Picking the right scale
A difference-of-Gaussians response computed live at the centre of five bright discs. Each disc answers strongest at the sigma that matches its own radius, which is what "scale selection" means before any pyramid is built.
Try this
Sweep sigma from small to large and watch the peak response walk from the smallest disc to the largest.
Where this lab comes from
Scale Space and the Difference of GaussiansScale space is a stack of copies of a photo, each blurred a little more, which lets a detector find features of every size and say how big each one is. Discs of radius 4, 8 and 16 px answer most strongly at a blur of 2.77, 5.61 and 11.34 px, close to r/√2; subtracting neighbouring blurs reproduces the scale-normalized Laplacian with a correlation of 0.99; and on a harbour photo SIFT's 9 octaves give each point a size that halves when the photo does.
More in edges and features
- An edge, and its two derivatives — One row of pixels, with the first and second derivative stacked underneath.
- Canny, one stage per panel — Smoothing and the two hysteresis thresholds, on a real photo.
- Flat, edge, or corner — Drag a window over a photo; two eigenvalues deliver the verdict.