/*
-----------------------------------------------------------------------------
This source file is part of OGRE
(Object-oriented Graphics Rendering Engine)
For the latest info, see http://www.ogre3d.org/

Copyright (c) 2000-2014 Torus Knot Software Ltd

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
-----------------------------------------------------------------------------
*/
#include "OgreStableHeaders.h"
#include "OgreTangentSpaceCalc.h"
#include "OgreHardwareBufferManager.h"
#include "OgreLogManager.h"
#include "OgreException.h"

namespace Ogre
{
    //---------------------------------------------------------------------
    TangentSpaceCalc::TangentSpaceCalc()
        : mVData(0)
        , mSplitMirrored(false)
        , mSplitRotated(false)
        , mStoreParityInW(false)
    {
    }

    //---------------------------------------------------------------------
    void TangentSpaceCalc::clear()
    {
        mIDataList.clear();
        mOpTypes.clear();
        mVData = 0;
    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::setVertexData(VertexData* v_in)
    {
        mVData = v_in;
    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::addIndexData(IndexData* i_in, RenderOperation::OperationType op)
    {
        if (op != RenderOperation::OT_TRIANGLE_FAN && 
            op != RenderOperation::OT_TRIANGLE_LIST && 
            op != RenderOperation::OT_TRIANGLE_STRIP)
        {
            OGRE_EXCEPT(Exception::ERR_INVALIDPARAMS,
                "Only indexed triangle (list, strip, fan) render operations are supported.",
                "TangentSpaceCalc::addIndexData");

        }
        mIDataList.push_back(i_in);
        mOpTypes.push_back(op);
    }
    //---------------------------------------------------------------------
    TangentSpaceCalc::Result TangentSpaceCalc::build(
        VertexElementSemantic targetSemantic,
        unsigned short sourceTexCoordSet, unsigned short index)
    {
        Result res;

        // Pull out all the vertex components we'll need
        populateVertexArray(sourceTexCoordSet);

        // Now process the faces and calculate / add their contributions
        processFaces(res);

        // Now normalise & orthogonalise
        normaliseVertices();

        // Create new final geometry
        // First extend existing buffers to cope with new vertices
        extendBuffers(res.vertexSplits);

        // Alter indexes
        remapIndexes(res);

        // Create / identify target & write tangents
        insertTangents(res, targetSemantic, sourceTexCoordSet, index);

        return res;


    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::extendBuffers(VertexSplits& vertexSplits)
    {
        if (!vertexSplits.empty())
        {
            // ok, need to increase the vertex buffer size, and alter some indexes

            // vertex buffers first
            VertexBufferBinding* newBindings = HardwareBufferManager::getSingleton().createVertexBufferBinding();
            const VertexBufferBinding::VertexBufferBindingMap& bindmap = 
                mVData->vertexBufferBinding->getBindings();
            for (VertexBufferBinding::VertexBufferBindingMap::const_iterator i = 
                bindmap.begin(); i != bindmap.end(); ++i)
            {
                HardwareVertexBufferSharedPtr srcbuf = i->second;
                // Derive vertex count from buffer not vertex data, in case using
                // the vertexStart option in vertex data
                size_t newVertexCount = srcbuf->getNumVertices() + vertexSplits.size();
                // Create new buffer & bind
                HardwareVertexBufferSharedPtr newBuf = 
                    HardwareBufferManager::getSingleton().createVertexBuffer(
                    srcbuf->getVertexSize(), newVertexCount, srcbuf->getUsage(), 
                    srcbuf->hasShadowBuffer());
                newBindings->setBinding(i->first, newBuf);

                // Copy existing contents (again, entire buffer, not just elements referenced)
                newBuf->copyData(*(srcbuf.get()), 0, 0, srcbuf->getNumVertices() * srcbuf->getVertexSize(), true);

                // Split vertices, read / write from new buffer
                char* pBase = static_cast<char*>(newBuf->lock(HardwareBuffer::HBL_NORMAL));
                for (VertexSplits::iterator spliti = vertexSplits.begin(); 
                    spliti != vertexSplits.end(); ++spliti)
                {
                    const char* pSrcBase = pBase + spliti->first * newBuf->getVertexSize();
                    char* pDstBase = pBase + spliti->second * newBuf->getVertexSize();
                    memcpy(pDstBase, pSrcBase, newBuf->getVertexSize());
                }
                newBuf->unlock();

