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72 lines
1.9 KiB
Markdown
72 lines
1.9 KiB
Markdown
---
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title: "06-homographies"
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tags:
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- lecture
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- cosc342
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---
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Homographies and mosaics
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- A homography* is:
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- A linear map between two planes (or views of a plane)
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- A 3 x 3 matrix, up to a scale
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- Two images of a scene are related by a homography
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- If the scene is planar, or
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- If the camera only rotates
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- 
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Homographies for planar scenes
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- Take a line in one view
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- This projects to a plane
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- That intersects the scene (another plane) at a line
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- That projects to a line in the other image
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- So lines are preserved
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- 
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> [!INFO] can create a "straight" view of a picture!
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[|300](https://i.imgur.com/BEjOoFL.png)
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> [!INFO] homography: maps points from one image to another
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> [!INFO] rephotography, mathing old photographs onto the current view of the user
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homography for rotating camera
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> [!INFO] K matric describe parameters of camera. R matric describes rotation of camera. T is zero because the camera is only rotating. H is the homograhy matrix
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not homographies
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- If we have both:
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- Non-planar scene, and
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- Translating camera
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- There is no homography between the images
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- Lines may not be preserved
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- Cannot make a mosaic
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- But we can do stereo
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making mosiac
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- To make a mosaic image:
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- Need to warp images to align
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- This warping is a homography
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- How to find the homography?
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- If a feature at in one image matches to in the other, then
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HOMOGRAPHY ESTIMATION IN OPENCV
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- https://docs.opencv.org/4.4.0/d9/d0c/group__calib3d.html#ga4abc2ece9fab9398f2e560d53c8c9780
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- pts1 – Points in first image (std::vector)
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- pts2 – Points in second image (std::vector)
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- cv::RANSAC - method to use (we’ll just use RANSAC)
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- 3.0 – RANSAC threshold
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cv::Mat H = cv::findHomography(pts1, pts2, cv::RANSAC, 3.0);
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