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Region proposal layer for Faster R-CNN


A region proposal layer outputs bounding boxes around potential objects in an image as part of the region proposal network (RPN) within Faster R-CNN. These outputs are further refined by additional layers within Faster R-CNN to produce the final object detection results.

There are two inputs to this layer:

  • 'scores' — The classification scores produced by the RPN classification branch

  • 'boxDeltas' — The bounding box deltas produced by the RPN regression branch

Use the input names when connecting or disconnecting the region proposal layer to other layers using connectLayers (Deep Learning Toolbox) or disconnectLayers (Deep Learning Toolbox) (requires Deep Learning Toolbox™).



layer = regionProposalLayer(anchorBoxes) creates a region proposal layer for building Faster R-CNN object detection networks, and sets the AnchorBoxes property.


layer = regionProposalLayer(anchorBoxes,'Name',Name) creates a region proposal layer and sets the optional Name property.


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Anchor boxes, specified as an M-by-2 matrix defining the [height width] of M anchor boxes.

Anchor boxes are predefined bounding box templates of fixed size. The size of each anchor box is typically determined based on a priori knowledge of the scale and aspect ratio of objects in the training dataset. An RPN network is trained to predict the translation and rescaling needed to align the anchor boxes with the ground truth bounding boxes. [1]

Layer name, specified as a character vector or a string scalar. For Layer array input, the trainnet (Deep Learning Toolbox) and dlnetwork (Deep Learning Toolbox) functions automatically assign names to layers with the name "".

The RegionProposalLayer object stores this property as a character vector.

Data Types: char | string

Number of inputs of the layer. This layer has two inputs.

Data Types: double

Input names of the layer. This layer has two inputs, named 'scores' and 'boxDeltas'.

Data Types: cell

This property is read-only.

Number of outputs from the layer, returned as 1. This layer has a single output only.

Data Types: double

This property is read-only.

Output names, returned as {'out'}. This layer has a single output only.

Data Types: cell


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Define three square anchor boxes for the region proposal layer.

anchorBoxes = [
    16 16
    64 64
    128 128

Create a region proposal layer with the name 'region_proposal'.

regionProposal = regionProposalLayer(anchorBoxes,'Name','region_proposal');


[1] Ren, S., K. He, R. Girshick, and J. Sun. "Faster R-CNN: Towards Real-Time Object Detection with Region Proposal Networks." Advances in Neural Information Processing Systems. Vol. 28, 2015.

Version History

Introduced in R2018b