When we compute the complexity of an image processing algorithm, we get an $O(N)$.does the $N$ refers to the number of pixels in the image or to the height/width of the image, I mean it is computed related to $O(nm)$ with $n\times m$ the size of the image ?  please, cite some references.
When we compute the complexity of a given algorithm related to image processing does the N refers to the number of Pixels in the image?
algorithms;complexity