- 文章转为微信公众号「机器学习炼丹术」
- 作者:炼丹兄(已授权)
- 作者联系方式:微信cyx645016617(欢迎交流)
- 论文名字:“RAFT: Recurrent All-Pairs Field Transforms for Optical Flow”
- 论文代码已经公开: github.com/princeton-v…
「前言」:本文结合官方提供的代码,对模型结构做一个解说。
模型结构
- 包含Feature Encoder,Context Encoder;
- 之后就是循环结构了
class RAFT(nn.Module):
def __init__(self, args):
super(RAFT, self).__init__()
self.args = args
if args.small:
self.hidden_dim = hdim = 96
self.context_dim = cdim = 64
args.corr_levels = 4
args.corr_radius = 3
else:
self.hidden_dim = hdim = 128
self.context_dim = cdim = 128
args.corr_levels = 4
args.corr_radius = 4
if 'dropout' not in self.args:
self.args.dropout = 0
if 'alternate_corr' not in self.args:
self.args.alternate_corr = False
# feature network, context network, and update block
if args.small:
self.fnet = SmallEncoder(output_dim=128, norm_fn='instance', dropout=args.dropout)
self.cnet = SmallEncoder(output_dim=hdim+cdim, norm_fn='none', dropout=args.dropout)
self.update_block = SmallUpdateBlock(self.args, hidden_dim=hdim)
else:
self.fnet = BasicEncoder(output_dim=256, norm_fn='instance', dropout=args.dropout)
self.cnet = BasicEncoder(output_dim=hdim+cdim, norm_fn='batch', dropout=args.dropout)
self.update_block = BasicUpdateBlock(self.args, hidden_dim=hdim)
def freeze_bn(self):
for m in self.modules():
if isinstance(m, nn.BatchNorm2d):
m.eval()
def initialize_flow(self, img):
""" Flow is represented as difference between two coordinate grids flow = coords1 - coords0"""
N, C, H, W = img.shape
coords0 = coords_grid(N, H//8, W//8).to(img.device)
coords1 = coords_grid(N, H//8, W//8).to(img.device)
# optical flow computed as difference: flow = coords1 - coords0
return coords0, coords1
def upsample_flow(self, flow, mask):
""" Upsample flow field [H/8, W/8, 2] -> [H, W, 2] using convex combination """
N, _, H, W = flow.shape
mask = mask.view(N, 1, 9, 8, 8, H, W)
mask = torch.softmax(mask, dim=2)
up_flow = F.unfold(8 * flow, [3,3], padding=1)
up_flow = up_flow.view(N, 2, 9, 1, 1, H, W)
up_flow = torch.sum(mask * up_flow, dim=2)
up_flow = up_flow.permute(0, 1, 4, 2, 5, 3)
return up_flow.reshape(N, 2, 8*H, 8*W)
def forward(self, image1, image2, iters=12, flow_init=None, upsample=True, test_mode=False):
""" Estimate optical flow between pair of frames """
image1 = 2 * (image1 / 255.0) - 1.0
image2 = 2 * (image2 / 255.0) - 1.0
image1 = image1.contiguous()
image2 = image2.contiguous()
hdim = self.hidden_dim
cdim = self.context_dim
# run the feature network
with autocast(enabled=self.args.mixed_precision):
fmap1, fmap2 = self.fnet([image1, image2])
fmap1 = fmap1.float()
fmap2 = fmap2.float()
if self.args.alternate_corr:
corr_fn = AlternateCorrBlock(fmap1, fmap2, radius=self.args.corr_radius)
else:
corr_fn = CorrBlock(fmap1, fmap2, radius=self.args.corr_radius)
# run the context network
with autocast(enabled=self.args.mixed_precision):
cnet = self.cnet(image1)
net, inp = torch.split(cnet, [hdim, cdim], dim=1)
net = torch.tanh(net)
inp = torch.relu(inp)
coords0, coords1 = self.initialize_flow(image1)
if flow_init is not None:
