btHeightfieldTerrainShape.cpp

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/*
Bullet Continuous Collision Detection and Physics Library
Copyright (c) 2003-2009 Erwin Coumans  http://bulletphysics.org
 
This software is provided 'as-is', without any express or implied warranty.
In no event will the authors be held liable for any damages arising from the use of this software.
Permission is granted to anyone to use this software for any purpose, 
including commercial applications, and to alter it and redistribute it freely, 
subject to the following restrictions:
 
1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.
*/
 
#include "btHeightfieldTerrainShape.h"
 
#include "LinearMath/btTransformUtil.h"
 
btHeightfieldTerrainShape::btHeightfieldTerrainShape(
    int heightStickWidth, int heightStickLength, const void* heightfieldData,
    btScalar heightScale, btScalar minHeight, btScalar maxHeight, int upAxis,
    PHY_ScalarType hdt, bool flipQuadEdges)
    :m_userIndex2(-1),
    m_userValue3(0),
    m_triangleInfoMap(0)
{
    initialize(heightStickWidth, heightStickLength, heightfieldData,
               heightScale, minHeight, maxHeight, upAxis, hdt,
               flipQuadEdges);
}
 
btHeightfieldTerrainShape::btHeightfieldTerrainShape(int heightStickWidth, int heightStickLength, const void* heightfieldData, btScalar maxHeight, int upAxis, bool useFloatData, bool flipQuadEdges)
    :m_userIndex2(-1),
    m_userValue3(0),
    m_triangleInfoMap(0)
{
    // legacy constructor: support only float or unsigned char,
    //  and min height is zero
    PHY_ScalarType hdt = (useFloatData) ? PHY_FLOAT : PHY_UCHAR;
    btScalar minHeight = 0.0f;
 
    // previously, height = uchar * maxHeight / 65535.
    // So to preserve legacy behavior, heightScale = maxHeight / 65535
    btScalar heightScale = maxHeight / 65535;
 
    initialize(heightStickWidth, heightStickLength, heightfieldData,
               heightScale, minHeight, maxHeight, upAxis, hdt,
               flipQuadEdges);
}
 
void btHeightfieldTerrainShape::initialize(
    int heightStickWidth, int heightStickLength, const void* heightfieldData,
    btScalar heightScale, btScalar minHeight, btScalar maxHeight, int upAxis,
    PHY_ScalarType hdt, bool flipQuadEdges)
{
    // validation
    btAssert(heightStickWidth > 1);   // && "bad width");
    btAssert(heightStickLength > 1);  // && "bad length");
    btAssert(heightfieldData);        // && "null heightfield data");
    // btAssert(heightScale) -- do we care?  Trust caller here
    btAssert(minHeight <= maxHeight);                                    // && "bad min/max height");
    btAssert(upAxis >= 0 && upAxis < 3);                                 // && "bad upAxis--should be in range [0,2]");
    btAssert(hdt != PHY_UCHAR || hdt != PHY_FLOAT || hdt != PHY_SHORT);  // && "Bad height data type enum");
 
    // initialize member variables
    m_shapeType = TERRAIN_SHAPE_PROXYTYPE;
    m_heightStickWidth = heightStickWidth;
    m_heightStickLength = heightStickLength;
    m_minHeight = minHeight;
    m_maxHeight = maxHeight;
    m_width = (btScalar)(heightStickWidth - 1);
    m_length = (btScalar)(heightStickLength - 1);
    m_heightScale = heightScale;
    m_heightfieldDataUnknown = heightfieldData;
    m_heightDataType = hdt;
    m_flipQuadEdges = flipQuadEdges;
    m_useDiamondSubdivision = false;
    m_useZigzagSubdivision = false;
    m_flipTriangleWinding = false;
    m_upAxis = upAxis;
    m_localScaling.setValue(btScalar(1.), btScalar(1.), btScalar(1.));
    
    m_vboundsChunkSize = 0;
    m_vboundsGridWidth = 0;
    m_vboundsGridLength = 0;
 
