. What does thresholding do to a grayscale image?
Thresholding is a per-pixel comparison against a cutoff value, producing a binary image.
. What criterion does Otsu's method use to automatically choose a threshold?
Otsu searches every possible cutoff for the one that best separates the histogram into two low-variance classes.
. What effect does erosion have on a white region in a binary image?
Erosion only keeps a pixel white if the entire structuring element fits inside the white region, so regions shrink.
. What is "opening" (erosion followed by dilation) typically used for?
Erosion first wipes out small specks entirely; the following dilation grows the surviving (larger) regions back toward their original size.