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

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>

Core/Scripts/BlueprintFormat.cs

@@ -49,7 +49,9 @@ namespace IslaApocalypse.Core
 		// current) inserts the DEPOSIT CAP governor after the carve cap — deposition is
 		// now brush-spread and per-cell bounded. Same reader rule as TDTL: an unknown
 		// body version is skipped whole rather than misread into plausible nonsense.
-		public const ushort EROS_VERSION = 2;
+		// 3 (task 19, current) replaces the single crater exclusion factor with the
+		// protected-core factor, a feather-band factor, and the crater mode byte.
+		public const ushort EROS_VERSION = 3;
 
 		// WSRF quantization: u16, 0 reserved as the no-water sentinel. A real level L
 		// (raw blueprint height units) encodes as 1 + round(L × 32768), so a genuine

Core/Scripts/BlueprintWriter.cs

@@ -176,7 +176,9 @@ namespace IslaApocalypse.Core
 			writer.Write(e.MinSlopeM);                                        // f32
 			writer.Write(e.ErodeRate); writer.Write(e.DepositRate);           // 2 × f32
 			writer.Write(e.Evaporation); writer.Write(e.Gravity);             // 2 × f32
-			writer.Write(e.CraterExclFactor);                                 // f32
+			writer.Write(e.CraterCoreFactor);                                 // f32
+			writer.Write(e.CraterFeatherFactor);                              // f32
+			writer.Write(e.CraterMode);                                       // u8
 		}
 
 		private static void WriteWaterBodyIds(BinaryWriter writer, WorldBlueprint bp)

Core/Scripts/ConfigManager.cs

@@ -79,6 +79,32 @@ namespace IslaApocalypse.Core // Change this if your namespace is different
 		public static float ErosionEvaporation = 0.004f;
 		public static float ErosionGravity = 4.0f;
 
+		// How erosion treats the crater surrounds (task 19). The task-17 hard
+		// 1.2 × CraterRadius cutoff left a visible un-eroded disc: measured, the carve
+		// writes only inside 0.80 × and its displacement is exactly 0 beyond that, so
+		// 620 811 LAND cells of ordinary terrain were being held smooth for no
+		// geometric reason. Now only the deep strike core is protected.
+		//   "full"    — full-strength erosion right up to the core boundary. The
+		//               crater formed after the terrain and has weathered since.
+		//   "feather" — erosion ramps 0→full across CraterErosionCore →
+		//               CraterErosionFeather (the detail pass's shape), so the crater
+		//               reads as younger, less-weathered, and there is no seam at all.
+		// Radii are FACTORS of CraterRadius. The flooded bay and its sea connection do
+		// NOT depend on these: below-sea cells are read-only in both directions.
+		public static string CraterErosionMode = "feather";
+		public static float CraterErosionCore = CRATER_EROSION_CORE_DEFAULT;
+		public static float CraterErosionFeather = CRATER_EROSION_FEATHER_DEFAULT;
+		// 0.80 = the carve's OWN extent (MapGenerator's physicalCraterRadius). Keeping the
+		// core at least this wide is what makes erosion and the carve touch DISJOINT cells,
+		// which is what keeps the post-carve flood guard exactly zero: the carve runs after
+		// erosion and scales height toward the sea target, so it AMPLIFIES any erosion delta
+		// inside its radius and can push a hair-above-sea cell across the waterline. Measured
+		// at a 0.50 core: 79 cells newly below the rendered sea, 113 310 below-sea cells
+		// disturbed. A smaller core reclaims nothing extra either — the over-protected annulus
+		// is 0.80x-1.2x, entirely outside the carve.
+		public const float CRATER_EROSION_CORE_DEFAULT = 0.80f;
+		public const float CRATER_EROSION_FEATHER_DEFAULT = 1.05f;
+
 		// Island falloff shaping (task 11).
 		//
 		// CoastProfile: "wide" adds the submarine shelf — the height curve is identity
@@ -246,6 +272,29 @@ namespace IslaApocalypse.Core // Change this if your namespace is different
 			ErosionCarveCap = Mathf.Clamp(ErosionCarveCap, 0f, 60f);
 			if (rawCount != ErosionDropletCount || rawLife != ErosionDropletLifetime || rawCap != ErosionCarveCap)
 				GD.PrintErr($"[ConfigManager] Erosion governor out of bounds — clamped: count {rawCount}->{ErosionDropletCount}, lifetime {rawLife}->{ErosionDropletLifetime}, cap {rawCap}->{ErosionCarveCap} m.");
+			// Crater erosion treatment (task 19)
+			if (data.ContainsKey("CraterErosionMode"))
+			{
+				string cm = (string)data["CraterErosionMode"];
+				if (cm == "full" || cm == "feather")
+					CraterErosionMode = cm;
+				else
+					GD.PrintErr($"[ConfigManager] Unknown CraterErosionMode '{cm}'. Keeping '{CraterErosionMode}'.");
+			}
+			if (data.ContainsKey("CraterErosionCore")) CraterErosionCore = (float)data["CraterErosionCore"];
+			if (data.ContainsKey("CraterErosionFeather")) CraterErosionFeather = (float)data["CraterErosionFeather"];
+			CraterErosionCore = Mathf.Clamp(CraterErosionCore, 0f, 3f);
+			CraterErosionFeather = Mathf.Clamp(CraterErosionFeather, 0f, 4f);
+			// A feather band that does not extend past the core is not a band; say so
+			// rather than silently behaving like "full".
+			if (CraterErosionMode == "feather" && CraterErosionFeather <= CraterErosionCore)
+			{
+				GD.PrintErr($"[ConfigManager] CraterErosionFeather {CraterErosionFeather:F2} must exceed " +
+					$"CraterErosionCore {CraterErosionCore:F2} — the ramp would have zero width. " +
+					$"Restoring {CRATER_EROSION_FEATHER_DEFAULT:F2}.");
+				CraterErosionFeather = CRATER_EROSION_FEATHER_DEFAULT;
+			}
+
 			// Negative is meaningless; 0 is the documented "unbounded" escape hatch.
 			ErosionDepositCap = Mathf.Clamp(ErosionDepositCap, 0f, 60f);
 			ErosionSeaMargin = Mathf.Clamp(ErosionSeaMargin, 0f, 5f);

