Bilinear interpolation

(M=2)

O[2m,2n]=I[m,n]  
O[2m,2n+1]=(I[m,n]+I[m,n+1])/2 O[2m+1,2n]=(I[m,n]+I[m+1,n])/2 O[2m+1,2n+1]=(I[m,n]+I[m,n+1]+I[m+1,n]+I[m+1,n+1])/4

Equivalent 1D Filter h=[0.5, 1, 0.5]

cv2.INTER_LINEAR used in Gaussian pyramid

References:

1. https://theailearner.com/2018/12/29/image-processing-bilinear-interpolation/