    // determine min/max axis-aligned bounding box (aabb) values
    switch (m_upAxis)
    {
        case 0:
        {
            m_localAabbMin.setValue(m_minHeight, 0, 0);
            m_localAabbMax.setValue(m_maxHeight, m_width, m_length);
            break;
        }
        case 1:
        {
            m_localAabbMin.setValue(0, m_minHeight, 0);
            m_localAabbMax.setValue(m_width, m_maxHeight, m_length);
            break;
        };
        case 2:
        {
            m_localAabbMin.setValue(0, 0, m_minHeight);
            m_localAabbMax.setValue(m_width, m_length, m_maxHeight);
            break;
        }
        default:
        {
            //need to get valid m_upAxis
            btAssert(0);  // && "Bad m_upAxis");
        }
    }
 
    // remember origin (defined as exact middle of aabb)
    m_localOrigin = btScalar(0.5) * (m_localAabbMin + m_localAabbMax);
}
 
btHeightfieldTerrainShape::~btHeightfieldTerrainShape()
{
    clearAccelerator();
}
 
void btHeightfieldTerrainShape::getAabb(const btTransform& t, btVector3& aabbMin, btVector3& aabbMax) const
{
    btVector3 halfExtents = (m_localAabbMax - m_localAabbMin) * m_localScaling * btScalar(0.5);
 
    btVector3 localOrigin(0, 0, 0);
    localOrigin[m_upAxis] = (m_minHeight + m_maxHeight) * btScalar(0.5);
    localOrigin *= m_localScaling;
 
    btMatrix3x3 abs_b = t.getBasis().absolute();
    btVector3 center = t.getOrigin();
    btVector3 extent = halfExtents.dot3(abs_b[0], abs_b[1], abs_b[2]);
    extent += btVector3(getMargin(), getMargin(), getMargin());
 
    aabbMin = center - extent;
    aabbMax = center + extent;
}
 
btScalar
btHeightfieldTerrainShape::getRawHeightFieldValue(int x, int y) const
{
    btScalar val = 0.f;
    switch (m_heightDataType)
    {
        case PHY_FLOAT:
        {
            val = m_heightfieldDataFloat[(y * m_heightStickWidth) + x];
            break;
        }
 
        case PHY_UCHAR:
        {
            unsigned char heightFieldValue = m_heightfieldDataUnsignedChar[(y * m_heightStickWidth) + x];
            val = heightFieldValue * m_heightScale;
            break;
        }
 
        case PHY_SHORT:
        {
            short hfValue = m_heightfieldDataShort[(y * m_heightStickWidth) + x];
            val = hfValue * m_heightScale;
            break;
        }
 
        default:
        {
            btAssert(!"Bad m_heightDataType");
        }
    }
 
    return val;
}
 
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 = getRawHeightFieldValue(x, y);
 
    switch (m_upAxis)
    {
        case 0:
        {
            vertex.setValue(
                height - m_localOrigin.getX(),
                (-m_width / btScalar(2.0)) + x,
                (-m_length / btScalar(2.0)) + y);
            break;
        }
        case 1:
        {
            vertex.setValue(
                (-m_width / btScalar(2.0)) + x,
                height - m_localOrigin.getY(),
                (-m_length / btScalar(2.0)) + y);
            break;
        };
        case 2:
        {
            vertex.setValue(
                (-m_width / btScalar(2.0)) + x,
                (-m_length / btScalar(2.0)) + y,
                height - m_localOrigin.getZ());
            break;
        }
        default:
        {
            //need to get valid m_upAxis
            btAssert(0);
        }
    }
 
    vertex *= m_localScaling;
}
 
static inline int
getQuantized(
    btScalar x)
{
    if (x < 0.0)
    {
        return (int)(x - 0.5);
    }
    return (int)(x + 0.5);
}
 
