From cd126aae8f5d172e91df96c0dfd8bfc6f121f0c2 Mon Sep 17 00:00:00 2001
From: "john.mccutchan" <john.mccutchan@08e121b0-ae19-0410-a57b-3be3395fd4fd>
Date: Mon, 9 Feb 2009 16:28:44 +0000
Subject: [PATCH] Height field terrain shape fixes from tomva1@yahoo.com

---
 .../btHeightfieldTerrainShape.cpp             | 20 ++++++++++---------
 .../btHeightfieldTerrainShape.h               | 13 +++++++++++-
 2 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.cpp b/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.cpp
index cc5a04e82..875d58ef8 100644
--- a/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.cpp
+++ b/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.cpp
@@ -144,7 +144,12 @@ void btHeightfieldTerrainShape::getAabb(const btTransform& t,btVector3& aabbMin,
 	aabbMax = center + extent;
 }
 
-btScalar	btHeightfieldTerrainShape::getHeightFieldValue(int x,int y) const
+
+/// This returns the "raw" (user's initial) height, not the actual height.
+/// The actual height needs to be adjusted to be relative to the center
+///   of the heightfield's AABB.
+btScalar
+btHeightfieldTerrainShape::getRawHeightFieldValue(int x,int y) const
 {
 	btScalar val = 0.f;
 	switch (m_heightDataType)
@@ -181,24 +186,22 @@ btScalar	btHeightfieldTerrainShape::getHeightFieldValue(int x,int y) const
 
 
 
-
+/// this returns the vertex in bullet-local coordinates
 void	btHeightfieldTerrainShape::getVertex(int x,int y,btVector3& vertex) const
 {
-
 	btAssert(x>=0);
 	btAssert(y>=0);
 	btAssert(x<m_heightStickWidth);
 	btAssert(y<m_heightStickLength);
 
-
-	btScalar	height = getHeightFieldValue(x,y);
+	btScalar	height = getRawHeightFieldValue(x,y);
 
 	switch (m_upAxis)
 	{
 	case 0:
 		{
 		vertex.setValue(
-			height,
+			height - m_localOrigin.getX(),
 			(-m_width/btScalar(2.0)) + x,
 			(-m_length/btScalar(2.0) ) + y
 			);
@@ -208,7 +211,7 @@ void	btHeightfieldTerrainShape::getVertex(int x,int y,btVector3& vertex) const
 		{
 			vertex.setValue(
 			(-m_width/btScalar(2.0)) + x,
-			height,
+			height - m_localOrigin.getY(),
 			(-m_length/btScalar(2.0)) + y
 			);
 			break;
@@ -218,7 +221,7 @@ void	btHeightfieldTerrainShape::getVertex(int x,int y,btVector3& vertex) const
 			vertex.setValue(
 			(-m_width/btScalar(2.0)) + x,
 			(-m_length/btScalar(2.0)) + y,
-			height
+			height - m_localOrigin.getZ()
 			);
 			break;
 		}
@@ -230,7 +233,6 @@ void	btHeightfieldTerrainShape::getVertex(int x,int y,btVector3& vertex) const
 	}
 
 	vertex*=m_localScaling;
-	
 }
 
 
diff --git a/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.h b/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.h
index 32a7353a7..41f8a0374 100644
--- a/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.h
+++ b/src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.h
@@ -30,6 +30,17 @@ subject to the following restrictions:
   center (as determined by width and length and height, with each
   axis multiplied by the localScaling).
 
+  \b NOTE: be careful with coordinates.  If you have a heightfield with a local
+  min height of -100m, and a max height of +500m, you may be tempted to place it
+  at the origin (0,0) and expect the heights in world coordinates to be
+  -100 to +500 meters.
+  Actually, the heights will be -300 to +300m, because bullet will re-center
+  the heightfield based on its AABB (which is determined by the min/max
+  heights).  So keep in mind that once you create a btHeightfieldTerrainShape
+  object, the heights will be adjusted relative to the center of the AABB.  This
+  is different to the behavior of many rendering engines, but is useful for
+  physics engines.
+
   Most (but not all) rendering and heightfield libraries assume upAxis = 1
   (that is, the y-axis is "up").  This class allows any of the 3 coordinates
   to be "up".  Make sure your choice of axis is consistent with your rendering
@@ -88,7 +99,7 @@ protected:
 	
 	btVector3	m_localScaling;
 
-	virtual btScalar	getHeightFieldValue(int x,int y) const;
+	virtual btScalar	getRawHeightFieldValue(int x,int y) const;
 	void		quantizeWithClamp(int* out, const btVector3& point,int isMax) const;
 	void		getVertex(int x,int y,btVector3& vertex) const;