torchvision.models.detection.maskrcnn_resnet50_fpn(pretrained=False, progress=True, num_classes=91, pretrained_backbone=True, trainable_backbone_layers=None, **kwargs)[source]

Constructs a Mask R-CNN model with a ResNet-50-FPN backbone.

Reference: “Mask R-CNN”.

The input to the model is expected to be a list of tensors, each of shape [C, H, W], one for each image, and should be in 0-1 range. Different images can have different sizes.

The behavior of the model changes depending if it is in training or evaluation mode.

During training, the model expects both the input tensors, as well as a targets (list of dictionary), containing:

  • boxes (FloatTensor[N, 4]): the ground-truth boxes in [x1, y1, x2, y2] format, with 0 <= x1 < x2 <= W and 0 <= y1 < y2 <= H.

  • labels (Int64Tensor[N]): the class label for each ground-truth box

  • masks (UInt8Tensor[N, H, W]): the segmentation binary masks for each instance

The model returns a Dict[Tensor] during training, containing the classification and regression losses for both the RPN and the R-CNN, and the mask loss.

During inference, the model requires only the input tensors, and returns the post-processed predictions as a List[Dict[Tensor]], one for each input image. The fields of the Dict are as follows, where N is the number of detected instances:

  • boxes (FloatTensor[N, 4]): the predicted boxes in [x1, y1, x2, y2] format, with 0 <= x1 < x2 <= W and 0 <= y1 < y2 <= H.

  • labels (Int64Tensor[N]): the predicted labels for each instance

  • scores (Tensor[N]): the scores or each instance

  • masks (UInt8Tensor[N, 1, H, W]): the predicted masks for each instance, in 0-1 range. In order to obtain the final segmentation masks, the soft masks can be thresholded, generally with a value of 0.5 (mask >= 0.5)

For more details on the output and on how to plot the masks, you may refer to Instance segmentation models.

Mask R-CNN is exportable to ONNX for a fixed batch size with inputs images of fixed size.


>>> model = torchvision.models.detection.maskrcnn_resnet50_fpn(pretrained=True)
>>> model.eval()
>>> x = [torch.rand(3, 300, 400), torch.rand(3, 500, 400)]
>>> predictions = model(x)
>>> # optionally, if you want to export the model to ONNX:
>>> torch.onnx.export(model, x, "mask_rcnn.onnx", opset_version = 11)
  • pretrained (bool) – If True, returns a model pre-trained on COCO train2017

  • progress (bool) – If True, displays a progress bar of the download to stderr

  • num_classes (int) – number of output classes of the model (including the background)

  • pretrained_backbone (bool) – If True, returns a model with backbone pre-trained on Imagenet

  • trainable_backbone_layers (int) – number of trainable (not frozen) resnet layers starting from final block. Valid values are between 0 and 5, with 5 meaning all backbone layers are trainable. If None is passed (the default) this value is set to 3.

Examples using maskrcnn_resnet50_fpn:


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