An image may be defined as a 2-dimensional function f(x, y) where x and y are…

In digital image processing, a monochrome image is modeled as a two-dimensional function f(x, y). The independent variables x and y specify a position within the image plane -- they are the image's spatial coordinates. The value the function takes at any such position is a physical light measurement, called the intensity (or gray level) of the image at that point.

Matching the stem to this standard definition: the first blank refers to x and y themselves, which are positions in the plane, so it is filled by "coordinates"; the second blank refers to the amplitude of f at the point (x, y), which is the physical light measurement at that position, so it is filled by "intensity". This gives the pairing "Coordinates; intensity".

• "Intensity; coordinates" swaps the two roles -- it calls the position variables x, y an intensity and calls the function's value a set of coordinates, exactly the reverse of what each quantity actually is.

• "Intensity; brightness" repeats the same role-swap for the first blank, and its second term, brightness, names the perceived visual sensation of light rather than the specific physical value the definition assigns to f(x, y) -- the two are related but not interchangeable in this technical definition.

• "Brightness; coordinates" again swaps the roles, and the light-related term it picks for the first blank is not even the specific quantity the definition reserves for the second blank.

This matches the classic definition used in digital image processing textbooks (for example, Gonzalez and Woods): x, y are the spatial coordinates of the image plane, and the amplitude of f at (x, y) is called the intensity of the image at that point. So the correct pairing is "Coordinates; intensity".