 
void btHeightfieldTerrainShape::quantizeWithClamp(int* out, const btVector3& point, int /*isMax*/) const
{
    btVector3 clampedPoint(point);
    clampedPoint.setMax(m_localAabbMin);
    clampedPoint.setMin(m_localAabbMax);
 
    out[0] = getQuantized(clampedPoint.getX());
    out[1] = getQuantized(clampedPoint.getY());
    out[2] = getQuantized(clampedPoint.getZ());
}
 
 
void btHeightfieldTerrainShape::processAllTriangles(btTriangleCallback* callback, const btVector3& aabbMin, const btVector3& aabbMax) const
{
    // scale down the input aabb's so they are in local (non-scaled) coordinates
    btVector3 localAabbMin = aabbMin * btVector3(1.f / m_localScaling[0], 1.f / m_localScaling[1], 1.f / m_localScaling[2]);
    btVector3 localAabbMax = aabbMax * btVector3(1.f / m_localScaling[0], 1.f / m_localScaling[1], 1.f / m_localScaling[2]);
 
    // account for local origin
    localAabbMin += m_localOrigin;
    localAabbMax += m_localOrigin;
 
    //quantize the aabbMin and aabbMax, and adjust the start/end ranges
    int quantizedAabbMin[3];
    int quantizedAabbMax[3];
    quantizeWithClamp(quantizedAabbMin, localAabbMin, 0);
    quantizeWithClamp(quantizedAabbMax, localAabbMax, 1);
 
    // expand the min/max quantized values
    // this is to catch the case where the input aabb falls between grid points!
    for (int i = 0; i < 3; ++i)
    {
        quantizedAabbMin[i]--;
        quantizedAabbMax[i]++;
    }
 
    int startX = 0;
    int endX = m_heightStickWidth - 1;
    int startJ = 0;
    int endJ = m_heightStickLength - 1;
 
    switch (m_upAxis)
    {
        case 0:
        {
            if (quantizedAabbMin[1] > startX)
                startX = quantizedAabbMin[1];
            if (quantizedAabbMax[1] < endX)
                endX = quantizedAabbMax[1];
            if (quantizedAabbMin[2] > startJ)
                startJ = quantizedAabbMin[2];
            if (quantizedAabbMax[2] < endJ)
                endJ = quantizedAabbMax[2];
            break;
        }
        case 1:
        {
            if (quantizedAabbMin[0] > startX)
                startX = quantizedAabbMin[0];
            if (quantizedAabbMax[0] < endX)
                endX = quantizedAabbMax[0];
            if (quantizedAabbMin[2] > startJ)
                startJ = quantizedAabbMin[2];
            if (quantizedAabbMax[2] < endJ)
                endJ = quantizedAabbMax[2];
            break;
        };
        case 2:
        {
            if (quantizedAabbMin[0] > startX)
                startX = quantizedAabbMin[0];
            if (quantizedAabbMax[0] < endX)
                endX = quantizedAabbMax[0];
            if (quantizedAabbMin[1] > startJ)
                startJ = quantizedAabbMin[1];
            if (quantizedAabbMax[1] < endJ)
                endJ = quantizedAabbMax[1];
            break;
        }
        default:
        {
            //need to get valid m_upAxis
            btAssert(0);
        }
    }
 
    // TODO If m_vboundsGrid is available, use it to determine if we really need to process this area
 
    for (int j = startJ; j < endJ; j++)
    {
        for (int x = startX; x < endX; x++)
        {
            btVector3 vertices[3];
            int indices[3] = { 0, 1, 2 };
            if (m_flipTriangleWinding)
            {
                indices[0] = 2;
                indices[2] = 0;
            }
 
