A z -> y -> x tree of u16 node offsets over 4x4x4-unit cells, each
populated cell naming one 128-byte mask of 1024 visibility bits that the
renderer fills from the camera's cell each frame.
Two sentinels carry meaning and one of them is easy to miss: a z or y
slot of 0 means "no subtree", and an x slot of 0xffff means "no cell".
Reading the latter as a mask index gives a grid whose highest mask is
65535 on every single level, which is the tell.
The check that makes this real is the mask array. It starts at
masks_offset and runs to (highest_mask + 1) * 0x80, and on every level
with a grid that ends at the block boundary exactly -- widest
unexplained tail across the disc is 0 bytes. A wrong tree shape or cell
size would leave the highest mask index elsewhere entirely.
412,521 populated cells over 140,939 masks, across the 24 levels that
have one.
An offset of 0 means the level has no occlusion data at all, which is
not the same as "at the start of the file" -- that is where the tfrags
block lives, and parsing it as a grid produces a mask array that fits
anyway, with 1600 bytes to spare. That is what made level 10 look
plausible on the first pass; three levels (10, 24, 25) carry none.
ReaperOfVeriod
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