Range measured: level 0, 2192576..2615232, the 422,656-byte sky region.
Periodicity start and end located by sliding an 8 KB window in 256-byte steps
through the region and measuring the 256-stride match rate.
Method: autocorrelation, then a periodicity scan. A random control of the
same size reads flat at 0.0000 for every stride tested.
The peak stride in the first 64 KB is 256 at a 0.772 match rate. Strides 255,
257 and 4 score nearly as high, which is what a genuine period looks like when
sampled rather than assumed -- a useful contrast with the 148-byte "stride"
that held for eight values and then broke.
Across levels the peak is 256 on 21, 512 on 5 and 128 on 1. 512 and 128 are a
harmonic and a sub-multiple of 256, so the fundamental period is 256 bytes.
Within level 0 the periodicity holds from +0 to about +60160 and then stops:
236 rows of 256 bytes, i.e. a 256-wide by 236-tall image at one byte per
pixel. The remaining ~362,240 bytes -- 86% of the region -- are not
256-periodic and stay unidentified. That is presumably what the layer offset
table at +0x20 points into.
The sky region is therefore not one thing. Its first ~60 KB is an image and
is the tractable half; the rest is still open, as is the non-uniform header
where level 2 differs from every other level.
ReaperOfVeriod
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