init

finetune/mmseg/models/decode_heads/ocr_head.py

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+# Copyright (c) OpenMMLab. All rights reserved.
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+from mmcv.cnn import ConvModule
+
+from mmseg.registry import MODELS
+from ..utils import SelfAttentionBlock as _SelfAttentionBlock
+from ..utils import resize
+from .cascade_decode_head import BaseCascadeDecodeHead
+
+
+class SpatialGatherModule(nn.Module):
+    """Aggregate the context features according to the initial predicted
+    probability distribution.
+
+    Employ the soft-weighted method to aggregate the context.
+    """
+
+    def __init__(self, scale):
+        super().__init__()
+        self.scale = scale
+
+    def forward(self, feats, probs):
+        """Forward function."""
+        batch_size, num_classes, height, width = probs.size()
+        channels = feats.size(1)
+        probs = probs.view(batch_size, num_classes, -1)
+        feats = feats.view(batch_size, channels, -1)
+        # [batch_size, height*width, num_classes]
+        feats = feats.permute(0, 2, 1)
+        # [batch_size, channels, height*width]
+        probs = F.softmax(self.scale * probs, dim=2)
+        # [batch_size, channels, num_classes]
+        ocr_context = torch.matmul(probs, feats)
+        ocr_context = ocr_context.permute(0, 2, 1).contiguous().unsqueeze(3)
+        return ocr_context
+
+
+class ObjectAttentionBlock(_SelfAttentionBlock):
+    """Make a OCR used SelfAttentionBlock."""
+
+    def __init__(self, in_channels, channels, scale, conv_cfg, norm_cfg,
+                 act_cfg):
+        if scale > 1:
+            query_downsample = nn.MaxPool2d(kernel_size=scale)
+        else:
+            query_downsample = None
+        super().__init__(
+            key_in_channels=in_channels,
+            query_in_channels=in_channels,
+            channels=channels,
+            out_channels=in_channels,
+            share_key_query=False,
+            query_downsample=query_downsample,
+            key_downsample=None,
+            key_query_num_convs=2,
+            key_query_norm=True,
+            value_out_num_convs=1,
+            value_out_norm=True,
+            matmul_norm=True,
+            with_out=True,
+            conv_cfg=conv_cfg,
+            norm_cfg=norm_cfg,
+            act_cfg=act_cfg)
+        self.bottleneck = ConvModule(
+            in_channels * 2,
+            in_channels,
+            1,
+            conv_cfg=self.conv_cfg,
+            norm_cfg=self.norm_cfg,
+            act_cfg=self.act_cfg)
+
+    def forward(self, query_feats, key_feats):
+        """Forward function."""
+        context = super().forward(query_feats, key_feats)
+        output = self.bottleneck(torch.cat([context, query_feats], dim=1))
+        if self.query_downsample is not None:
+            output = resize(query_feats)
+
+        return output
+
+
+@MODELS.register_module()
+class OCRHead(BaseCascadeDecodeHead):
+    """Object-Contextual Representations for Semantic Segmentation.
+
+    This head is the implementation of `OCRNet
+    <https://arxiv.org/abs/1909.11065>`_.
+
+    Args:
+        ocr_channels (int): The intermediate channels of OCR block.
+        scale (int): The scale of probability map in SpatialGatherModule in
+            Default: 1.
+    """
+
+    def __init__(self, ocr_channels, scale=1, **kwargs):
+        super().__init__(**kwargs)
+        self.ocr_channels = ocr_channels
+        self.scale = scale
+        self.object_context_block = ObjectAttentionBlock(
+            self.channels,
+            self.ocr_channels,
+            self.scale,
+            conv_cfg=self.conv_cfg,
+            norm_cfg=self.norm_cfg,
+            act_cfg=self.act_cfg)
+        self.spatial_gather_module = SpatialGatherModule(self.scale)
+
+        self.bottleneck = ConvModule(
+            self.in_channels,
+            self.channels,
+            3,
+            padding=1,
+            conv_cfg=self.conv_cfg,
+            norm_cfg=self.norm_cfg,
+            act_cfg=self.act_cfg)
+
+    def forward(self, inputs, prev_output):
+        """Forward function."""
+        x = self._transform_inputs(inputs)
+        feats = self.bottleneck(x)
+        context = self.spatial_gather_module(feats, prev_output)
+        object_context = self.object_context_block(feats, context)
+        output = self.cls_seg(object_context)
+
+        return output