import torch import torch.nn as nn import torch.nn.functional as F from .backbone import CNNEncoder from .transformer import FeatureTransformer, FeatureFlowAttention from .matching import global_correlation_softmax, local_correlation_softmax from .geometry import flow_warp from .utils import normalize_img, feature_add_position class GMFlow(nn.Module): def __init__(self, num_scales=1, upsample_factor=8, feature_channels=128, attention_type='swin', num_transformer_layers=6, ffn_dim_expansion=4, num_head=1, **kwargs, ): super(GMFlow, self).__init__() self.num_scales = num_scales self.feature_channels = feature_channels self.upsample_factor = upsample_factor self.attention_type = attention_type self.num_transformer_layers = num_transformer_layers # CNN backbone self.backbone = CNNEncoder(output_dim=feature_channels, num_output_scales=num_scales) # Transformer self.transformer = FeatureTransformer(num_layers=num_transformer_layers, d_model=feature_channels, nhead=num_head, attention_type=attention_type, ffn_dim_expansion=ffn_dim_expansion, ) # flow propagation with self-attn self.feature_flow_attn = FeatureFlowAttention(in_channels=feature_channels) # convex upsampling: concat feature0 and flow as input self.upsampler = nn.Sequential(nn.Conv2d(2 + feature_channels, 256, 3, 1, 1), nn.ReLU(inplace=True), nn.Conv2d(256, upsample_factor ** 2 * 9, 1, 1, 0)) def extract_feature(self, img0, img1): concat = torch.cat((img0, img1), dim=0) # [2B, C, H, W] features = self.backbone(concat) # list of [2B, C, H, W], resolution from high to low # reverse: resolution from low to high features = features[::-1] feature0, feature1 = [], [] for i in range(len(features)): feature = features[i] chunks = torch.chunk(feature, 2, 0) # tuple feature0.append(chunks[0]) feature1.append(chunks[1]) return feature0, feature1 def upsample_flow(self, flow, feature, bilinear=False, upsample_factor=8, ): if bilinear: up_flow = F.interpolate(flow, scale_factor=upsample_factor, mode='bilinear', align_corners=True) * upsample_factor else: # convex upsampling concat = torch.cat((flow, feature), dim=1) mask = self.upsampler(concat) b, flow_channel, h, w = flow.shape mask = mask.view(b, 1, 9, self.upsample_factor, self.upsample_factor, h, w) # [B, 1, 9, K, K, H, W] mask = torch.softmax(mask, dim=2) up_flow = F.unfold(self.upsample_factor * flow, [3, 3], padding=1) up_flow = up_flow.view(b, flow_channel, 9, 1, 1, h, w) # [B, 2, 9, 1, 1, H, W] up_flow = torch.sum(mask * up_flow, dim=2) # [B, 2, K, K, H, W] up_flow = up_flow.permute(0, 1, 4, 2, 5, 3) # [B, 2, K, H, K, W] up_flow = up_flow.reshape(b, flow_channel, self.upsample_factor * h, self.upsample_factor * w) # [B, 2, K*H, K*W] return up_flow def forward(self, img0, img1, attn_splits_list=None, corr_radius_list=None, prop_radius_list=None, pred_bidir_flow=False, **kwargs, ): results_dict = {} flow_preds = [] img0, img1 = normalize_img(img0, img1) # [B, 3, H, W] # resolution low to high feature0_list, feature1_list = self.extract_feature(img0, img1) # list of features flow = None assert len(attn_splits_list) == len(corr_radius_list) == len(prop_radius_list) == self.num_scales for scale_idx in range(self.num_scales): feature0, feature1 = feature0_list[scale_idx], feature1_list[scale_idx] if pred_bidir_flow and scale_idx > 0: # predicting bidirectional flow with refinement feature0, feature1 = torch.cat((feature0, feature1), dim=0), torch.cat((feature1, feature0), dim=0) upsample_factor = self.upsample_factor * (2 ** (self.num_scales - 1 - scale_idx)) if scale_idx > 0: flow = F.interpolate(flow, scale_factor=2, mode='bilinear', align_corners=True) * 2 if flow is not None: flow = flow.detach() feature1 = flow_warp(feature1, flow) # [B, C, H, W] attn_splits = attn_splits_list[scale_idx] corr_radius = corr_radius_list[scale_idx] prop_radius = prop_radius_list[scale_idx] # add position to features feature0, feature1 = feature_add_position(feature0, feature1, attn_splits, self.feature_channels) # Transformer feature0, feature1 = self.transformer(feature0, feature1, attn_num_splits=attn_splits) # correlation and softmax if corr_radius == -1: # global matching flow_pred = global_correlation_softmax(feature0, feature1, pred_bidir_flow)[0] else: # local matching flow_pred = local_correlation_softmax(feature0, feature1, corr_radius)[0] # flow or residual flow flow = flow + flow_pred if flow is not None else flow_pred # upsample to the original resolution for supervison if self.training: # only need to upsample intermediate flow predictions at training time flow_bilinear = self.upsample_flow(flow, None, bilinear=True, upsample_factor=upsample_factor) flow_preds.append(flow_bilinear) # flow propagation with self-attn if pred_bidir_flow and scale_idx == 0: feature0 = torch.cat((feature0, feature1), dim=0) # [2*B, C, H, W] for propagation flow = self.feature_flow_attn(feature0, flow.detach(), local_window_attn=prop_radius > 0, local_window_radius=prop_radius) # bilinear upsampling at training time except the last one if self.training and scale_idx < self.num_scales - 1: flow_up = self.upsample_flow(flow, feature0, bilinear=True, upsample_factor=upsample_factor) flow_preds.append(flow_up) if scale_idx == self.num_scales - 1: flow_up = self.upsample_flow(flow, feature0) flow_preds.append(flow_up) results_dict.update({'flow_preds': flow_preds}) return results_dict