            }

            // Update vertex data
            // Increase vertex count according to num splits
            mVData->vertexCount += vertexSplits.size();
            // Flip bindings over to new buffers (old buffers released)
            HardwareBufferManager::getSingleton().destroyVertexBufferBinding(mVData->vertexBufferBinding);
            mVData->vertexBufferBinding = newBindings;

            // If vertex size requires 32bit index buffer
            if (mVData->vertexCount > 65536)
            {
                for (size_t i = 0; i < mIDataList.size(); ++i)
                {
                    // check index size
                    IndexData* idata = mIDataList[i];
                    HardwareIndexBufferSharedPtr srcbuf = idata->indexBuffer;
                    if (srcbuf->getType() == HardwareIndexBuffer::IT_16BIT)
                    {
                        size_t indexCount = srcbuf->getNumIndexes();

                        // convert index buffer to 32bit.
                        HardwareIndexBufferSharedPtr newBuf =
                            HardwareBufferManager::getSingleton().createIndexBuffer(
                            HardwareIndexBuffer::IT_32BIT, indexCount,
                            srcbuf->getUsage(), srcbuf->hasShadowBuffer());

                        uint16* pSrcBase = static_cast<uint16*>(srcbuf->lock(HardwareBuffer::HBL_NORMAL));
                        uint32* pBase = static_cast<uint32*>(newBuf->lock(HardwareBuffer::HBL_NORMAL));

                        size_t j = 0;
                        while (j < indexCount)
                        {
                            *pBase++ = *pSrcBase++;
                            ++j;
                        }

                        srcbuf->unlock();
                        newBuf->unlock();

                        // assign new index buffer.
                        idata->indexBuffer = newBuf;
                    }
                }
            }
        }

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::remapIndexes(Result& res)
    {
        for (size_t i = 0; i < mIDataList.size(); ++i)
        {

            IndexData* idata = mIDataList[i];
            // Now do index data
            // no new buffer required, same size but some triangles remapped
            if (idata->indexBuffer->getType() == HardwareIndexBuffer::IT_32BIT)
            {
                uint32* p32 = static_cast<uint32*>(idata->indexBuffer->lock(HardwareBuffer::HBL_NORMAL));
                remapIndexes(p32, i, res);
            }
            else
            {
                uint16* p16 = static_cast<uint16*>(idata->indexBuffer->lock(HardwareBuffer::HBL_NORMAL));
                remapIndexes(p16, i, res);
            }
            idata->indexBuffer->unlock();
        }

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::normaliseVertices()
    {
        // Just run through our complete (possibly augmented) list of vertices
        // Normalise the tangents & binormals
        for (VertexInfoArray::iterator i = mVertexArray.begin(); i != mVertexArray.end(); ++i)
        {
            VertexInfo& v = *i;

            v.tangent.normalise();
            v.binormal.normalise();

            // Orthogonalise with the vertex normal since it's currently
            // orthogonal with the face normals, but will be close to ortho
            // Apply Gram-Schmidt orthogonalise
            Vector3 temp = v.tangent;
            v.tangent = temp - (v.norm * v.norm.dotProduct(temp));

            temp = v.binormal;
            v.binormal = temp - (v.norm * v.norm.dotProduct(temp));

            // renormalize 
            v.tangent.normalise();
            v.binormal.normalise();

        }
    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::processFaces(Result& result)
    {
        // Quick pre-check for triangle strips / fans
        for (OpTypeList::iterator ot = mOpTypes.begin(); ot != mOpTypes.end(); ++ot)
        {
            if (*ot != RenderOperation::OT_TRIANGLE_LIST)
            {
                // Can't split strips / fans
                setSplitMirrored(false);
                setSplitRotated(false);
            }
        }

        for (size_t i = 0; i < mIDataList.size(); ++i)
        {
            IndexData* i_in = mIDataList[i];
            RenderOperation::OperationType opType = mOpTypes[i];

            // Read data from buffers
            uint16 *p16 = 0;
            uint32 *p32 = 0;

