A river is sub-pixel at almost every Atlas scale (a 100 m channel against
Global's ~17.6 km per screen pixel), so drawing it to scale draws nothing.
The line becomes a symbol whose job is legibility: a fixed 5 px screen-space
stroke, contiguous through the river's own cell centres, never drawn over
water, and culled when too small to read.
The cull threshold derives from the stroke rather than being stipulated. A
mark reads as a line at roughly 3x its own width, so the minimum is 15 px of
on-screen length — and the kilometre thresholds then fall out of each rung's
scale: 264 km at Global, 2.8 km at Region, 28 m at District. A level-of-detail
ladder with no hand-tuned constants, which self-corrects if the width changes.
Measured on the visible extent, not total river length: a course crossing the
window always spans it and passes, so only a course wholly inside the view and
small is culled. Correct at both ends of the ladder, and no new wire field.
Records a tension that had to be resolved rather than split. Jeroen proposed a
flat 100 km cutoff, then noted that 5x5 px still reads as a dot and asked for
15-20. Those are incompatible: the km threshold depends only on the pixel
length, so 100 km implies ~6 px, which is dot-shaped at a 5 px stroke.
Admitting 100 km rivers AND keeping them line-shaped needs a ~2 px stroke.
Thick lines mean fewer rivers; thin lines mean more. Ruled for the bold stroke.
Client-side, and provably so: the cull is measured in screen pixels, and the
display ratio is a client-side viewport-dependent parameter that never reaches
a wire request or cache key, so the server cannot know how many pixels a
course occupies. Water-clipping needs nothing new either — the per-cell
classification is already in the adopted canvas.
Built on the 2026-07-27 measurement in D-258's amendment: 375 courses present,
458 of 518,400 pixels different. The rivers were not failing to render, they
were correctly beneath notice; the defect was drawing specks instead of
drawing properly or not at all.
T-1237 implements. T-1238 restores size-varying width as polish, deliberately
deferred — noting width and cull are coupled, since a thinner stroke lowers
its own visibility threshold.
Co-Authored-By: Claude <noreply@anthropic.com>
jpmschweitzerandClaude
f5044926da