Core/Scripts/MapDataParser.cs

@@ -118,7 +118,8 @@ namespace IslaApocalypse.Core
 		public float CarveCapM, DepositCapM, SeaMarginM;
 		public float Inertia, CapacityFactor, MinSlopeM;
 		public float ErodeRate, DepositRate, Evaporation, Gravity;
-		public float CraterExclFactor;
+		public float CraterCoreFactor, CraterFeatherFactor;
+		public byte CraterMode;   // 0 = full, 1 = feather
 	}
 
 	public class WorldBlueprint
@@ -515,7 +516,8 @@ namespace IslaApocalypse.Core
 			e.MinSlopeM = reader.ReadSingle();
 			e.ErodeRate = reader.ReadSingle(); e.DepositRate = reader.ReadSingle();
 			e.Evaporation = reader.ReadSingle(); e.Gravity = reader.ReadSingle();
-			e.CraterExclFactor = reader.ReadSingle();
+			e.CraterCoreFactor = reader.ReadSingle(); e.CraterFeatherFactor = reader.ReadSingle();
+			e.CraterMode = reader.ReadByte();
 			blueprint.Erosion = e;
 			return true;
 		}

Tools/Scripts/HydraulicErosion.cs

@@ -38,9 +38,12 @@ using System;
 /// read-only — never eroded, never deposited on. Land stays land, sea stays sea;
 /// the rendered coastline cannot move. Deposition only raises land cells.
 ///
-/// The crater exclusion: no cell within CraterExclRadius of the impact centre is
+/// The crater treatment (task 19): no cell within the protected strike CORE is
-/// modified (droplets may traverse). The carve remains the final authority on its
+/// modified (droplets may traverse), and outside it either FULL strength applies
-/// own terrain.
+/// immediately or FEATHER ramps in across a band. The carve remains the final
+/// authority on the deep bowl; the bay's sea connection is guaranteed by the sea
+/// clamp rather than by the exclusion, since below-sea cells are read-only in
+/// both directions.
 ///
 /// Heights in the array are raw blueprint units (1 unit = 251 m). All sediment
 /// accounting below is done in METRES and converted only when a delta is applied,
@@ -62,9 +65,25 @@ public static class HydraulicErosion
 
 	public const float M_PER_UNIT = 251f;
 
