Review: Geometry and Features

Geometry is what turns a picture into a measurement. It's often the part of vision that gets skipped, and it's where you can stand out. Let's check what you've kept.

• Pictures become measurements
• Often skipped
• Where you can stand out

Recap: geometry

Keypoints are distinctive spots that can be matched between images. The ratio test filters out poor matches, and RANSAC finds the transform that most matches agree on. A homography maps one flat plane onto another, from four points. Calibration measures a lens, from a checkerboard. And two cameras give you depth, by triangulation.

Question 1

Question 1. How many point pairs define a homography? A, Two. B, Three. C, Four. D, Nine. Pause and choose. The answer is C. Four pairs give the eight numbers that you need.

Question 2

Question 2. What does RANSAC assume? A, All data is good. B, Much of the data may be wrong. C, Images are flat. D, Data is sorted. Pause and choose. The answer is B. It looks for the largest group that agrees.

Question 3

Question 3. A homography only works perfectly for what? A, Flat surfaces, or pure rotation. B, Any scene. C, Moving objects. D, Black and white images. Pause and choose. The answer is A. 3D scenes break it.

Question 4

Question 4. What does a low reprojection error tell you? A, The photos are sharp. B, The camera model explains the data well. C, The lens is cheap. D, The image is small. Pause and choose. The answer is B. Under half a pixel is good.

Question 5

Question 5. What should you do after calibrating? A, Change the zoom. B, Lock the focus and zoom. C, Move the camera. D, Add noise. Pause and choose. The answer is B. Changing them means calibrating again.

Question 6

Question 6. What gives depth from two cameras? A, Disparity between views. B, Colour. C, Brightness. D, Compression. Pause and choose. The answer is A. Closer objects shift more.

Summary