            HardwareIndexBufferSharedPtr ibuf = i_in->indexBuffer;
            if (ibuf->getType() == HardwareIndexBuffer::IT_32BIT)
            {
                p32 = static_cast<uint32*>(
                    ibuf->lock(HardwareBuffer::HBL_READ_ONLY));
                // offset by index start
                p32 += i_in->indexStart;
            }
            else
            {
                p16 = static_cast<uint16*>(
                    ibuf->lock(HardwareBuffer::HBL_READ_ONLY));
                // offset by index start
                p16 += i_in->indexStart;
            }
            // current triangle
            size_t vertInd[3] = { 0, 0, 0 };
            // loop through all faces to calculate the tangents and normals
            size_t faceCount = opType == RenderOperation::OT_TRIANGLE_LIST ? 
                i_in->indexCount / 3 : i_in->indexCount - 2;
            for (size_t f = 0; f < faceCount; ++f)
            {
                bool invertOrdering = false;
                // Read 1 or 3 indexes depending on type
                if (f == 0 || opType == RenderOperation::OT_TRIANGLE_LIST)
                {
                    vertInd[0] = p32? *p32++ : *p16++;
                    vertInd[1] = p32? *p32++ : *p16++;
                    vertInd[2] = p32? *p32++ : *p16++;
                }
                else if (opType == RenderOperation::OT_TRIANGLE_FAN)
                {
                    // Element 0 always remains the same
                    // Element 2 becomes element 1
                    vertInd[1] = vertInd[2];
                    // read new into element 2
                    vertInd[2] = p32? *p32++ : *p16++;
                }
                else if (opType == RenderOperation::OT_TRIANGLE_STRIP)
                {
                    // Shunt everything down one, but also invert the ordering on 
                    // odd numbered triangles (== even numbered i's)
                    // we interpret front as anticlockwise all the time but strips alternate
                    if (f & 0x1)
                    {
                        // odd tris (index starts at 3, 5, 7)
                        invertOrdering = true;
                    }
                    vertInd[0] = vertInd[1];
                    vertInd[1] = vertInd[2];            
                    vertInd[2] = p32? *p32++ : *p16++;
                }

                // deal with strip inversion of winding
                size_t localVertInd[3];
                localVertInd[0] = vertInd[0];
                if (invertOrdering)
                {
                    localVertInd[1] = vertInd[2];
                    localVertInd[2] = vertInd[1];
                }
                else
                {
                    localVertInd[1] = vertInd[1];
                    localVertInd[2] = vertInd[2];
                }


                // For each triangle
                //   Calculate tangent & binormal per triangle
                //   Note these are not normalised, are weighted by UV area
                Vector3 faceTsU, faceTsV, faceNorm;
                calculateFaceTangentSpace(localVertInd, faceTsU, faceTsV, faceNorm);

                // Skip invalid UV space triangles
                if (faceTsU.isZeroLength() || faceTsV.isZeroLength())
                    continue;

                addFaceTangentSpaceToVertices(i, f, localVertInd, faceTsU, faceTsV, faceNorm, result);

            }


            ibuf->unlock();
        }

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::addFaceTangentSpaceToVertices(
        size_t indexSet, size_t faceIndex, size_t *localVertInd, 
        const Vector3& faceTsU, const Vector3& faceTsV, const Vector3& faceNorm, 
        Result& result)
    {
        // Calculate parity for this triangle
        int faceParity = calculateParity(faceTsU, faceTsV, faceNorm);
        // Now add these to each vertex referenced by the face
        for (int v = 0; v < 3; ++v)
        {
            // index 0 is vertex we're calculating, 1 and 2 are the others

            // We want to re-weight these by the angle the face makes with the vertex
            // in order to obtain tessellation-independent results
            Real angleWeight = calculateAngleWeight(localVertInd[v], 
                localVertInd[(v+1)%3], localVertInd[(v+2)%3]);


            VertexInfo* vertex = &(mVertexArray[localVertInd[v]]);

            // check parity (0 means not set)
            // Locate parity-version of vertex index, or create if doesn't exist
            // If parity-version of vertex index was different, record alteration
            // in triangle remap
            // in vertex split list
            bool splitVertex = false;
            size_t reusedOppositeParity = 0;
            bool splitBecauseOfParity = false;
            bool newVertex = false;
            if (!vertex->parity)
            {
                // init
                vertex->parity = faceParity;
                newVertex = true;
            }
            if (mSplitMirrored)
            {
                if (!newVertex && faceParity != calculateParity(vertex->tangent, vertex->binormal, vertex->norm))//vertex->parity != faceParity)
                {
                    // Check for existing alternative parity
                    if (vertex->oppositeParityIndex)
                    {
                        // Ok, have already split this vertex because of parity
                        // Use the same one again
                        reusedOppositeParity = vertex->oppositeParityIndex;
                        vertex = &(mVertexArray[reusedOppositeParity]);
                    }
                    else
                    {
                        splitVertex = true;
                        splitBecauseOfParity = true;