-	// Crater exclusion factor: erosion stays outside 1.2 × CraterRadius — fully
+	// --- Crater treatment (task 19) ---
-	// clear of both the physical carve (0.80×) and the detail feather (1.05×).
+	//
-	public const float CRATER_EXCL_FACTOR = 1.2f;
+	// Task 17 used a hard 1.2 × CraterRadius cutoff. Measured on seed 1280587109
+	// (task-19 radius dump): the carve writes only inside 0.80 × (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 — a
+	// visible un-eroded disc against dissected ground, with a hard edge.
+	//
+	// The protected core is now the deep strike zone only. The bay itself needs no
+	// exclusion: below-sea cells are read-only in both directions (the sea clamp),
+	// so erosion can neither carve the bay's sea connection open nor silt it shut.
+	// The core exists to stop the BOWL being dissected on seeds where it holds land
+	// (on 1280587109 there is no land at all inside 0.50 ×, so the core is
+	// functionally redundant there — the guard is for the general seed).
+	public const float CRATER_CORE_FACTOR_DEFAULT = 0.50f;    // ×CraterRadius
+	public const float CRATER_FEATHER_FACTOR_DEFAULT = 1.05f; // ×CraterRadius, FEATHER only
+
+	public const byte CRATER_MODE_FULL = 0;
+	public const byte CRATER_MODE_FEATHER = 1;
 
 	// Spawn: droplets source in the mountains, never the ocean. A land point is
 	// accepted with probability SPAWN_FLOOR + (1-SPAWN_FLOOR) · relative elevation,
@@ -91,6 +110,7 @@ public static class HydraulicErosion
 		public float DepositRate;     // fraction of surplus sediment dropped per step
 		public float Evaporation;     // water lost per step (fraction)
 		public float Gravity;         // speed gain per metre of drop
+		public byte CraterMode;       // CRATER_MODE_FULL | CRATER_MODE_FEATHER (task 19)
 		public int Seed;              // resolvedSeed + SEED_OFFSET
 	}
 
@@ -130,7 +150,7 @@ public static class HydraulicErosion
 	/// bound is violated on exit — the caller treats that as a build failure.
 	/// </summary>
 	public static Stats Apply(float[,] height, int mapSize, float[,] seaMap, float seaFlat,
-		float craterCx, float craterCy, float craterExclRadius, Params p)
+		float craterCx, float craterCy, float craterCoreRadius, float craterFeatherRadius, Params p)
 	{
 		var stats = new Stats();
 		var rng = new Pcg32(p.Seed);
@@ -173,12 +193,25 @@ public static class HydraulicErosion
 			for (int j = 0; j < brushN; j++) brushW[j] /= wSum;
 		}
 
-		float exclSq = craterExclRadius * craterExclRadius;
 		float SeaAt(int cx, int cy) => seaMap != null ? seaMap[cx, cy] : seaFlat;
-		bool Excluded(int cx, int cy)
+
+		// Crater weight (task 19): 0 inside the protected strike core, 1 where erosion
+		// runs at full strength. FULL steps straight to 1 at the core boundary; FEATHER
+		// ramps linearly out to craterFeatherRadius, mirroring the detail pass's shape,
+		// so the crater reads as younger/less-weathered with no seam. Amounts are SCALED
+		// by this rather than skipped, which is what makes FEATHER a one-liner.
+		float coreSq = craterCoreRadius * craterCoreRadius;
+		bool feather = p.CraterMode == CRATER_MODE_FEATHER
+			&& craterFeatherRadius > craterCoreRadius;
+		float CraterWeight(int cx, int cy)
 		{
 			float ddx = cx - craterCx, ddy = cy - craterCy;
-			return ddx * ddx + ddy * ddy < exclSq;
+			float d2 = ddx * ddx + ddy * ddy;
+			if (d2 < coreSq) return 0f;
+			if (!feather) return 1f;
+			float d = MathF.Sqrt(d2);
+			if (d >= craterFeatherRadius) return 1f;
+			return (d - craterCoreRadius) / (craterFeatherRadius - craterCoreRadius);
 		}
 
