Phase 8 #5 of REVIEWS/DART_PDF_VS_MAKEPAD_PDF_GAP_ANALYSIS.md
("Advanced transparency"). Design and merge criteria in
REVIEWS/adr/0009-pdf-transparency.md.
The headline defect was not that transparency was missing. `gs` was
MIS-PARSED as CloseStroke:
/GS0 gs 1 0 0 rg 100 100 200 200 re f
-> [CloseStroke, RgbFill, Rectangle, FillWinding]
so every page setting a graphics state gained a stroked path the
document never asked for, and lost its alpha, blend mode and soft mask
silently. Downstream everything was dead: StrokeExtGState/FillExtGState
were never constructed, set_fill_opacity was never called and emitted no
command when it was, and PdfPage::ext_gstate was read by no code at all.
New pdf-graphics/src/transparency.rs: full /ExtGState (ca, CA, BM,
SMask, LW, LC, LJ, ML, D, AIS, TK), all sixteen blend modes including
the four non-separable ones, soft masks with /S, /G, /BC and a /TR
evaluated through ADR 0006's PdfFunction, and /Group parsing. Wired end
to end: parser -> PdfOp::SetExtGState -> device -> RenderCommand ->
Makepad renderer.
Compositing is NOT claimed. Backdrop blending needs render-to-texture,
which an engine-neutral crate has no framebuffer for. Constant alpha is
applied because it needs no backdrop; blend modes and soft masks are
reported through TransparencyError::Unsupported rather than dropped,
because a silently ignored /Multiply looks exactly like a correct
/Normal.
The ADR required a test enumerating every multi-character operator, on
the grounds that fixing the third instance of a shadowing bug (after cm
and rg) without preventing the fourth is not a fix. It immediately found
three more live bugs, none of them transparency-related:
- `b` and `b*` dropped their close-path, mapping to FillStroke* instead
of CloseFillStroke*, so every closed-and-stroked path drew with a gap.
- Text operators within three bytes of the end of a content stream were
mis-parsed: the b'T' arm guarded `*i + 3 < len` while reading only two
bytes, so a stream ending in `/F1 12 Tf` parsed as FillWinding.
And the transparency-group fixture found a fourth:
- PdfPage::xobjects was empty for every page of every document.
extract_xobjects called doc.resolve(), which follows the reference,
then asked the resolved object for as_ref() - always None - and only
accepted a bare dict when every XObject is a stream. No `Do` operator
could be resolved through the page model. Same defect as the one that
once destroyed annotation object references; it now has its own
regression test.
T* and BMC are genuinely unimplemented and are deliberately excluded
from the guard's table rather than papered over.
7 corpus fixtures, 12 acceptance tests, 32 unit tests asserting the
§11.3.5 formulas (not our own output), and a parse_ext_gstate fuzz
target.
TEST_TARGET=pdf 451 -> 497, TEST_TARGET=pdf-ui 495 -> 541.
rustfmt and clippy -D warnings clean.