                        LogManager::getSingleton().stream(LML_TRIVIAL)
                            << "TSC parity split - Vpar: " << vertex->parity 
                            << " Fpar: " << faceParity
                            << " faceTsU: " << faceTsU
                            << " faceTsV: " << faceTsV
                            << " faceNorm: " << faceNorm
                            << " vertTsU:" << vertex->tangent
                            << " vertTsV:" << vertex->binormal
                            << " vertNorm:" << vertex->norm;

                    }
                }
            }

            if (mSplitRotated)
            {

                // deal with excessive tangent space rotations as well as mirroring
                // same kind of split behaviour appropriate
                if (!newVertex && !splitVertex)
                {
                    // If more than 90 degrees, split
                    Vector3 uvCurrent = vertex->tangent + vertex->binormal;

                    // project down to the plane (plane normal = face normal)
                    Vector3 vRotHalf = uvCurrent - faceNorm;
                    vRotHalf *= faceNorm.dotProduct(uvCurrent);

                    if ((faceTsU + faceTsV).dotProduct(vRotHalf) < 0.0f)
                    {
                        splitVertex = true;
                    }
                }
            }

            if (splitVertex)
            {
                size_t newVertexIndex = mVertexArray.size();
                VertexSplit splitInfo(localVertInd[v], newVertexIndex);
                result.vertexSplits.push_back(splitInfo);
                // re-point opposite parity
                if (splitBecauseOfParity)
                {
                    vertex->oppositeParityIndex = newVertexIndex;
                }
                // copy old values but reset tangent space
                VertexInfo locVertex = *vertex;
                locVertex.tangent = Vector3::ZERO;
                locVertex.binormal = Vector3::ZERO;
                locVertex.parity = faceParity;
                mVertexArray.push_back(locVertex);
                result.indexesRemapped.push_back(IndexRemap(indexSet, faceIndex, splitInfo));

                vertex = &(mVertexArray[newVertexIndex]);

            }
            else if (reusedOppositeParity)
            {
                // didn't split again, but we do need to record the re-used remapping
                VertexSplit splitInfo(localVertInd[v], reusedOppositeParity);
                result.indexesRemapped.push_back(IndexRemap(indexSet, faceIndex, splitInfo));

            }

            // Add weighted tangent & binormal
            vertex->tangent += (faceTsU * angleWeight);
            vertex->binormal += (faceTsV * angleWeight);


        }

    }
    //---------------------------------------------------------------------
    int TangentSpaceCalc::calculateParity(const Vector3& u, const Vector3& v, const Vector3& n)
    {
        // Note that this parity is the reverse of what you'd expect - this is
        // because the 'V' texture coordinate is actually left handed
        if (u.crossProduct(v).dotProduct(n) >= 0.0f)
            return -1;
        else
            return 1;

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::calculateFaceTangentSpace(const size_t* vertInd, 
        Vector3& tsU, Vector3& tsV, Vector3& tsN)
    {
        const VertexInfo& v0 = mVertexArray[vertInd[0]];
        const VertexInfo& v1 = mVertexArray[vertInd[1]];
        const VertexInfo& v2 = mVertexArray[vertInd[2]];
        Vector2 deltaUV1 = v1.uv - v0.uv;
        Vector2 deltaUV2 = v2.uv - v0.uv;
        Vector3 deltaPos1 = v1.pos - v0.pos;
        Vector3 deltaPos2 = v2.pos - v0.pos;

        // face normal
        tsN = deltaPos1.crossProduct(deltaPos2);
        tsN.normalise();


        Real uvarea = deltaUV1.crossProduct(deltaUV2) * 0.5f;
        if (Math::RealEqual(uvarea, 0.0f))
        {
            // no tangent, null uv area
            tsU = tsV = Vector3::ZERO;
        }
        else
        {

            // Normalise by uvarea
            Real a = deltaUV2.y / uvarea;
            Real b = -deltaUV1.y / uvarea;
            Real c = -deltaUV2.x / uvarea;
            Real d = deltaUV1.x / uvarea;

            tsU = (deltaPos1 * a) + (deltaPos2 * b);
            tsU.normalise();

            tsV = (deltaPos1 * c) + (deltaPos2 * d);
            tsV.normalise();

            Real abs_uvarea = Math::Abs(uvarea);
            tsU *= abs_uvarea;
            tsV *= abs_uvarea;