 		for (int drop = 0; drop < p.DropletCount; drop++)
@@ -261,12 +294,13 @@ public static class HydraulicErosion
 						{
 							int cx = xi + brushDx[b], cy = yi + brushDy[b];
 							if (cx < 0 || cx >= mapSize || cy < 0 || cy >= mapSize) continue;
-							if (Excluded(cx, cy)) continue;
+							float wCrater = CraterWeight(cx, cy);
+							if (wCrater <= 0f) continue;
 							float hCell = height[cx, cy];
 							// Below-sea cells are read-only in BOTH directions: no
 							// submarine deltas, so the rendered coastline cannot move.
 							if (hCell < SeaAt(cx, cy)) continue;
-							float give = amountM * brushW[b];
+							float give = amountM * brushW[b] * wCrater;
 							// Governor 4: the ledger read the other way. net is negative
 							// where the cell has already been built up, so the headroom
 							// is cap + net.
@@ -293,11 +327,12 @@ public static class HydraulicErosion
 						{
 							int cx = xi + brushDx[b], cy = yi + brushDy[b];
 							if (cx < 0 || cx >= mapSize || cy < 0 || cy >= mapSize) continue;
-							if (Excluded(cx, cy)) continue;
+							float wCrater = CraterWeight(cx, cy);
+							if (wCrater <= 0f) continue;
 							float sea = SeaAt(cx, cy);
 							float hCell = height[cx, cy];
 							if (hCell < sea) continue; // below-sea cells are read-only
-							float want = amountM * brushW[b];
+							float want = amountM * brushW[b] * wCrater;
 							float bySea = MathF.Max(0f, (hCell - (sea + p.SeaMarginM / M_PER_UNIT)) * M_PER_UNIT);
 							float byCap = MathF.Max(0f, p.CarveCapM - net[cx, cy]);
 							float take = MathF.Min(want, MathF.Min(bySea, byCap));

Tools/Scripts/MapGenerator.cs

@@ -53,6 +53,12 @@ public partial class MapGenerator : TextureRect
 	// Hydraulic erosion (task 17): output-only droplet pass on the RENDER map,
 	// after detail, before the crater carve. The classify map never sees it.
 	private bool _erosionOn;
+	private byte _craterErosionMode;
+
+	// The carve's radius as a fraction of CraterRadius — the ONLY place this number
+	// lives on the generator side. Erosion's protected core must be at least this
+	// wide or the two passes overlap; see the guard before the erosion call.
+	private const float CRATER_CARVE_FACTOR = 0.80f;
 
