基于Pytorch SSD模型分析

本文参考github上SSD实现,对模型进行分析,主要分析模型组成及输入输出大小.SSD网络结构如下图:

每输入的图像有8732个框输出;

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.autograd import Variable
#from layers import *
from data import voc, coco
import os

base = {
 '300': [64, 64, 'M', 128, 128, 'M', 256, 256, 256, 'C', 512, 512, 512, 'M',
   512, 512, 512],
 '512': [],
}
extras = {
 '300': [256, 'S', 512, 128, 'S', 256, 128, 256, 128, 256],
 '512': [],
}
mbox = {
 '300': [4, 6, 6, 6, 4, 4], # number of boxes per feature map location
 '512': [],
}

VGG基础网络结构:

def vgg(cfg, i, batch_norm=False):
 layers = []
 in_channels = i
 for v in cfg:
  if v == 'M':
   layers += [nn.MaxPool2d(kernel_size=2, stride=2)]
  elif v == 'C':
   layers += [nn.MaxPool2d(kernel_size=2, stride=2, ceil_mode=True)]
  else:
   conv2d = nn.Conv2d(in_channels, v, kernel_size=3, padding=1)
   if batch_norm:
    layers += [conv2d, nn.BatchNorm2d(v), nn.ReLU(inplace=True)]
   else:
    layers += [conv2d, nn.ReLU(inplace=True)]
   in_channels = v
 pool5 = nn.MaxPool2d(kernel_size=3, stride=1, padding=1)
 conv6 = nn.Conv2d(512, 1024, kernel_size=3, padding=6, dilation=6)
 conv7 = nn.Conv2d(1024, 1024, kernel_size=1)
 layers += [pool5, conv6,
    nn.ReLU(inplace=True), conv7, nn.ReLU(inplace=True)]
 return layers

size=300
vgg=vgg(base[str(size)], 3)
print(vgg)

输出为:

Conv2d(3, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
Conv2d(64, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=True)
Conv2d(256, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
ReLU(inplace)
MaxPool2d(kernel_size=3, stride=1, padding=1, dilation=1, ceil_mode=False)
Conv2d(512, 1024, kernel_size=(3, 3), stride=(1, 1), padding=(6, 6), dilation=(6, 6))
ReLU(inplace)
Conv2d(1024, 1024, kernel_size=(1, 1), stride=(1, 1))
ReLU(inplace)

SSD中添加的网络

add_extras函数构建基本的卷积层

def add_extras(cfg, i, batch_norm=False):
 # Extra layers added to VGG for feature scaling
 layers = []
 in_channels = i
 flag = False
 for k, v in enumerate(cfg):
  if in_channels != 'S':
   if v == 'S':
    layers += [nn.Conv2d(in_channels, cfg[k + 1],
       kernel_size=(1, 3)[flag], stride=2, padding=1)]
   else:
    layers += [nn.Conv2d(in_channels, v, kernel_size=(1, 3)[flag])]
   flag = not flag
  in_channels = v
 return layers

extra_layers=add_extras(extras[str(size)], 1024)
for layer in extra_layers:
 print(layer)

输出为:

Conv2d(1024, 256, kernel_size=(1, 1), stride=(1, 1))
Conv2d(256, 512, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1))
Conv2d(512, 128, kernel_size=(1, 1), stride=(1, 1))
Conv2d(128, 256, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1))
Conv2d(256, 128, kernel_size=(1, 1), stride=(1, 1))
Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1))
Conv2d(256, 128, kernel_size=(1, 1), stride=(1, 1))
Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1))

multibox函数得到每个特征图的默认box的位置计算网络和分类得分网络

def multibox(vgg, extra_layers, cfg, num_classes):
 loc_layers = []
 conf_layers = []
 vgg_source = [21, -2]
 for k, v in enumerate(vgg_source):
  loc_layers += [nn.Conv2d(vgg[v].out_channels,
         cfg[k] * 4, kernel_size=3, padding=1)]
  conf_layers += [nn.Conv2d(vgg[v].out_channels,
      cfg[k] * num_classes, kernel_size=3, padding=1)]
 for k, v in enumerate(extra_layers[1::2], 2):
  loc_layers += [nn.Conv2d(v.out_channels, cfg[k]
         * 4, kernel_size=3, padding=1)]
  conf_layers += [nn.Conv2d(v.out_channels, cfg[k]
         * num_classes, kernel_size=3, padding=1)]
 return vgg, extra_layers, (loc_layers, conf_layers)

base_, extras_, head_ = multibox(vgg(base[str(size)], 3), ## 产生vgg19基本模型
          add_extras(extras[str(size)], 1024), 
          mbox[str(size)], num_classes)
#mbox[str(size)]为:[4, 6, 6, 6, 4, 4]

