One-Stage vs Two-Stage Instance Segmentation

calendar Mar 4, 2022 · 5 min read · image segmentation machine learning computer vision data science  ·
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In computer vision, image segmentation refers to the process of dividing an image into distinct regions or segments, each corresponding to a different object or background. There are two main approaches to image segmentation: one-stage and two-stage.

One-stage image segmentation methods aim to directly predict a segmentation mask for the entire image in a single pass. These methods are typically faster and more efficient than two-stage methods, but they may be less accurate and less flexible.

Two-stage image segmentation methods, on the other hand, use a two-step process to generate a segmentation mask. In the first step, these methods generate a set of candidate object proposals, which are potential locations and sizes of objects in the image. In the second step, these proposals are refined and combined to generate the final segmentation mask. Two-stage methods are typically more accurate and flexible than one-stage methods, but they may be slower and more computationally expensive.

Overall, the choice between one-stage and two-stage image segmentation methods depends on the specific application and the trade-offs between speed, accuracy, and flexibility.

Two-Stage Segmentation Examples

Some examples of two-stage image segmentation methods include:

  • Region proposal networks (RPNs): RPNs are a type of object detection method that can be used for image segmentation. RPNs use a convolutional neural network to generate a set of candidate object proposals, which are potential locations and sizes of objects in the image. The proposals are then passed to a separate network for classification and refinement, which produces the final segmentation mask.
  • Selective search: Selective search is a bottom-up approach to object proposal generation, which can be used in conjunction with a convolutional neural network for image segmentation. Selective search uses a combination of color, texture, and shape information to generate a set of candidate object proposals, which are then passed to the convolutional neural network for classification and refinement.
  • EdgeBoxes: EdgeBoxes is another bottom-up approach to object proposal generation, which can be used for image segmentation. EdgeBoxes uses edge information in the image to generate a set of candidate object proposals, which are then passed to a convolutional neural network for classification and refinement.
Models
  • Faster R-CNN: Faster R-CNN is a popular two-stage object detection model that can be used for image segmentation. Faster R-CNN uses a region proposal network (RPN) to generate candidate object proposals, which are then passed to a separate network for classification and refinement. The final bounding boxes can be used to generate the segmentation mask.
  • Mask R-CNN: Mask R-CNN is a two-stage image segmentation model based on the Faster R-CNN object detection model. Mask R-CNN adds a branch to the Faster R-CNN network to predict segmentation masks for each detected object.
  • Cascade R-CNN: Cascade R-CNN is a two-stage object detection model that improves on the Faster R-CNN model by adding a cascade of classifiers to the region proposal network. This allows the model to refine the object proposals and improve the accuracy of the final segmentation masks.
  • YOLACT: YOLACT is a two-stage image segmentation model that uses a single shot detector (SSD) network to generate object proposals, and then refines the proposals using a separate network to generate the final segmentation masks.

One-stage Segmentation Examples

Some examples of one-stage image segmentation methods include:

  • Fully convolutional networks (FCNs): FCNs are a type of deep neural network that can take an image of arbitrary size as input and produce a corresponding segmentation mask. FCNs use a series of convolutional and pooling layers to extract features from the input image, and then use a deconvolutional layer to upsample the features to the size of the input image and generate the final segmentation mask.
  • Single shot detectors (SSDs): SSDs are a type of object detection method that can be used for image segmentation. SSDs use a convolutional neural network to simultaneously predict object classes and locations in the image, using a single pass through the network. The predicted bounding boxes can then be used to generate the segmentation mask.
  • Region-based convolutional neural networks (RCNNs): RCNNs are another type of object detection method that can be used for image segmentation. RCNNs use a two-stage process, where the first stage generates object proposals and the second stage uses a convolutional neural network to refine the proposals and generate the final segmentation mask. However, unlike two-stage segmentation methods, the proposal generation and refinement stages are both performed using a single network, rather than two separate networks.
Models
  • Mask R-CNN: Mask R-CNN is a popular one-stage image segmentation model based on the Faster R-CNN object detection model. Mask R-CNN adds a branch to the Faster R-CNN network to predict segmentation masks for each detected object.
  • U-Net: U-Net is a one-stage image segmentation model based on a fully convolutional network (FCN) architecture. U-Net uses a series of convolutional and deconvolutional layers to extract features from the input image and generate the final segmentation mask.
  • DeepLab: DeepLab is another one-stage image segmentation model based on an FCN architecture. DeepLab uses a combination of atrous convolutions and spatial pyramid pooling to extract features from the input image and generate the final segmentation mask.
  • PSPNet: PSPNet is a one-stage image segmentation model based on the ResNet-101 convolutional neural network. PSPNet uses spatial pyramid pooling to extract features from the input image and generate the final segmentation mask.

Author: Sadman Kabir Soumik