 	// Task-10 detail passes (shelf micro-relief + shelf-edge variation): gated by
 	// TerrainDetail, active only with the curve on (both are defined in terms of
@@ -114,6 +120,8 @@ public partial class MapGenerator : TextureRect
 		_curveKnots = ConfigManager.TerrainCurve == "v5" ? HeightCurve.V5 : null;
 		_curveOn = _curveKnots != null;
 		_erosionOn = ConfigManager.Erosion == "v1";
+		_craterErosionMode = ConfigManager.CraterErosionMode == "feather"
+			? HydraulicErosion.CRATER_MODE_FEATHER : HydraulicErosion.CRATER_MODE_FULL;
 		// (The monotonicity assertion now runs inside GenerateTopography, against the
 		// effective per-seed curve, once hMaxSeed is known.)
 		// Classify gets its own array whenever ANY render-only pass diverges the two
@@ -392,7 +400,9 @@ public partial class MapGenerator : TextureRect
 				DepositRate = ConfigManager.ErosionDepositRate,
 				Evaporation = ConfigManager.ErosionEvaporation,
 				Gravity = ConfigManager.ErosionGravity,
-				CraterExclFactor = HydraulicErosion.CRATER_EXCL_FACTOR
+				CraterCoreFactor = ConfigManager.CraterErosionCore,
+				CraterFeatherFactor = ConfigManager.CraterErosionFeather,
+				CraterMode = _craterErosionMode
 			} : null,
 			TerrainDetail = _detailOn ? new TerrainDetailInfo
 			{
@@ -638,7 +648,7 @@ public partial class MapGenerator : TextureRect
 		// `raw`; neither can move a column below the red ceiling or above the cap,
 		// because K2 and K6 never move and the curve is monotonic between them.
 		float reliefAmpRaw = ConfigManager.ShelfReliefAmp / 251f;
-		float physicalCraterRadius = _impactRadius * 0.80f;
+		float physicalCraterRadius = _impactRadius * CRATER_CARVE_FACTOR;
 		for (int x = 0; x < MapSize; x++)
 		{
 			for (int y = 0; y < MapSize; y++)
@@ -700,6 +710,17 @@ public partial class MapGenerator : TextureRect
 					for (int y = 0; y < MapSize; y++)
 						seaMap[x, y] = GetSeaLevel(_tempMap[x, y]);
 			}
+			// Erosion's protected core must cover the carve, or the two passes touch the
+			// same cells and the carve — which runs AFTER erosion and scales height
+			// toward the sea target — amplifies erosion's delta across the waterline.
+			// Refusing to fail silently: the in-pass flood guard below cannot see this,
+			// because it measures before the carve.
+			if (ConfigManager.CraterErosionCore < CRATER_CARVE_FACTOR)
+				GD.PrintErr($"[MapGenerator] ⚠ CraterErosionCore {ConfigManager.CraterErosionCore:F2} is " +
+					$"inside the carve radius ({CRATER_CARVE_FACTOR:F2} × CraterRadius). Erosion will modify " +
+					"carve-authored terrain, and the crater carve will then amplify those deltas — expect a " +
+					"handful of rendered-waterline crossings inside the bay that the erosion flood guard cannot see.");
+
 			long wetBefore = CountRenderWaterPixels(seaMap, seaFlat);
 
 			var p = new HydraulicErosion.Params
@@ -717,17 +738,22 @@ public partial class MapGenerator : TextureRect
 				DepositRate = ConfigManager.ErosionDepositRate,
 				Evaporation = ConfigManager.ErosionEvaporation,
 				Gravity = ConfigManager.ErosionGravity,
+				CraterMode = _craterErosionMode,
 				Seed = _noise.Seed + HydraulicErosion.SEED_OFFSET
 			};
 			var st = HydraulicErosion.Apply(_heightMap, MapSize, seaMap, seaFlat,
 				_impactCenter.X, _impactCenter.Y,
-				_impactRadius * HydraulicErosion.CRATER_EXCL_FACTOR, p);
+				_impactRadius * ConfigManager.CraterErosionCore,
+				_impactRadius * ConfigManager.CraterErosionFeather, p);
 
 			long wetAfter = CountRenderWaterPixels(seaMap, seaFlat);
 			if (wetAfter != wetBefore)
 				throw new System.InvalidOperationException(
 					$"[MapGenerator] EROSION FLOOD-GUARD VIOLATION: render-map water pixels {wetBefore} -> {wetAfter}. Refusing to generate.");
 
+			GD.Print($"{T()} [Erosion] crater '{ConfigManager.CraterErosionMode}': core {_impactRadius * ConfigManager.CraterErosionCore:F0} px" +
+				(ConfigManager.CraterErosionMode == "feather"
+					? $" -> feather to {_impactRadius * ConfigManager.CraterErosionFeather:F0} px" : " (hard edge, full strength beyond)") + ".");
 			GD.Print($"{T()} [Erosion] v1: {st.Spawned} droplets ({st.SkippedNoLand} skipped), {st.Steps} steps, " +
 				$"{(Time.GetTicksMsec() - tEro0) / 1000.0:F1}s wall. Eroded {st.ErodedVolumeM3:F0} m³ over {st.ModifiedCells} touched cells " +
 				$"(max cell carve {st.MaxCellErosionM:F2} m vs cap {p.CarveCapM:F2} m), deposited {st.DepositedVolumeM3:F0} m³ " +

Tools/Scripts/RoundTripHarness.cs

@@ -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; }