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fix: shrink erosion's crater exclusion to the strike core; add FULL/FEATHER modes (terrain-water task 19)

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Task 17's hard 1.2 x CraterRadius cutoff left a visible un-eroded disc with a hard
edge. Measured (radius dump, seed 1280587109): the carve writes only inside 0.80 x
(640 px) and its displacement is EXACTLY 0 beyond that, so the 640-960 px annulus
was 620,811 LAND cells of ordinary terrain held smooth for no geometric reason.
That annulus is now eroded — 98% of those cells are touched.

The protected core is the carve's own extent (CraterErosionCore, 0.80 x).
CraterErosionMode selects the transition: "full" applies full strength at the core
boundary (older-crater look), "feather" ramps 0->full out to CraterErosionFeather
(1.05 x, mirroring the detail pass) for a younger-crater look with no seam.
Implemented as a WEIGHT that scales carve and deposit amounts, not a skip, which is
what makes feather a one-liner. Default "feather" pending the gate.

Core = the carve radius is load-bearing, not tidy: the carve runs AFTER erosion and
scales height toward the sea target, so it amplifies any erosion delta inside its
radius. Measured at a 0.50 core: 79 cells newly below the rendered sea and 113,310
below-sea cells disturbed — the in-pass flood guard cannot see this because it
measures before the carve. At 0.80 the two passes touch disjoint cells and the
guarantee is exact again (0/0/0). A narrower core also reclaims nothing extra, since
the over-protected annulus lies entirely outside the carve. MapGenerator now owns
CRATER_CARVE_FACTOR as the single source of that 0.80 and warns loudly if the
configured core is narrower.

Verified both modes, seed 1280587109: 1_biomes/0_water md5-identical to erosion-OFF,
BIOM/WBID bitwise equal, 0 newly below/above sea, 0 below-sea cells modified, 0
cells modified inside the core, island top 457.65 m unchanged, isotropy 1.005/1.007.
Bay-to-ocean connection demonstrated explicitly by flood fill through rendered
water to the map's north border (2,122,935 cells), not merely argued from the sea
clamp.

EROS body version -> 3: craterExclFactor replaced by craterCoreFactor +
craterFeatherFactor + craterMode. Round-trip harness PASS with a real v3 payload.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Stewart Howe 2026-08-10 05:50:22 -04:00
parent 85e7bceb72
commit f810ff44dc
7 changed files with 141 additions and 24 deletions
Download patch file Download diff file Expand all files Collapse all files

7
Tools/Scripts/RoundTripHarness.cs
View file

@ -257,11 +257,12 @@ public partial class RoundTripHarness : Node
var ea = a.Erosion; var eb = b.Erosion;
bool same = ea.Version == eb.Version
&& ea.DropletCount == eb.DropletCount && ea.Lifetime == eb.Lifetime
&& ea.BrushRadius == eb.BrushRadius && ea.SeedOffset == eb.SeedOffset;
&& ea.BrushRadius == eb.BrushRadius && ea.SeedOffset == eb.SeedOffset
&& ea.CraterMode == eb.CraterMode;
float[] fa = { ea.CarveCapM, ea.DepositCapM, ea.SeaMarginM, ea.Inertia, ea.CapacityFactor, ea.MinSlopeM,
ea.ErodeRate, ea.DepositRate, ea.Evaporation, ea.Gravity, ea.CraterExclFactor };
ea.ErodeRate, ea.DepositRate, ea.Evaporation, ea.Gravity, ea.CraterCoreFactor, ea.CraterFeatherFactor };
float[] fb = { eb.CarveCapM, eb.DepositCapM, eb.SeaMarginM, eb.Inertia, eb.CapacityFactor, eb.MinSlopeM,
eb.ErodeRate, eb.DepositRate, eb.Evaporation, eb.Gravity, eb.CraterExclFactor };
eb.ErodeRate, eb.DepositRate, eb.Evaporation, eb.Gravity, eb.CraterCoreFactor, eb.CraterFeatherFactor };
for (int i = 0; i < fa.Length; i++)
if (System.BitConverter.SingleToInt32Bits(fa[i]) != System.BitConverter.SingleToInt32Bits(fb[i])) same = false;
if (!same) { GD.PrintErr("[Harness] EROS fields differ."); return false; }