            if (m_flipQuadEdges || (m_useDiamondSubdivision && !((j + x) & 1)) || (m_useZigzagSubdivision && !(j & 1)))
            {
                //first triangle
                getVertex(x, j, vertices[indices[0]]);
                getVertex(x, j + 1, vertices[indices[1]]);
                getVertex(x + 1, j + 1, vertices[indices[2]]);
                callback->processTriangle(vertices, 2 * x, j);
                //second triangle
                //  getVertex(x,j,vertices[0]);//already got this vertex before, thanks to Danny Chapman
                getVertex(x + 1, j + 1, vertices[indices[1]]);
                getVertex(x + 1, j, vertices[indices[2]]);
                callback->processTriangle(vertices, 2 * x+1, j);
            }
            else
            {
                //first triangle
                getVertex(x, j, vertices[indices[0]]);
                getVertex(x, j + 1, vertices[indices[1]]);
                getVertex(x + 1, j, vertices[indices[2]]);
                callback->processTriangle(vertices, 2 * x, j);
                //second triangle
                getVertex(x + 1, j, vertices[indices[0]]);
                //getVertex(x,j+1,vertices[1]);
                getVertex(x + 1, j + 1, vertices[indices[2]]);
                callback->processTriangle(vertices, 2 * x+1, j);
            }
        }
    }
}
 
void btHeightfieldTerrainShape::calculateLocalInertia(btScalar, btVector3& inertia) const
{
    //moving concave objects not supported
 
    inertia.setValue(btScalar(0.), btScalar(0.), btScalar(0.));
}
 
void btHeightfieldTerrainShape::setLocalScaling(const btVector3& scaling)
{
    m_localScaling = scaling;
}
const btVector3& btHeightfieldTerrainShape::getLocalScaling() const
{
    return m_localScaling;
}
 
namespace
{
    struct GridRaycastState
    {
        int x;  // Next quad coords
        int z;
        int prev_x;  // Previous quad coords
        int prev_z;
        btScalar param;      // Exit param for previous quad
        btScalar prevParam;  // Enter param for previous quad
        btScalar maxDistanceFlat;
        btScalar maxDistance3d;
    };
}
 
// TODO Does it really need to take 3D vectors?
template <typename Action_T>
void gridRaycast(Action_T& quadAction, const btVector3& beginPos, const btVector3& endPos, int indices[3])
{
    GridRaycastState rs;
    rs.maxDistance3d = beginPos.distance(endPos);
    if (rs.maxDistance3d < 0.0001)
    {
        // Consider the ray is too small to hit anything
        return;
    }
    
 
    btScalar rayDirectionFlatX = endPos[indices[0]] - beginPos[indices[0]];
    btScalar rayDirectionFlatZ = endPos[indices[2]] - beginPos[indices[2]];
    rs.maxDistanceFlat = btSqrt(rayDirectionFlatX * rayDirectionFlatX + rayDirectionFlatZ * rayDirectionFlatZ);
 
    if (rs.maxDistanceFlat < 0.0001)
    {
        // Consider the ray vertical
        rayDirectionFlatX = 0;
        rayDirectionFlatZ = 0;
    }
    else
    {
        rayDirectionFlatX /= rs.maxDistanceFlat;
        rayDirectionFlatZ /= rs.maxDistanceFlat;
    }
 
    const int xiStep = rayDirectionFlatX > 0 ? 1 : rayDirectionFlatX < 0 ? -1 : 0;
    const int ziStep = rayDirectionFlatZ > 0 ? 1 : rayDirectionFlatZ < 0 ? -1 : 0;
 
    const float infinite = 9999999;
    const btScalar paramDeltaX = xiStep != 0 ? 1.f / btFabs(rayDirectionFlatX) : infinite;
    const btScalar paramDeltaZ = ziStep != 0 ? 1.f / btFabs(rayDirectionFlatZ) : infinite;
 
    // pos = param * dir
    btScalar paramCrossX;  // At which value of `param` we will cross a x-axis lane?
    btScalar paramCrossZ;  // At which value of `param` we will cross a z-axis lane?
 