coords1 = coords1 + flow_init
flow_predictions = []
for itr in range(iters):
coords1 = coords1.detach()
corr = corr_fn(coords1) # index correlation volume
flow = coords1 - coords0
with autocast(enabled=self.args.mixed_precision):
net, up_mask, delta_flow = self.update_block(net, inp, corr, flow)
# F(t+1) = F(t) + \Delta(t)
coords1 = coords1 + delta_flow
# upsample predictions
if up_mask is None:
# upflow8是用线性插值的方法上采样8倍。
flow_up = upflow8(coords1 - coords0)
else:
flow_up = self.upsample_flow(coords1 - coords0, up_mask)
flow_predictions.append(flow_up)
if test_mode:
return coords1 - coords0, flow_up
return flow_predictions
可以看到模型运行的步骤为:
- 输入两个图片image1和image2;
- 两个图片一起输入到fnet中,得到两个图片的特征图fmap1和fmap2;
- fmap1和fmap2输入到CorrBlock中,得到两者的相关特征矩阵corr;这个CorrBlock是需要用cuda编程构建,作者已经提供对应的cpp和cu文件;
- image1通过context network(cnet),得到net和inp两个特征,cnet和fnet的结构基本相同;
- 然后就是初始化光流和对应网格,然后就是进入循环部分;
- 每一次都是把net,inp,corr,flow放入到updateblock中,计算出来net,up_mask,和delta_flow;
Encoder
作者提供了一个BasicEncoder,也好心的为显存不够大的朋友提供了一个smallEncoder,这里仅解说BasicEncoder的结构:
class BasicEncoder(nn.Module):
def __init__(self, output_dim=128, norm_fn='batch', dropout=0.0):
super(BasicEncoder, self).__init__()
self.norm_fn = norm_fn
if self.norm_fn == 'group':
self.norm1 = nn.GroupNorm(num_groups=8, num_channels=64)
elif self.norm_fn == 'batch':
self.norm1 = nn.BatchNorm2d(64)
elif self.norm_fn == 'instance':
self.norm1 = nn.InstanceNorm2d(64)
elif self.norm_fn == 'none':
self.norm1 = nn.Sequential()
self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3)
self.relu1 = nn.ReLU(inplace=True)
self.in_planes = 64
self.layer1 = self._make_layer(64, stride=1)
self.layer2 = self._make_layer(96, stride=2)
self.layer3 = self._make_layer(128, stride=2)
# output convolution
self.conv2 = nn.Conv2d(128, output_dim, kernel_size=1)
self.dropout = None
if dropout > 0:
self.dropout = nn.Dropout2d(p=dropout)
for m in self.modules():
if isinstance(m, nn.Conv2d):
nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu')
elif isinstance(m, (nn.BatchNorm2d, nn.InstanceNorm2d, nn.GroupNorm)):
if m.weight is not None:
nn.init.constant_(m.weight, 1)
if m.bias is not None:
nn.init.constant_(m.bias, 0)
def _make_layer(self, dim, stride=1):
layer1 = ResidualBlock(self.in_planes, dim, self.norm_fn, stride=stride)
layer2 = ResidualBlock(dim, dim, self.norm_fn, stride=1)
layers = (layer1, layer2)
self.in_planes = dim
return nn.Sequential(*layers)
def forward(self, x):
# 这里输入的图片是两个图片,是需要配准的两个图片
is_list = isinstance(x, tuple) or isinstance(x, list)
if is_list:
batch_dim = x[0].shape[0]
x = torch.cat(x, dim=0)
x = self.conv1(x) # 这是一个简单的卷积层,放大通道数从3到64
x = self.norm1(x) # 默认使用的是batchnorm
x = self.relu1(x)
x = self.layer1(x) # 是由两个残差block构成。
x = self.layer2(x)
x = self.layer3(x)
x = self.conv2(x)
if self.training and self.dropout is not None:
x = self.dropout(x)
if is_list:
x = torch.split(x, [batch_dim, batch_dim], dim=0)
return x
- BasicEncoder中图片下采样了3次,也就是原来的八分之一;
- 除了第一层和最后一层是简单的卷积层,其他的都是由残差模块组成,每个残差模块包含两个卷积层;
updateblock
class BasicMotionEncoder(nn.Module):
