Signs built from hundreds of overlapping paint-stroke drawings (masked
smartphone screens and similar typesetting) sum to far more bitmap area
than the paged ALPHA_8 atlas can hold: the issue sample needs 5 pages of
16M px at 1080p and 19 at 4K against the 4-page cap, so the packer
dropped the painter-order tail - the sign's text and late mask strokes.
Move the packer out of the JNI file into AssPack.c (pure C, compilable
against a desktop libass for verification) and add a composite fallback:
when a frame can never fit MAX_ATLAS_PAGES pages or the vertex budget,
blend the image list CPU-side into one premultiplied RGBA rect over the
union bounding box - O(frame area) instead of O(sum of image areas) -
and draw it as a single quad through a new MODE_COMPOSITE path in the
GL renderer. Oversized composites reuse the existing grow-and-re-render
contract; the atlas fast path is byte-identical for every frame that fits.
Verified with a desktop harness compiling the shipped AssPack.c against
fork libass 0.18.3 and the issue sample: all atlas-mode frames byte-match
the previous packer, the sign's frames composite with zero truncation and
byte-match a reference full-frame blend at 1080p and 4K, and the
multi-page composite grow path round-trips.
close#1868
edde746
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