    // paramCrossX and paramCrossZ are initialized as being the first cross
    // X initialization
    if (xiStep != 0)
    {
        if (xiStep == 1)
        {
            paramCrossX = (ceil(beginPos[indices[0]]) - beginPos[indices[0]]) * paramDeltaX;
        }
        else
        {
            paramCrossX = (beginPos[indices[0]] - floor(beginPos[indices[0]])) * paramDeltaX;
        }
    }
    else
    {
        paramCrossX = infinite;  // Will never cross on X
    }
 
    // Z initialization
    if (ziStep != 0)
    {
        if (ziStep == 1)
        {
            paramCrossZ = (ceil(beginPos[indices[2]]) - beginPos[indices[2]]) * paramDeltaZ;
        }
        else
        {
            paramCrossZ = (beginPos[indices[2]] - floor(beginPos[indices[2]])) * paramDeltaZ;
        }
    }
    else
    {
        paramCrossZ = infinite;  // Will never cross on Z
    }
 
    rs.x = static_cast<int>(floor(beginPos[indices[0]]));
    rs.z = static_cast<int>(floor(beginPos[indices[2]]));
 
    // Workaround cases where the ray starts at an integer position
    if (paramCrossX == 0.0)
    {
        paramCrossX += paramDeltaX;
        // If going backwards, we should ignore the position we would get by the above flooring,
        // because the ray is not heading in that direction
        if (xiStep == -1)
        {
            rs.x -= 1;
        }
    }
 
    if (paramCrossZ == 0.0)
    {
        paramCrossZ += paramDeltaZ;
        if (ziStep == -1)
            rs.z -= 1;
    }
 
    rs.prev_x = rs.x;
    rs.prev_z = rs.z;
    rs.param = 0;
 
    while (true)
    {
        rs.prev_x = rs.x;
        rs.prev_z = rs.z;
        rs.prevParam = rs.param;
 
        if (paramCrossX < paramCrossZ)
        {
            // X lane
            rs.x += xiStep;
            // Assign before advancing the param,
            // to be in sync with the initialization step
            rs.param = paramCrossX;
            paramCrossX += paramDeltaX;
        }
        else
        {
            // Z lane
            rs.z += ziStep;
            rs.param = paramCrossZ;
            paramCrossZ += paramDeltaZ;
        }
 
        if (rs.param > rs.maxDistanceFlat)
        {
            rs.param = rs.maxDistanceFlat;
            quadAction(rs);
            break;
        }
        else
        {
            quadAction(rs);
        }
    }
}
 
struct ProcessTrianglesAction
{
    const btHeightfieldTerrainShape* shape;
    bool flipQuadEdges;
    bool useDiamondSubdivision;
    int width;
    int length;
    btTriangleCallback* callback;
 
    void exec(int x, int z) const
    {
        if (x < 0 || z < 0 || x >= width || z >= length)
        {
            return;
        }
 
        btVector3 vertices[3];
 
        // TODO Since this is for raycasts, we could greatly benefit from an early exit on the first hit
 
        // Check quad
        if (flipQuadEdges || (useDiamondSubdivision && (((z + x) & 1) > 0)))
        {
            // First triangle
            shape->getVertex(x, z, vertices[0]);
            shape->getVertex(x + 1, z, vertices[1]);
            shape->getVertex(x + 1, z + 1, vertices[2]);
            callback->processTriangle(vertices, x, z);
 
            // Second triangle
            shape->getVertex(x, z, vertices[0]);
            shape->getVertex(x + 1, z + 1, vertices[1]);
            shape->getVertex(x, z + 1, vertices[2]);
            callback->processTriangle(vertices, x, z);
        }
        else
        {
            // First triangle
            shape->getVertex(x, z, vertices[0]);
            shape->getVertex(x, z + 1, vertices[1]);
            shape->getVertex(x + 1, z, vertices[2]);
            callback->processTriangle(vertices, x, z);
 
