proc3d — the canonical A2 font-metric divergence
Accepted delta — structural-match. proc3d (graphs-proc3d / share-proc3d / windows-proc3d) is the textbook A2 font-metric case: a sub-pixel text-measurement difference shifts node x-positions by a few points, and the graph stays structural-match. This page is the standalone work-up referenced from the divergences list.
Input
| Engine | dot |
| Source | tests/graphs/proc3d.gv (from the upstream Graphviz test corpus) — 443 lines |
| Key attrs | fontname=Courier, orientation=land, size="10,7.5", ranksep=1.0 |
Why it diverges (root cause)
The x-network-simplex layout is faithful; the only difference is that the port's font measurer reports some wide labels a fraction of a point wider than FreeType. proc3d's widest labels are the file-path ovals — e.g. /home/ek/work/src/lefty/lefty.c, the exact string known-divergences.md §A2 measures at +0.75 pt (+0.43%). A wider label makes a slightly wider node, whose half-width feeds the ROUND()-ed left-to-right separation constraints; the network simplex then picks a marginally different (equally optimal) integer x-assignment. The result is a near-uniform ≤ 3.55 pt x-shift over a ~2620 pt drawing — rank, order, topology and y-coordinates identical.
The delta — golden vs ours, overlaid
Golden (green) and ours (red) superimposed in the same frame. At full scale they blend to brown — the shift is sub-perceptual (hence structural-match).
Zoomed, the green/red fringe appears almost entirely on the long file-path oval labels — exactly the wide strings the measurer over-measures. The code/box nodes stay coincident:
Full drawings — golden first, ours second
Golden — native dot | Ours — graphviz-ts |
|---|---|
 |  |
Numbers
| metric | value |
|---|---|
| verdict | structural-match |
| maxDelta (port vs native) | 3.55 pt |
| labels shifted in x | 73 / 73 (near-uniform) |
| drawing x-extent | ~2620 pt → shift is 0.13% |
| rank / order / topology / y | identical to C |
Reproduce
The native oracle runs under the headless GVBINDIR (/tmp/ghl, from test/corpus/gen-headless-gvbindir.sh) so both sides use the same estimate_textspan_size measurer — see §A2 “Isolating the algorithm from the font backend”.
# port
GV_TEXT_MEASURER=estimate \
npx tsx test/corpus/render-one.ts ~/git/graphviz/tests/graphs/proc3d.gv dot
# native C oracle (headless, estimate measurer)
GVBINDIR=/tmp/ghl \
~/git/graphviz/build/cmd/dot/dot -Tsvg ~/git/graphviz/tests/graphs/proc3d.gv