where x could represent a spatial coordinate, as in image processing. But if x {\displaystyle x\,} represents time ( t ) {\displaystyle (t)\,} , then a moving average defined that way is non-causal (also called non-realizable), because f ( t ) {\displaystyle f(t)\,} depends on future inputs, such as s ( t + 1 ) {\displaystyle s(t+1)\,} . A realizable output is