得到的输出为:

base_为上述描述的vgg网络,extras_为extra_layers网络,head_为:

([Conv2d(512, 16, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(1024, 24, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(512, 24, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 24, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 16, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 16, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))],
 [Conv2d(512, 84, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(1024, 126, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(512, 126, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 126, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 84, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
 Conv2d(256, 84, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))])

SSD网络及forward函数为:

class SSD(nn.Module):
 """Single Shot Multibox Architecture
 The network is composed of a base VGG network followed by the
 added multibox conv layers. Each multibox layer branches into
  1) conv2d for class conf scores
  2) conv2d for localization predictions
  3) associated priorbox layer to produce default bounding
   boxes specific to the layer's feature map size.
 See: https://arxiv.org/pdf/1512.02325.pdf for more details.

 Args:
  phase: (string) Can be "test" or "train"
  size: input image size
  base: VGG16 layers for input, size of either 300 or 500
  extras: extra layers that feed to multibox loc and conf layers
  head: "multibox head" consists of loc and conf conv layers
 """

 def __init__(self, phase, size, base, extras, head, num_classes):
  super(SSD, self).__init__()
  self.phase = phase
  self.num_classes = num_classes 
  self.cfg = (coco, voc)[num_classes == 21]
  self.priorbox = PriorBox(self.cfg)
  self.priors = Variable(self.priorbox.forward(), volatile=True)
  self.size = size

  # SSD network
  self.vgg = nn.ModuleList(base)
  # Layer learns to scale the l2 normalized features from conv4_3
  self.L2Norm = L2Norm(512, 20)
  self.extras = nn.ModuleList(extras)

  self.loc = nn.ModuleList(head[0])
  self.conf = nn.ModuleList(head[1])

  if phase == 'test':
   self.softmax = nn.Softmax(dim=-1)
   self.detect = Detect(num_classes, 0, 200, 0.01, 0.45)

 def forward(self, x):
  """Applies network layers and ops on input image(s) x.

  Args:
   x: input image or batch of images. Shape: [batch,3,300,300].

  Return:
   Depending on phase:
   test:
    Variable(tensor) of output class label predictions,
    confidence score, and corresponding location predictions for
    each object detected. Shape: [batch,topk,7]

   train:
    list of concat outputs from:
     1: confidence layers, Shape: [batch*num_priors,num_classes]
     2: localization layers, Shape: [batch,num_priors*4]
     3: priorbox layers, Shape: [2,num_priors*4]
  """
  sources = list()
  loc = list()
  conf = list()

  # apply vgg up to conv4_3 relu
  for k in range(23):
   x = self.vgg[k](x) ##得到的x尺度为[1,512,38,38]

  s = self.L2Norm(x)
  sources.append(s)

  # apply vgg up to fc7
  for k in range(23, len(self.vgg)):
   x = self.vgg[k](x) ##得到的x尺寸为[1,1024,19,19]
  sources.append(x)

  # apply extra layers and cache source layer outputs
  for k, v in enumerate(self.extras):
   x = F.relu(v(x), inplace=True)
   if k % 2 == 1:
    sources.append(x)
  '''
  上述得到的x输出分别为:
  torch.Size([1, 512, 10, 10])
  torch.Size([1, 256, 5, 5])
  torch.Size([1, 256, 3, 3])
  torch.Size([1, 256, 1, 1])
  '''

  # apply multibox head to source layers
  for (x, l, c) in zip(sources, self.loc, self.conf):
   loc.append(l(x).permute(0, 2, 3, 1).contiguous())
   conf.append(c(x).permute(0, 2, 3, 1).contiguous())

  loc = torch.cat([o.view(o.size(0), -1) for o in loc], 1)
  conf = torch.cat([o.view(o.size(0), -1) for o in conf], 1)
  if self.phase == "test":
   output = self.detect(
    loc.view(loc.size(0), -1, 4),     # loc preds
    self.softmax(conf.view(conf.size(0), -1,
        self.num_classes)),    # conf preds
    self.priors.type(type(x.data))     # default boxes
   )
  else:
   output = (
    loc.view(loc.size(0), -1, 4), #[1,8732,4]
    conf.view(conf.size(0), -1, self.num_classes),#[1,8732,21]
    self.priors
   )
  return output

上述代码中sources中保存的数据输出如下,即用于边框提取的特征图:

torch.Size([1, 512, 38, 38])
torch.Size([1, 1024, 19, 19])
torch.Size([1, 512, 10, 10])
torch.Size([1, 256, 5, 5])
torch.Size([1, 256, 3, 3])
torch.Size([1, 256, 1, 1])

模型输入为

x=Variable(torch.randn(1,3,300,300))

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