            // tangent (tsU) and binormal (tsV) are now weighted by uv area


        }

    }
    //---------------------------------------------------------------------
    Real TangentSpaceCalc::calculateAngleWeight(size_t vidx0, size_t vidx1, size_t vidx2)
    {
        const VertexInfo& v0 = mVertexArray[vidx0];
        const VertexInfo& v1 = mVertexArray[vidx1];
        const VertexInfo& v2 = mVertexArray[vidx2];

        Vector3 diff0 = v1.pos - v0.pos;
        Vector3 diff1 = v2.pos - v1.pos;

        // Weight is just the angle - larger == better
        return diff0.angleBetween(diff1).valueRadians();

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::populateVertexArray(unsigned short sourceTexCoordSet)
    {
        // Just pull data out into more friendly structures
        VertexDeclaration *dcl = mVData->vertexDeclaration;
        VertexBufferBinding *bind = mVData->vertexBufferBinding;

        // Get the incoming UV element
        const VertexElement* uvElem = dcl->findElementBySemantic(
            VES_TEXTURE_COORDINATES, sourceTexCoordSet);

        if (!uvElem || uvElem->getType() != VET_FLOAT2)
        {
            OGRE_EXCEPT(Exception::ERR_INVALIDPARAMS,
                "No 2D texture coordinates with selected index, cannot calculate tangents.",
                "TangentSpaceCalc::build");
        }

        HardwareVertexBufferSharedPtr uvBuf, posBuf, normBuf;
        unsigned char *pUvBase, *pPosBase, *pNormBase;
        size_t uvInc, posInc, normInc;

        uvBuf = bind->getBuffer(uvElem->getSource());
        pUvBase = static_cast<unsigned char*>(
            uvBuf->lock(HardwareBuffer::HBL_READ_ONLY));
        uvInc = uvBuf->getVertexSize();
        // offset for vertex start
        pUvBase += mVData->vertexStart * uvInc;

        // find position
        const VertexElement *posElem = dcl->findElementBySemantic(VES_POSITION);
        if (posElem->getSource() == uvElem->getSource())
        {
            pPosBase = pUvBase;
            posInc = uvInc;
        }
        else
        {
            // A different buffer
            posBuf = bind->getBuffer(posElem->getSource());
            pPosBase = static_cast<unsigned char*>(
                posBuf->lock(HardwareBuffer::HBL_READ_ONLY));
            posInc = posBuf->getVertexSize();
            // offset for vertex start
            pPosBase += mVData->vertexStart * posInc;
        }
        // find a normal buffer
        const VertexElement *normElem = dcl->findElementBySemantic(VES_NORMAL);
        if (!normElem)
            OGRE_EXCEPT(Exception::ERR_ITEM_NOT_FOUND, 
            "No vertex normals found", 
            "TangentSpaceCalc::build");

        if (normElem->getSource() == uvElem->getSource())
        {
            pNormBase = pUvBase;
            normInc = uvInc;
        }
        else if (normElem->getSource() == posElem->getSource())
        {
            // normals are in the same buffer as position
            // this condition arises when an animated(skeleton) mesh is not built with 
            // an edge list buffer ie no shadows being used.
            pNormBase = pPosBase;
            normInc = posInc;
        }
        else
        {
            // A different buffer
            normBuf = bind->getBuffer(normElem->getSource());
            pNormBase = static_cast<unsigned char*>(
                normBuf->lock(HardwareBuffer::HBL_READ_ONLY));
            normInc = normBuf->getVertexSize();
            // offset for vertex start
            pNormBase += mVData->vertexStart * normInc;
        }

        // Preinitialise vertex info
        mVertexArray.clear();
        mVertexArray.resize(mVData->vertexCount);

        float* pFloat;
        VertexInfo* vInfo = &(mVertexArray[0]);
        for (size_t v = 0; v < mVData->vertexCount; ++v, ++vInfo)
        {
            posElem->baseVertexPointerToElement(pPosBase, &pFloat);
            vInfo->pos.x = *pFloat++;
            vInfo->pos.y = *pFloat++;
            vInfo->pos.z = *pFloat++;
            pPosBase += posInc;

            normElem->baseVertexPointerToElement(pNormBase, &pFloat);
            vInfo->norm.x = *pFloat++;
            vInfo->norm.y = *pFloat++;
            vInfo->norm.z = *pFloat++;
            pNormBase += normInc;

            uvElem->baseVertexPointerToElement(pUvBase, &pFloat);
            vInfo->uv.x = *pFloat++;
            vInfo->uv.y = *pFloat++;
            pUvBase += uvInc;


