Answers.Body: <p>The reason is probably that the segmentation polygons have various shapes and complexities. You don't know how many points you need per polygon so defining a proper output that specifies the polygons is not straight forward. In contrast, bounding boxes are always defined by 4 coordinates (2 coordinates for lower left and upper right corner are already enough).</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2022-07-14T14:28:24.400Z
Answers.LastActivityDate: 2022-07-14T14:28:24.400Z
Answers.UserDisplayName: Chillston
Body: <p>I'm wondering why many model architectures use binary mask reconstruction for segmentational CNNs, and not regression of mask polygon coordinates? Many object detectors use regression to find coordinates of bounding boxes.</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-07-13T11:17:22.783Z
Comments.Text: If I understand correctly, the reason is probably that the segmentation polygons have various shapes and complexities. You don't know how many points you need per polygon. Bounding boxes however are always defined by 4 coordinates.
Comments.UserDisplayName: Chillston
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-07-14T13:25:00.580Z
Comments.Text: @Chillston This is the correct answer, and should be posted as an answer so it can be upvoted and the OP can accept it.
Comments.UserDisplayName: Kroshtan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-07-14T14:28:47.127Z
Comments.Text: You are right, thank you - I put it as an answer
Comments.UserDisplayName: Chillston
Tags: <object-detection><image-segmentation><mask-rcnn><instance-segmentation>
Title: Why instance segmentation architectures using reconstruction masks but not regression?
UserDisplayName: Dmitry  Sokolov
ThreadText: # Why instance segmentation architectures using reconstruction masks but not regression?
  
  (Asked by: username_0 on 2022-07-13)
  
  I'm wondering why many model architectures use binary mask reconstruction for segmentational CNNs, and not regression of mask polygon coordinates? Many object detectors use regression to find coordinates of bounding boxes.
  
  **Tags:** object-detection, image-segmentation, mask-rcnn, instance-segmentation
  > **Comments:**
  > **username_1**: If I understand correctly, the reason is probably that the segmentation polygons have various shapes and complexities. You don't know how many points you need per polygon. Bounding boxes however are always defined by 4 coordinates.  
  > **username_2**: @Chillston This is the correct answer, and should be posted as an answer so it can be upvoted and the OP can accept it.  
  > **username_1**: You are right, thank you - I put it as an answer  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_1 on 2022-07-14. Score: 1 | Accepted answer
  
  The reason is probably that the segmentation polygons have various shapes and complexities. You don't know how many points you need per polygon so defining a proper output that specifies the polygons is not straight forward. In contrast, bounding boxes are always defined by 4 coordinates (2 coordinates for lower left and upper right corner are already enough).
  
  
  ---
  
  