--- a
+++ b/opengait/modeling/backbones/plain.py
@@ -0,0 +1,63 @@
+"""The plain backbone.
+
+    The plain backbone only contains the BasicConv2d, FocalConv2d and MaxPool2d and LeakyReLU layers.
+"""
+
+import torch.nn as nn
+from ..modules import BasicConv2d, FocalConv2d
+
+
+class Plain(nn.Module):
+    """
+    The Plain backbone class.
+
+    An implicit LeakyRelu appended to each layer except maxPooling. 
+    The kernel size, stride and padding of the first convolution layer are 5, 1, 2, the ones of other layers are 3, 1, 1.
+
+    Typical usage: 
+    - BC-64: Basic conv2d with output channel 64. The input channel is the output channel of previous layer.
+
+    - M: nn.MaxPool2d(kernel_size=2, stride=2)].
+
+    - FC-128-1: Focal conv2d with output channel 64 and halving 1(divided to 2^1=2 parts).
+
+    Use it in your configuration file.
+    """
+
+    def __init__(self, layers_cfg, in_channels=1):
+        super(Plain, self).__init__()
+        self.layers_cfg = layers_cfg
+        self.in_channels = in_channels
+
+        self.feature = self.make_layers()
+
+    def forward(self, seqs):
+        out = self.feature(seqs)
+        return out
+
+    def make_layers(self):
+        """
+        Reference: torchvision/models/vgg.py
+        """
+        def get_layer(cfg, in_c, kernel_size, stride, padding):
+            cfg = cfg.split('-')
+            typ = cfg[0]
+            if typ not in ['BC', 'FC']:
+                raise ValueError('Only support BC or FC, but got {}'.format(typ))
+            out_c = int(cfg[1])
+
+            if typ == 'BC':
+                return BasicConv2d(in_c, out_c, kernel_size=kernel_size, stride=stride, padding=padding)
+            return FocalConv2d(in_c, out_c, kernel_size=kernel_size, stride=stride, padding=padding, halving=int(cfg[2]))
+
+        Layers = [get_layer(self.layers_cfg[0], self.in_channels,
+                            5, 1, 2), nn.LeakyReLU(inplace=True)]
+        in_c = int(self.layers_cfg[0].split('-')[1])
+        for cfg in self.layers_cfg[1:]:
+            if cfg == 'M':
+                Layers += [nn.MaxPool2d(kernel_size=2, stride=2)]
+            else:
+                conv2d = get_layer(cfg, in_c, 3, 1, 1)
+                Layers += [conv2d, nn.LeakyReLU(inplace=True)]
+                in_c = int(cfg.split('-')[1])
+        return nn.Sequential(*Layers)