        }

        // unlock buffers
        uvBuf->unlock();
        if (posBuf)
        {
            posBuf->unlock();
        }
        if (normBuf)
        {
            normBuf->unlock();
        }

    }
    //---------------------------------------------------------------------
    void TangentSpaceCalc::insertTangents(Result& res,
        VertexElementSemantic targetSemantic, unsigned short sourceTexCoordSet, 
        unsigned short index)
    {
        // Make a new tangents semantic or find an existing one
        VertexDeclaration *vDecl = mVData->vertexDeclaration ;
        VertexBufferBinding *vBind = mVData->vertexBufferBinding ;

        const VertexElement *tangentsElem = vDecl->findElementBySemantic(targetSemantic, index);
        bool needsToBeCreated = false;
        VertexElementType tangentsType = mStoreParityInW ? VET_FLOAT4 : VET_FLOAT3;

        if (!tangentsElem)
        { // no tex coords with index 1
            needsToBeCreated = true ;
        }
        else if (tangentsElem->getType() != tangentsType)
        {
            //  buffer exists, but not 3D
            OGRE_EXCEPT(Exception::ERR_INVALIDPARAMS,
                "Target semantic set already exists but is not of the right size, therefore "
                "cannot contain tangents. You should delete this existing entry first. ",
                "TangentSpaceCalc::insertTangents");
        }

        HardwareVertexBufferSharedPtr targetBuffer, origBuffer;
        unsigned char* pSrc = NULL;

        if (needsToBeCreated)
        {
            // To be most efficient with our vertex streams,
            // tack the new tangents onto the same buffer as the
            // source texture coord set
            const VertexElement* prevTexCoordElem =
                mVData->vertexDeclaration->findElementBySemantic(
                VES_TEXTURE_COORDINATES, sourceTexCoordSet);
            if (!prevTexCoordElem)
            {
                OGRE_EXCEPT(Exception::ERR_ITEM_NOT_FOUND,
                    "Cannot locate the first texture coordinate element to "
                    "which to append the new tangents.", 
                    "Mesh::orgagniseTangentsBuffer");
            }
            // Find the buffer associated with  this element
            origBuffer = mVData->vertexBufferBinding->getBuffer(
                prevTexCoordElem->getSource());
            // Now create a new buffer, which includes the previous contents
            // plus extra space for the 3D coords
            targetBuffer = HardwareBufferManager::getSingleton().createVertexBuffer(
                origBuffer->getVertexSize() + VertexElement::getTypeSize(tangentsType),
                origBuffer->getNumVertices(),
                origBuffer->getUsage(),
                origBuffer->hasShadowBuffer() );
            // Add the new element
            tangentsElem = &(vDecl->addElement(
                prevTexCoordElem->getSource(),
                origBuffer->getVertexSize(),
                tangentsType,
                targetSemantic,
                index));
            // Set up the source pointer
            pSrc = static_cast<unsigned char*>(
                origBuffer->lock(HardwareBuffer::HBL_READ_ONLY));
            // Rebind the new buffer
            vBind->setBinding(prevTexCoordElem->getSource(), targetBuffer);
        }
        else
        {
            // space already there
            origBuffer = mVData->vertexBufferBinding->getBuffer(
                tangentsElem->getSource());
            targetBuffer = origBuffer;
        }


        unsigned char* pDest = static_cast<unsigned char*>(
            targetBuffer->lock(HardwareBuffer::HBL_DISCARD));
        size_t origVertSize = origBuffer->getVertexSize();
        size_t newVertSize = targetBuffer->getVertexSize();
        for (size_t v = 0; v < origBuffer->getNumVertices(); ++v)
        {
            if (needsToBeCreated)
            {
                // Copy original vertex data as well 
                memcpy(pDest, pSrc, origVertSize);
                pSrc += origVertSize;
            }
            // Write in the tangent
            float* pTangent;
            tangentsElem->baseVertexPointerToElement(pDest, &pTangent);
            VertexInfo& vertInfo = mVertexArray[v];
            *pTangent++ = vertInfo.tangent.x;
            *pTangent++ = vertInfo.tangent.y;
            *pTangent++ = vertInfo.tangent.z;
            if (mStoreParityInW)
                *pTangent++ = (float)vertInfo.parity;

            // Next target vertex
            pDest += newVertSize;

        }
        targetBuffer->unlock();

        if (needsToBeCreated)
        {
            origBuffer->unlock();
        }
    }

}
