learnharness/docs/vision-edge-glyph.md

Vision — The edge glyph: follows vs supports

Ideation settled 2026-09-14. Not yet planned, not yet assigned. Companion: vision.md § "Edges" (the relation types) and § "Rendering".

One line: the two relation types become visually distinct — follows renders as a double-shaft arrow (two parallel tails side by side, the implication glyph ⟹), supports stays a single line. Arrows keep their existing direction: prerequisite → dependent.

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Why double-shaft (DECIDED 2026-09-14)

• Semantically exact. follows means prerequisite ⇒ successor — derivability. The implication glyph is the relation; nothing encodes "derivable from" more literally.
• Weight where the structure is. The DAG backbone (follows) gets the heavy glyph; the majority relation (supports is over half of all edges) stays light. The structural skeleton reads first.
• Shape survives everything color can't. Cluster regions already own the color channel; a shape code collides with nothing and survives colorblindness.
• Scale-stable. Pure geometry degrades nowhere — see the rejected alternative below.

Why not dashed (rejected 2026-09-14)

Dash patterns break under our constant screen-space stroke width: the dash array is re-projected across zoom levels, so the pattern pops and crawls instead of scaling cleanly (the same class of artifact as the non-integer blur radii). And with supports the majority relation, dashes would dominate the canvas rather than mark the exception. Dashed = wrong twice.

Geometry

The renderer is the custom 2D canvas; edges draw in the edge layer with shafts + arrowheads from arrowGeometry (sigma-style head, prerequisite → dependent). For follows:

• Both shafts offset along the edge normal by a fixed screen-space gap, converging on a shared single arrowhead.
• The gap is proportional to the constant stroke width — a constant visual ratio at every zoom level.
• No LOD collapse (withdrawn 2026-09-14). The earlier idea of collapsing to a single shaft below a stroke-width threshold assumed width shrinks with zoom — it doesn't; stroke width is constant on screen, so the double shaft renders identically at every zoom level. Simpler than proposed: nothing to gate.

Legend and scope

• Add a legend entry to the existing graph legend: ⟹ follows · → supports.
• Main graph view only. The plan-block renderer (dagre) is untouched — the plan DAG is single-relation, so there is nothing to distinguish there.

Lane

lh-graph's renderer files (graph-canvas.ts, graph-view.tsx). Not assigned, not scheduled.