3-dimensional array to list?

[Medium]

Arrays are already the most compact structure here. And it's not even the arrays (memorywise) that are your problem, the problem is, that the algorithm is constrained to a certain size. (You didn't write it yourself i guess.)

Currently, your arrays are sized like this:
Noise: 8*127*127
Layers: 8*128*128
These are fixed values, and do not depend on image size anywhere in the code you've shown. But because Layers[a, x, y] sometimes is accessed with image width and height for x and y, you run out of bounds on images larger than 128 pixel in any axis. (InterpolateRect() is such a candidate for example, if the passed Rect is >128².) You either need to make the current algo depend on image size, or upsample from 128x128. A list however, won't do anything at all. Especially nothing good.