. What artifact can occur when an image is downsampled by simply keeping every k-th pixel, without blurring first?
Fine regular detail (like brick mortar lines) that oscillates faster than the new pixel spacing can represent aliases into noisy-looking fake patterns.
. Why does blurring before downsampling prevent aliasing?
Smoothing away fine detail before subsampling avoids the fold-in effect that causes aliasing.
. What happens if the Gaussian kernel size is too small for the requested sigma?
A too-small kernel can't represent a wide bell curve, so cv2.getGaussianKernel renormalizes it into something close to a box average.
. What is a Gaussian pyramid?
Each pyramid level is a properly anti-aliased, half-size version of the previous level, produced by blur-then-downsample.