2016-08-30 21:58:39.820596 21429 net.cpp:540] Forwarding mbox_conf_flatten
[INFO 2016-08-30 21:58:39.820647 21429 net.cpp:540] Forwarding detection_out
[INFO 2016-08-30 21:58:39.832866 21429 net.cpp:540] Forwarding detection_eval
SSD 网络使用了大量的小的卷积核(1x1, 3x3)，不仅用于分类而且用于bounding box的位置回归，通过一些滤波实现不同长宽比的目标检测，并进而用于在后续的不同feature map下的多尺度的检测。
SSD设计了一个bounding box集合， 包含4个：长的 、宽的、大正方、小正方，分布在不同尺寸(4x4,8x8)的feature map的每个位置, 即用卷积的方式覆盖了一个m*n*p的feature map的m*n个位置。在训练时，对这些box与groundtruth box进行匹配，即对每个box计算和groundtruth的位移和分类概率，获得了4个位移值和c个分类概率值，并根据groundtruth的类别获得TP和FP，最终通过计算加权位置损失和分类置信度损失获得模型整体损失， 并通过非极大值抑制来获得最终的检测结果。
不同形状的box，及其在多分辨率feature map下的应用，实现了box的参数空间的离散化从而提高计算效率。groudtruth的信息, 包括类别和位置都需要明确地附给那些网络输出，使得损失函数和反向传播是end to end的。在训练时，需要将groundtruh和box对应起来，只要和groundtruth的jaccard覆盖率大于0.5，就能和该groundtruth对应上，每个groundtruh必须至少有一个box与其对应。另外，当候选box数量很多时，往往FP也很多，导致TP和FP的数量不平衡。于是，根据分类置信度对候选box进行排序，取top个候选使得FP和TP的比例在3：1。
关于如何识别多尺度目标。我们知道，低层的feature map对图像的细节表达出色从而可以提高语义分割质量，高层的feature map可以平滑分割结果。于是，综合底层和高层的feature map进行检测。不同层的feature map有不同的感受野尺寸，这个很关键，可以参考Zhou, B., Khosla, A., Lapedriza, A., Oliva, A., Torralba, A.: Object detectors emerge in deep scene cnns. In: ICLR. (2015)。然而不需要给某一层feature map构建不同尺寸的box， 而是某层的feature map只学习检测某个尺度的对象，所以某一层的feature map只有一个尺度的box。举个例子，在8x8的feature map中的box是无法检测到尺寸较大的狗的（如下图)。从低层到高层，box的缩放比均匀地分布在0.2~0.95之间。进一步为了解决长宽比的问题，每层的box又生成了{1,1+,2,3,1/2,1/3} 6个不同长宽比的扩展box。
SSD从某种意义上是结合了RPN和YOLO的思想。即
1）RPN的anchor思想，在feature map上运用256 个 3x3 的滤波器，事实上是在feature map的每个位置，从256个维度来表达9种anchor box特征。滤波器滑动窗的位置提供了相对原图的定位信息。回归框提供了相对该滑动窗的更精细的定位信息。RPN使得计算降低256倍（即从基于原图的操作转为基于特征图的操作）。
2）YOLO的回归思想，即用特征回归出目标的位置和了类别, 而没有使用ROI pooling进行分类和提取。