            // Second triangle
            shape->getVertex(x + 1, z, vertices[0]);
            shape->getVertex(x, z + 1, vertices[1]);
            shape->getVertex(x + 1, z + 1, vertices[2]);
            callback->processTriangle(vertices, x, z);
        }
    }
 
    void operator()(const GridRaycastState& bs) const
    {
        exec(bs.prev_x, bs.prev_z);
    }
};
 
struct ProcessVBoundsAction
{
    const btAlignedObjectArray<btHeightfieldTerrainShape::Range>& vbounds;
    int width;
    int length;
    int chunkSize;
 
    btVector3 rayBegin;
    btVector3 rayEnd;
    btVector3 rayDir;
 
    int* m_indices;
    ProcessTrianglesAction processTriangles;
 
    ProcessVBoundsAction(const btAlignedObjectArray<btHeightfieldTerrainShape::Range>& bnd, int* indices)
        : vbounds(bnd),
        m_indices(indices)
    {
    }
    void operator()(const GridRaycastState& rs) const
    {
        int x = rs.prev_x;
        int z = rs.prev_z;
 
        if (x < 0 || z < 0 || x >= width || z >= length)
        {
            return;
        }
 
        const btHeightfieldTerrainShape::Range chunk = vbounds[x + z * width];
 
        btVector3 enterPos;
        btVector3 exitPos;
 
        if (rs.maxDistanceFlat > 0.0001)
        {
            btScalar flatTo3d = chunkSize * rs.maxDistance3d / rs.maxDistanceFlat;
            btScalar enterParam3d = rs.prevParam * flatTo3d;
            btScalar exitParam3d = rs.param * flatTo3d;
            enterPos = rayBegin + rayDir * enterParam3d;
            exitPos = rayBegin + rayDir * exitParam3d;
 
            // We did enter the flat projection of the AABB,
            // but we have to check if we intersect it on the vertical axis
            if (enterPos[1] > chunk.max && exitPos[m_indices[1]] > chunk.max)
            {
                return;
            }
            if (enterPos[1] < chunk.min && exitPos[m_indices[1]] < chunk.min)
            {
                return;
            }
        }
        else
        {
            // Consider the ray vertical
            // (though we shouldn't reach this often because there is an early check up-front)
            enterPos = rayBegin;
            exitPos = rayEnd;
        }
 
        gridRaycast(processTriangles, enterPos, exitPos, m_indices);
        // Note: it could be possible to have more than one grid at different levels,
        // to do this there would be a branch using a pointer to another ProcessVBoundsAction
    }
};
 
// TODO How do I interrupt the ray when there is a hit? `callback` does not return any result
void btHeightfieldTerrainShape::performRaycast(btTriangleCallback* callback, const btVector3& raySource, const btVector3& rayTarget) const
{
    // Transform to cell-local
    btVector3 beginPos = raySource / m_localScaling;
    btVector3 endPos = rayTarget / m_localScaling;
    beginPos += m_localOrigin;
    endPos += m_localOrigin;
 
    ProcessTrianglesAction processTriangles;
    processTriangles.shape = this;
    processTriangles.flipQuadEdges = m_flipQuadEdges;
    processTriangles.useDiamondSubdivision = m_useDiamondSubdivision;
    processTriangles.callback = callback;
    processTriangles.width = m_heightStickWidth - 1;
    processTriangles.length = m_heightStickLength - 1;
 
    // TODO Transform vectors to account for m_upAxis
    int indices[3] = { 0, 1, 2 };
    if (m_upAxis == 2)
    {
        indices[1] = 2;
        indices[2] = 1;
    }
    int iBeginX = static_cast<int>(floor(beginPos[indices[0]]));
    int iBeginZ = static_cast<int>(floor(beginPos[indices[2]]));
    int iEndX = static_cast<int>(floor(endPos[indices[0]]));
    int iEndZ = static_cast<int>(floor(endPos[indices[2]]));
 
    if (iBeginX == iEndX && iBeginZ == iEndZ)
    {
        // The ray will never cross quads within the plane,
        // so directly process triangles within one quad
        // (typically, vertical rays should end up here)
        processTriangles.exec(iBeginX, iEndZ);
        return;
    }
 