def __init__(self, args):
super(BasicMotionEncoder, self).__init__()
cor_planes = args.corr_levels * (2*args.corr_radius + 1)**2
self.convc1 = nn.Conv2d(cor_planes, 256, 1, padding=0)
self.convc2 = nn.Conv2d(256, 192, 3, padding=1)
self.convf1 = nn.Conv2d(2, 128, 7, padding=3)
self.convf2 = nn.Conv2d(128, 64, 3, padding=1)
self.conv = nn.Conv2d(64+192, 128-2, 3, padding=1)
def forward(self, flow, corr):
cor = F.relu(self.convc1(corr))
cor = F.relu(self.convc2(cor))
flo = F.relu(self.convf1(flow))
flo = F.relu(self.convf2(flo))
cor_flo = torch.cat([cor, flo], dim=1)
out = F.relu(self.conv(cor_flo))
return torch.cat([out, flow], dim=1)
class SepConvGRU(nn.Module):
def __init__(self, hidden_dim=128, input_dim=192+128):
super(SepConvGRU, self).__init__()
self.convz1 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (1,5), padding=(0,2))
self.convr1 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (1,5), padding=(0,2))
self.convq1 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (1,5), padding=(0,2))
self.convz2 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (5,1), padding=(2,0))
self.convr2 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (5,1), padding=(2,0))
self.convq2 = nn.Conv2d(hidden_dim+input_dim, hidden_dim, (5,1), padding=(2,0))
def forward(self, h, x):
# horizontal
hx = torch.cat([h, x], dim=1)
z = torch.sigmoid(self.convz1(hx))
r = torch.sigmoid(self.convr1(hx))
q = torch.tanh(self.convq1(torch.cat([r*h, x], dim=1)))
h = (1-z) * h + z * q
# vertical
hx = torch.cat([h, x], dim=1)
z = torch.sigmoid(self.convz2(hx))
r = torch.sigmoid(self.convr2(hx))
q = torch.tanh(self.convq2(torch.cat([r*h, x], dim=1)))
h = (1-z) * h + z * q
return h
class FlowHead(nn.Module):
def __init__(self, input_dim=128, hidden_dim=256):
super(FlowHead, self).__init__()
self.conv1 = nn.Conv2d(input_dim, hidden_dim, 3, padding=1)
self.conv2 = nn.Conv2d(hidden_dim, 2, 3, padding=1)
self.relu = nn.ReLU(inplace=True)
def forward(self, x):
return self.conv2(self.relu(self.conv1(x)))
class BasicUpdateBlock(nn.Module):
def __init__(self, args, hidden_dim=128, input_dim=128):
super(BasicUpdateBlock, self).__init__()
self.args = args
self.encoder = BasicMotionEncoder(args)
self.gru = SepConvGRU(hidden_dim=hidden_dim, input_dim=128+hidden_dim)
self.flow_head = FlowHead(hidden_dim, hidden_dim=256)
self.mask = nn.Sequential(
nn.Conv2d(128, 256, 3, padding=1),
nn.ReLU(inplace=True),
nn.Conv2d(256, 64*9, 1, padding=0))
def forward(self, net, inp, corr, flow, upsample=True):
motion_features = self.encoder(flow, corr)
inp = torch.cat([inp, motion_features], dim=1)
net = self.gru(net, inp)
delta_flow = self.flow_head(net)
# scale mask to balence gradients
mask = .25 * self.mask(net)
return net, mask, delta_flow
BasicMotionEncoder
:输入的相关性矩阵和光流做特征融合;SepConvGRU
: 这个有意思,是卷积中的GRU结构。-
这里有两个输入,分别是net和inp;
-
两个输入拼接起来,放到卷积层中,得到update和reset门的输出;
-
然后用reset*hidden再拼接上x,得到新的hidden;
-
然后根据update的权重来更新hidden层。
-
也就是说,模型中的net变量,其实是GRP循环网络中的hidden隐变量;
-
总结
- 卷积的GRU结构我学到了。我觉得这个GRU结构可以和很多的地方做结合。