Rivers across the plains preset #167

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opened 2026-10-11 20:23:52 +01:00 by liamjd · 0 comments

liamjd commented

2026-10-11 20:23:52 +01:00

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Milestone 2, terrain goal: "Plains map could have rivers running through it".

Proposal

Unlike the canyon (#165), plains have no predefined channel, so the river's path has to be derived:

Choose a source on one map edge and a mouth on another, deterministically from the seed.
Trace the path downhill: a cheapest-path search where climbing costs a lot, rather than straight lines. Rivers that run uphill read wrong instantly, even on a stylised map.
Carve it with #165's shared river step (path + width → lowered, water-masked tiles with stepped falls).
river_count and a width on the plains preset.

The island and crater could use the same step later (a river to the sea, a stream into the lake). Keep the tracing independent of the plains shape so that stays possible.

Decisions needed

Crossing: the same as #165. A plains river splits the map in two, and bridges are out of scope. Fords again are the recommendation, for consistency.
Count and width: one river or several, and whether rivers may merge. Start with one, and don't build merging until it is wanted.

Acceptance criteria

With river_count = 1, a continuous channel runs edge to edge, and its surface never steps up along its course.
Same seed, same river.
#162's reachability and #163's landing guarantee both hold with the river on.
Unit tests for the monotonic-descent property and for continuity (every channel tile is 8-adjacent to the next).
Headed screenshot.

Depends on

#164, and #165 for the shared carving step.

Milestone 2, terrain goal: "Plains map could have rivers running through it". ## Proposal Unlike the canyon (#165), plains have no predefined channel, so the river's path has to be derived: - Choose a source on one map edge and a mouth on another, deterministically from the seed. - Trace the path **downhill**: a cheapest-path search where climbing costs a lot, rather than straight lines. Rivers that run uphill read wrong instantly, even on a stylised map. - Carve it with #165's shared river step (path + width → lowered, water-masked tiles with stepped falls). - `river_count` and a width on the plains preset. The island and crater could use the same step later (a river to the sea, a stream into the lake). Keep the tracing independent of the plains shape so that stays possible. ### Decisions needed - **Crossing:** the same as #165. A plains river splits the map in two, and bridges are out of scope. Fords again are the recommendation, for consistency. - **Count and width:** one river or several, and whether rivers may merge. Start with one, and don't build merging until it is wanted. ## Acceptance criteria - [ ] With `river_count = 1`, a continuous channel runs edge to edge, and its surface never steps **up** along its course. - [ ] Same seed, same river. - [ ] #162's reachability and #163's landing guarantee both hold with the river on. - [ ] Unit tests for the monotonic-descent property and for continuity (every channel tile is 8-adjacent to the next). - [ ] Headed screenshot. ## Depends on #164, and #165 for the shared carving step.

liamjd added this to the Terrain Generation milestone

2026-10-11 20:23:52 +01:00

liamjd added the

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2026-10-11 20:23:52 +01:00