    
 
    if (m_vboundsGrid.size()==0)
    {
        // Process all quads intersecting the flat projection of the ray
        gridRaycast(processTriangles, beginPos, endPos, &indices[0]);
    }
    else
    {
        btVector3 rayDiff = endPos - beginPos;
        btScalar flatDistance2 = rayDiff[indices[0]] * rayDiff[indices[0]] + rayDiff[indices[2]] * rayDiff[indices[2]];
        if (flatDistance2 < m_vboundsChunkSize * m_vboundsChunkSize)
        {
            // Don't use chunks, the ray is too short in the plane
            gridRaycast(processTriangles, beginPos, endPos, &indices[0]);
        }
 
        ProcessVBoundsAction processVBounds(m_vboundsGrid, &indices[0]);
        processVBounds.width = m_vboundsGridWidth;
        processVBounds.length = m_vboundsGridLength;
        processVBounds.rayBegin = beginPos;
        processVBounds.rayEnd = endPos;
        processVBounds.rayDir = rayDiff.normalized();
        processVBounds.processTriangles = processTriangles;
        processVBounds.chunkSize = m_vboundsChunkSize;
        // The ray is long, run raycast on a higher-level grid
        gridRaycast(processVBounds, beginPos / m_vboundsChunkSize, endPos / m_vboundsChunkSize, indices);
    }
}
 
void btHeightfieldTerrainShape::buildAccelerator(int chunkSize)
{
    if (chunkSize <= 0)
    {
        clearAccelerator();
        return;
    }
 
    m_vboundsChunkSize = chunkSize;
    int nChunksX = m_heightStickWidth / chunkSize;
    int nChunksZ = m_heightStickLength / chunkSize;
 
    if (m_heightStickWidth % chunkSize > 0)
    {
        ++nChunksX;  // In case terrain size isn't dividable by chunk size
    }
    if (m_heightStickLength % chunkSize > 0)
    {
        ++nChunksZ;
    }
 
    if (m_vboundsGridWidth != nChunksX || m_vboundsGridLength != nChunksZ)
    {
        clearAccelerator();
        m_vboundsGridWidth = nChunksX;
        m_vboundsGridLength = nChunksZ;
    }
 
    if (nChunksX == 0 || nChunksZ == 0)
    {
        return;
    }
 
    // This data structure is only reallocated if the required size changed
    m_vboundsGrid.resize(nChunksX * nChunksZ);
    
    // Compute min and max height for all chunks
    for (int cz = 0; cz < nChunksZ; ++cz)
    {
        int z0 = cz * chunkSize;
 
        for (int cx = 0; cx < nChunksX; ++cx)
        {
            int x0 = cx * chunkSize;
 
            Range r;
 
            r.min = getRawHeightFieldValue(x0, z0);
            r.max = r.min;
 
            // Compute min and max height for this chunk.
            // We have to include one extra cell to account for neighbors.
            // Here is why:
            // Say we have a flat terrain, and a plateau that fits a chunk perfectly.
            //
            //   Left        Right
            // 0---0---0---1---1---1
            // |   |   |   |   |   |
            // 0---0---0---1---1---1
            // |   |   |   |   |   |
            // 0---0---0---1---1---1
            //           x
            //
            // If the AABB for the Left chunk did not share vertices with the Right,
            // then we would fail collision tests at x due to a gap.
            //
            for (int z = z0; z < z0 + chunkSize + 1; ++z)
            {
                if (z >= m_heightStickLength)
                {
                    continue;
                }
 
                for (int x = x0; x < x0 + chunkSize + 1; ++x)
                {
                    if (x >= m_heightStickWidth)
                    {
                        continue;
                    }
 
                    btScalar height = getRawHeightFieldValue(x, z);
 
                    if (height < r.min)
                    {
                        r.min = height;
                    }
                    else if (height > r.max)
                    {
                        r.max = height;
                    }
                }
            }
 
            m_vboundsGrid[cx + cz * nChunksX] = r;
        }
    }
}
 
void btHeightfieldTerrainShape::clearAccelerator()
{
    m_vboundsGrid.clear();
}