I still don't understand how this type of parallax scrolling is possible. There don't appear to be any layers of sprites, and the background looks like all one solid 3D object.
How exactly was this accomplished?
How exactly, I can't say (wasn't any of the programmers).
Well, I assume the background isn't really 3d and in that case there can't be
any perspective projection of 3d points. So I think the effect rests on the
principle of pseudo perspective. That is to say; if you know how an object
scales under perspective projection (the moon in this case, it's part of a
sphere), then you can derive scaling factors which mimics the scale of the
object on the screen and use these scaling factors to scale/distort the
original 2d image creating a pseudo perspective. That means, instead of using
the real scaling factors for creating a perspective projection, i.e. in using
the real depth coordinate, you simply create your own depth based on some
knowledge.
For example. Standard perspective projection of a point (x,y,z) boils down to
x' = x/z
y' = y/z
with (x',y') its 2d projection.
If we don't know z, let's build our own one, i.e. z = f(x,y), hence
x' = x/f(x,y)
y' = y/f(x,y)
You now have to find an f(x,y) that somehow mimics the perspective projection
of a part of a sphere. One thing is for sure, the scaling isn't linear. A
simple (and easy to compute) function is a parabolic one, which has enough
curvature to fake the curvature of a sphere when being close enough. So you
will basically fiddle with some quadratic terms to get the scaling done in
such a way that it mimics some 3d curvature.
Don't know whether it was done this way, but that's how I would do it. It's
actually pretty simple, because you just have to find the right scaling
factors and scale your image with it while it scrolls into view.
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