In Images as Signals, I tried to show how an image like this:
could be thought of as a signal of lightness ... a measure of how light each pixel is. I also drew a graph like this:
of that signal. Each of the ramps in that graph is one horizontal row of pixels, read from left to right (darker to lighter). So if you think about two pixels next to each other on the same row, their lightness measurements on the signal might be like this:
In other words, the pixel to the left, which is slightly darker, occurs in the signal just before the slightly lighter pixel to the right of it.
If, on the other hand, we want to locate two pixels, one above the other, with the same lightness, their positions on the signal might look like this:
5 days ago