feat: mark_sf(merge = TRUE) dissolves adjacent same-fill regions - #142
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A choropleth built from many small features (counties grouped into regions) draws a border between every feature, meshing each colour block with internal seams. `merge = TRUE` dissolves each same-fill style group into one region: each feature's rings are unioned with vellum::vl_path_op() (interpreted even-odd, so holes and multipart features stay correct), producing a single winding path whose shared interior borders are gone -- one crisp outline per region, exact in PDF, no double-stroked seams. Raw coordinates are unioned and the result mapped once (the same map-once discipline as the batched emit). With fewer than two features in a group there is nothing to union, so it falls back to the ordinary emit; and it is skipped on an interactive layer, where the per-feature keys must survive. Docs: NEWS, a spatial-and-networks section, inst/examples/33-merged-regions.R, tests in test-sf-merge.R. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Wires the merged-choropleth half of WI-9:
vl_path_op()as region geometry. (The other half — clipping-as-geometry — stays a documented follow-up, as agreed; it's an architectural change to the viewport-mask pipeline and only benefits vector marks.)What
A choropleth built from many small features (counties grouped into regions) draws a border between every feature, meshing each colour block with internal seams.
mark_sf(merge = TRUE)dissolves each same-fill style group into one region.In
.emit_sf, each feature's rings become an operand (interpreted even-odd, so holes and multipart features stay correct) and are folded withvellum::vl_path_op(op = "union")into a single winding path — shared interior borders gone, one crisp outline per region, exact in PDF. Raw coordinates are unioned and the result mapped once (the same map-once discipline as the batched emit), reusing the venn.R unwrap/rewrap pattern.Boundaries
Verification (WI-0)
Rendered and looked at plain vs merged NC choropleths (internal county borders present → dissolved to one outline per region); confirmed a merged map renders to a valid PDF (
%PDF-, crisp geometry) and throughvellumwidget::as_widget();inst/examples/33-merged-regions.Rruns end to end.Deterministic tests in
test-sf-merge.R: two adjacent squares sharing an edge lose that edge when merged (border-pixel count halves), distinct fills are untouched, a single feature is a no-op, and an NC choropleth renders to PNG/PDF/scene. Full suite[ FAIL 0 | WARN 1 pre-existing | SKIP 0 | PASS 2880 ].No new exports (a
merge =argument onmark_sf()); requires the already-declared vellum >= 0.6.5.🤖 Generated with Claude Code