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BIF reduced levels stitched via the openslide subtile model — DP + legacy iScan (#83, #80)

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v0.56.0
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BIF reduced pyramid levels are stitched via the openslide subtile model — DP
and legacy iScan, correctly this time (#83, #80).

Fixed

  • BIF reduced levels (L1+) now composite via the openslide Ventana subtile
    model     (#83 DP + #80 legacy). A stored reduced tile is the downsample of a
    2ⁱ×2ⁱ block of L0 camera frames, with the inter-frame overlap baked inside
    its pixels. The earlier whole-tile placement (v0.55.0) could only remove
    overlap at tile boundaries, so it over-compacted the dense legacy case
    ("very broken on zoom", reverted in v0.55.1). Reading openslide's
    openslide-vendor-ventana.c showed the correct approach: decompose each
    reduced tile into per-L0-frame subtiles and place each subtile at that
    frame's own compacted position (L0 origin >> i), sourced from stored tile
    (col>>i, row>>i) cropped to its (col%2ⁱ, row%2ⁱ) quadrant. Every frame
    lands at its stitched spot, removing all overlap (internal + boundary) →
    an exact 2× pyramid that matches L0 downsampled (validated: Ventana-1 L1
    content MAD 3.8 vs L0÷2; OS-1 legacy L1 MAD 2.7, vs the broken whole-tile
    layout's 33). DP and legacy now share one path. Size = L0 hull
    floor-halved; Grid + tile bytes unchanged.
  • Legacy iScan reduced-level Size / Downsample are stitched again (the
    v0.55.1 revert to the raw frame grid is superseded): OS-1 L1 52909×46962
    (= L0 hull ÷ 2, the openslide content extent), inter-level ratio ~2.0.

Added

  • subtileLayout — an internal optional extension of the regionLayout
    capability: UnitSize(level) + SubtileSource(level, col, row). The shared
    compositors (ReadRegion / StitchedTile and the ScaledStrips iterator)
    decode the mapped source tile and blit its cropped quadrant, so all three
    pixel-reassembly paths handle subtiles uniformly.

Changed

  • BIF reduced levels report Level.Overlapping = true for both DP and
    legacy iScan (whenever L0 was overlap-compacted). Consumers on the #71
    Overlapping contract route reduced-level reads through the region API and
    get the stitch-aligned output. Tile bytes and Grid are unchanged.
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