features
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Fundamentals Corners: The Structure Tensor, Harris, Shi-Tomasi and Förstner
Lesson 5 of the Edge Detection unit. A contour says where a boundary runs but not where you are along it. Two eigenvalues of a 2x2 matrix fix that: measured on a real photo, flat background scores 0.001/0.000, an edge 0.411/0.019 and a corner 0.442/0.318. Harris, Shi-Tomasi and Förstner all read that same matrix, and on the edge patch Harris comes out positive at k = 0.04 and negative at k = 0.06, so k decides the answer rather than the data.
Fundamentals What Makes a Point Matchable? SIFT, From Pixels to 128 Numbers
How SIFT turns a patch of pixels into 128 numbers that survive a moving camera. Measured on a real photo: a 45-level difference-of-Gaussians pyramid finds 791 keypoints spanning 1.8 to 112 px, each becomes a 128-dimensional descriptor, and Lowe's ratio test turns 791 candidates into 237 trustworthy matches. The invariances are real but bounded: 98.8% precision through a 45° rotation, and the scene is lost by 38° of viewpoint change. Every number has an artifact.