🔧 Insights

1. Models architecture

All segmentation models in SMP (this library short name) are made of:

  • encoder (feature extractor, a.k.a backbone)

  • decoder (features fusion block to create segmentation mask)

  • segmentation head (final head to reduce number of channels from decoder and upsample mask to preserve input-output spatial resolution identity)

  • classification head (optional head which build on top of deepest encoder features)

2. Creating your own encoder

Encoder is a “classification model” which extarct features from image and pass it to decoder. Each encoder should have following attributes and methods and be inherited from segmetation_models_pytorch.encoders._base.EncoderMixin

class MyEncoder(torch.nn.Module, EncoderMixin):

    def __init__(self, **kwargs):

        # A number of channels for each encoder feature tensor, list of integers
        self._out_channels: List[int] = [3, 16, 64, 128, 256, 512]

        # A number of stages in decoder (in other words number of downsampling operations), integer
        # use in in forward pass to reduce number of returning fatures
        self._depth: int = 5

        # Default number of input channels in first Conv2d layer for encoder (usually 3)
        self._in_channels: int = 3

        # Define enoder modules below

    def forward(self, x: torch.Tensor) -> List[torch.Tensor]:
        """Produce list of features of different spatial resolutions, each feature is a 4D torch.tensor of
        shape NCHW (features should be sorted in descending order according to spatial resolution, starting
        with resolution same as input `x` tensor).

        Input: `x` with shape (1, 3, 64, 64)
        Output: [f0, f1, f2, f3, f4, f5] - features with corresponding shapes
                [(1, 3, 64, 64), (1, 64, 32, 32), (1, 128, 16, 16), (1, 256, 8, 8),
                (1, 512, 4, 4), (1, 1024, 2, 2)] (C - dim may differ)

        also should support number of features according to specified depth, e.g. if depth = 5,
        number of feature tensors = 6 (one with same resolution as input and 5 downsampled),
        depth = 3 -> number of feature tensors = 4 (one with same resolution as input and 3 downsampled).

        return [feat1, feat2, feat3, feat4, feat5, feat6]

When you write your own Encoder class register its build parameters

smp.encoders.encoders["my_awesome_encoder"] = {
    "encoder": MyEncoder, # enocoder class here
    "pretrained_settings": {
        "imagenet": {
            "mean": [0.485, 0.456, 0.406],
            "std": [0.229, 0.224, 0.225],
            "url": "https://some-url.com/my-model-weights",
            "input_space": "RGB",
            "input_range": [0, 1],
    "params": {
        # init params for encoder if any

Now you can use your encoder

model = smp.Unet(encoder_name="my_awesome_encoder")

For better understanding see more examples of encoder in smp.encoders module.


If it works fine, don`t forget to contribute your work and make a PR to SMP 😉

3. Aux classification output

All models support aux_params parameter, which is default set to None. If aux_params = None than classification auxiliary output is not created, else model produce not only mask, but also label output with shape (N, C).

Classification head consist of following layers:

  1. GlobalPooling

  2. Dropout (optional)

  3. Linear

  4. Activation (optional)


    pooling='avg',             # one of 'avg', 'max'
    dropout=0.5,               # dropout ratio, default is None
    activation='sigmoid',      # activation function, default is None
    classes=4,                 # define number of output labels

model = smp.Unet('resnet34', classes=4, aux_params=aux_params)
mask, label = model(x)

mask.shape, label.shape
# (N, 4, H, W), (N, 4)