Gaussian pyramid: A series of down-sampled images are obtained by Gaussian smoothing and subsampling, that is, the K+1 layer of Gaussian images is obtained by smoothing and subsampling the K-th Gaussian pyramid. The image of the pyramid is as follows:
Downsampling pyrDown: Convolving the image Gi with a Gaussian kernel and removing all even-numbered rows and columns to obtain an image of Gi+1, and the resulting image is only a quarter of the original image.
Upsampling pyrUp: The image is doubled in each direction, and the new rows and columns are padded with 0s. Convolve the enlarged image with the same kernel as before (multiplied by 4) to get an approximation of the newly added pixels
Laplacian Pyramid: The Laplacian Pyramid is constructed by subtracting a sequence of images from the source image that is first zoomed out and then zoomed in
