I think I better understand what you are asking now. Noise is
not random - it's random-looking but is completely based on a
mathematical formula and is repeatable. All the information is
encoded in the formula. This means that you can have a formula
that potentially covers an infinite area, and just use the
formula on the coordinates of the area you need. When you need
an adjacent area, you just re-use the formula on the new
coordinates, and since the formula yields continuous values, the
areas will join seamlessly. Here's a simplified example, using
sine instead of perlin noise for the height generation, and
imagining the world is infinite in the X axis but only 1 unit
high in the Y and Z axes. height(x,y) = sin(x/20) The game
starts, and we generate heights for the nearby area, ie. (0,0)
to (9,0): [0.0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.34, 0.39,
0.43] We have a hill, rising up towards the right. Let's say we
walk to the end of it and need to generate the values from (10,0
to 19,0) now: [0.48, 0.52, 0.5