From ed04ba2d4e20c7ee6821c4133721f82fe9c092bd Mon Sep 17 00:00:00 2001
From: Cyd <cyd@falloutshelterarcade.com>
Date: Mon, 3 Aug 2026 13:41:39 -0500
Subject: [PATCH] BT410 5.3.98: the ground snap -- the mech walks the arena's
 actual terrain, and there is no gravity

The probe half of the master performance's ground model (binary
@4aa630-4aa6cc, via BT411's raw-asm decode), placed after Simulate's position
integration:

  place the collision volume; drop a probe from the volume's authored bottom
  (origin.y + collisionTemplate->minY); ask the zone's box tree what surface
  lies under it (BoundingBoxTreeNode::FindBoundingBoxUnder); and if the query
  hits (h > 1e-4), put the origin ON that surface exactly --
  origin.y -= (h - lift).

The design is worth stating because it is not the obvious one: THERE IS NO
GRAVITY ANYWHERE IN THE MECH.  Placement is absolute each frame.  Walking
up-slope rides the lift window (an implicit step allowance), walking off a
roof drops instantly, and a probe MISS holds Y -- so a falling-through-the-
world runaway is structurally impossible rather than merely guarded against.

The engine side needed NOTHING reconstructed: Mover's own ctor already
builds collisionTemplate/collisionVolume from the model's BoxedSolidStream
resource and the zone tree was live all along.  One include (boxsolid.hpp --
mover.hpp only forward-declares BoxedSolid, so the extents were an
incomplete type).

LIVE, arena run, no fault -- the y track tells the story by itself:

    y = 20 -> 11.747 -> 10 -> 10 -> 0 -> 19.4049 -> 0

Spawn deck, down a ramp (caught mid-slope), an intermediate level, the arena
floor, up onto a structure, and off it.  Every previous run had y pinned at
its spawn constant for the whole mission.

Deferred with the collision increment: frame rejection on blocking hits, the
crushable-icon sentinel, the crash clip (SetLegAnimation 0x20) on hard
impacts, and the gyro crunch feed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 restoration/source410/BT/MECH.CPP | 63 +++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/restoration/source410/BT/MECH.CPP b/restoration/source410/BT/MECH.CPP
index bb089aee..fbe89f5b 100644
--- a/restoration/source410/BT/MECH.CPP
+++ b/restoration/source410/BT/MECH.CPP
@@ -53,6 +53,9 @@
 #include <joint.hpp>		// Joint / JointSubsystem -- ResolveJoint
 #include <segment.hpp>		// EntitySegment -- the skeleton segment table
 #include <mechdmg.hpp>		// Mech::DamageZone -- the hull zone fill (Pass 3)
+#if !defined(BOXSOLID_HPP)
+#	include <boxsolid.hpp>	// BoxedSolid extents -- the ground-snap probe
+#endif
 #include <dmgtable.hpp>		// DamageLookupTable -- the cylinder hit table
 #include <player.hpp>		// Player::VehicleDeadMessage -- the death notification
 #include <btplayer.hpp>		// BTPlayer::ScoreMessage -- the kill credit
@@ -1527,6 +1530,66 @@ void
 	);
 	localToWorld = localOrigin;
 
+	//
+	//-----------------------------------------------------------------------
+	// THE GROUND SNAP -- the probe half of the master performance's ground
+	// model (binary @4aa630-4aa6cc, decoded from raw asm; there is NO
+	// gravity anywhere in the mech).  Place the collision volume, drop a
+	// probe from the volume's authored bottom, ask the zone's box tree for
+	// the surface under it, and place the origin ON that surface exactly.
+	// Walking up-slope rides the lift window; walking off a roof drops
+	// instantly; a probe MISS (h == -1) holds Y, so a runaway is
+	// structurally impossible.
+	//
+	// The COLLISION half (frame rejection, crush sentinel, crash clip) is a
+	// separate increment -- it hangs off ProcessCollisionList.
+	//
+	// Gated on the engine having built a collision volume for this model
+	// (Mover's ctor does when the resource carries one); logged once if
+	// absent so a silent no-op is visible.
+	//-----------------------------------------------------------------------
+	//
+	if (
+		GetCollisionVolumeCount() > 0 &&
+		collisionVolume != NULL &&
+		collisionTemplate != NULL
+	)
+	{
+		MoveCollisionVolume();
+
+		BoundingBoxTreeNode
+			*ground_node = GetMoverCollisionRoot();
+		if (ground_node != NULL)
+		{
+			Point3D
+				probe = localOrigin.linearPosition;
+			probe.y += collisionTemplate->minY;
+
+			Scalar
+				height = -1.0f;
+			ground_node->FindBoundingBoxUnder(probe, &height);
+
+			if (height > 0.0001f)
+			{
+				Scalar
+					drop = height - collisionTemplate->minY;
+				localOrigin.linearPosition.y -= drop;
+				collisionVolume->minY -= drop;
+				collisionVolume->maxY -= drop;
+				localToWorld = localOrigin;
+			}
+		}
+	}
+	else if (getenv("BT_MECH_LOG"))
+	{
+		static int noVolumeOnce = 0;
+		if (!noVolumeOnce++)
+		{
+			DEBUG_STREAM << "[ground] mech has NO collision volume -- "
+				<< "snap inactive (model streams none?)" << endl << flush;
+		}
+	}
+
 	if (getenv("BT_MECH_LOG"))
 	{
 		static Scalar reportAccum = 0.0f;