Character was speeding up beyond the requested speed when sliding along a wall (#507)

After normal movement a stair walk could slide along a wall instead of stepping over it, this caused the total movement to be larger than what should be possible during normal movement.

Jolt/Physics/Character/CharacterVirtual.cpp

@@ -1168,13 +1168,42 @@ bool CharacterVirtual::WalkStairs(float inDeltaTime, Vec3Arg inStepUp, Vec3Arg i
 		DebugRenderer::sInstance->DrawArrow(mPosition, up_position, Color::sWhite, 0.01f);
 #endif // JPH_DEBUG_RENDERER
 
+	// Collect normals of steep slopes that we would like to walk stairs on.
+	// We need to do this before calling MoveShape because it will update mActiveContacts.
+	Vec3 character_velocity = inStepForward / inDeltaTime;
+	Vec3 horizontal_velocity = character_velocity - character_velocity.Dot(mUp) * mUp;
+	std::vector<Vec3, STLTempAllocator<Vec3>> steep_slope_normals(inAllocator);
+	steep_slope_normals.reserve(mActiveContacts.size());
+	for (const Contact &c : mActiveContacts)
+		if (c.mHadCollision
+			&& c.mSurfaceNormal.Dot(horizontal_velocity - c.mLinearVelocity) < 0.0f // Pushing into the contact
+			&& IsSlopeTooSteep(c.mSurfaceNormal)) // Slope too steep
+			steep_slope_normals.push_back(c.mSurfaceNormal);
+	JPH_ASSERT(!steep_slope_normals.empty(), "CanWalkStairs should have returned false!");
+
 	// Horizontal movement
 	RVec3 new_position = up_position;
-	MoveShape(new_position, inStepForward / inDeltaTime, inDeltaTime, nullptr, inBroadPhaseLayerFilter, inObjectLayerFilter, inBodyFilter, inShapeFilter, inAllocator);
-	float horizontal_movement_sq = Vec3(new_position - up_position).LengthSq();
+	MoveShape(new_position, character_velocity, inDeltaTime, nullptr, inBroadPhaseLayerFilter, inObjectLayerFilter, inBodyFilter, inShapeFilter, inAllocator);
+	Vec3 horizontal_movement = Vec3(new_position - up_position);
+	float horizontal_movement_sq = horizontal_movement.LengthSq();
 	if (horizontal_movement_sq < 1.0e-8f)
 		return false; // No movement, cancel
 
+	// Check if we made any progress towards any of the steep slopes, if not we just slid along the slope
+	// so we need to cancel the stair walk or else we will move faster than we should as we've done
+	// normal movement first and then stair walk.
+	bool made_progress = false;
+	float max_dot = -0.05f * inStepForward.Length();
+	for (const Vec3 &normal : steep_slope_normals)
+		if (normal.Dot(horizontal_movement) < max_dot)
+		{
+			// We moved more than 5% of the forward step against a steep slope, accept this as progress
+			made_progress = true;
+			break;
+		}
+	if (!made_progress)
+		return false;
+
 #ifdef JPH_DEBUG_RENDERER
 	// Draw horizontal sweep
 	if (sDrawWalkStairs)

UnitTests/Physics/CharacterVirtualTests.cpp

@@ -7,6 +7,7 @@
 #include <Jolt/Physics/Collision/Shape/CapsuleShape.h>
 #include <Jolt/Physics/Collision/Shape/RotatedTranslatedShape.h>
 #include <Jolt/Physics/Collision/Shape/MeshShape.h>
+#include <Jolt/Physics/Collision/Shape/BoxShape.h>
 #include <Jolt/Physics/Character/CharacterVirtual.h>
 #include "Layers.h"
 
@@ -614,4 +615,33 @@ TEST_SUITE("CharacterVirtualTests")
 		}
 		CHECK_APPROX_EQUAL(character.mCharacter->GetPosition(), RVec3(cCylinderLength, 0, 1), 1.0e-4f);
 	}
+
+	TEST_CASE("TestStairWalkAlongWall")
+	{
+		// Stair stepping is very delta time sensitive, so test various update frequencies
+		float frequencies[] = { 60.0f, 120.0f, 240.0f, 360.0f };
+		for (float frequency : frequencies)
+		{
+			float time_step = 1.0f / frequency;
+
+			PhysicsTestContext c(time_step);
+			c.CreateFloor();
+
+			// Create character
+			Character character(c);
+			character.Create();
+
+			// Create a wall
+			const float cWallHalfThickness = 0.05f;
+			c.GetBodyInterface().CreateAndAddBody(BodyCreationSettings(new BoxShape(Vec3(50.0f, 1.0f, cWallHalfThickness)), RVec3(0, 1.0_r, Real(-character.mRadiusStanding - character.mCharacter->GetCharacterPadding() - cWallHalfThickness)), Quat::sIdentity(), EMotionType::Static, Layers::NON_MOVING), EActivation::DontActivate);
+
+			// Start moving along the wall, if the stair stepping algorithm is working correctly it should not trigger and not apply extra speed to the character
+			character.mHorizontalSpeed = Vec3(5.0f, 0, -1.0f);
+			character.Simulate(1.0f);
+
+			// We should have moved along the wall at the desired speed
+			CHECK(character.mCharacter->GetGroundState() == CharacterBase::EGroundState::OnGround);
+			CHECK_APPROX_EQUAL(character.mCharacter->GetPosition(), RVec3(5.0f, 0, 0), 1.0e-2f);
+		}
+	}
 }