TriStrip_tri_stripper.cpp

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/* -*-c++-*- OpenSceneGraph - Copyright (C) 1998-2006 Robert Osfield 
 *
 * This library is open source and may be redistributed and/or modified under  
 * the terms of the OpenSceneGraph Public License (OSGPL) version 0.0 or 
 * (at your option) any later version.  The full license is in LICENSE file
 * included with this distribution, and on the openscenegraph.org website.
 * 
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
 * OpenSceneGraph Public License for more details.
*/
// tri_stripper.cpp: implementation of the Tri Stripper class.
//
// Copyright (C) 2002 Tanguy Fautré.
// For conditions of distribution and use,
// see copyright notice in tri_stripper.h
//

#include "TriStrip_tri_stripper.h"



// namespace triangle_stripper
namespace triangle_stripper {




// Construction/Destruction



// Members Functions

void tri_stripper::Strip(primitives_vector * out_pPrimitivesVector)
{
    // verify that the number of indices is correct
    if (m_TriIndices.size() % 3 != 0)
        throw triangles_indices_error();

    // clear possible garbage
    m_PrimitivesVector.clear();
    out_pPrimitivesVector->clear();

    // Initialize the triangle graph
    InitTriGraph();

    // Initialize the triangle priority queue
    InitTriHeap();

    // Initialize the cache simulator
    InitCache();

    // Launch the triangle strip generator
    Stripify();

    // Add the triangles that couldn't be stripped
    AddLeftTriangles();

    // Free ressources
    m_Triangles.clear();
    
    // Put the results into the user's vector
    std::swap(m_PrimitivesVector, (* out_pPrimitivesVector));
//    m_PrimitivesVector.swap(* out_pPrimitivesVector);
}



void tri_stripper::InitTriGraph()
{
    // Set up the graph size and complete the triangles data
    // note: setsize() completely resets the graph as well as the node markers
    m_Triangles.setsize(m_TriIndices.size() / 3);

    size_t i;
    for (i = 0; i < m_Triangles.size(); ++i)
        m_Triangles[i] = triangle(m_TriIndices[i * 3 + 0], m_TriIndices[i * 3 + 1], m_TriIndices[i * 3 + 2]);

    // Build the edges lookup table
    triangle_edges TriInterface;
    TriInterface.reserve(m_Triangles.size() * 3);

    for (i = 0; i < m_Triangles.size(); ++i) {
        TriInterface.push_back(triangle_edge(m_Triangles[i]->A(), m_Triangles[i]->B(), i)); 
        TriInterface.push_back(triangle_edge(m_Triangles[i]->B(), m_Triangles[i]->C(), i)); 
        TriInterface.push_back(triangle_edge(m_Triangles[i]->C(), m_Triangles[i]->A(), i)); 
    }

    // Sort the lookup table for faster searches
    std::sort(TriInterface.begin(), TriInterface.end(), _cmp_tri_interface_lt());

    // Link neighbour triangles together using the edges lookup table
    for (i = 0; i < m_Triangles.size(); ++i) {

        const triangle_edge EdgeBA(m_Triangles[i]->B(), m_Triangles[i]->A(), i);
        const triangle_edge EdgeCB(m_Triangles[i]->C(), m_Triangles[i]->B(), i);
        const triangle_edge EdgeAC(m_Triangles[i]->A(), m_Triangles[i]->C(), i);

        LinkNeighboursTri(TriInterface, EdgeBA);
        LinkNeighboursTri(TriInterface, EdgeCB);
        LinkNeighboursTri(TriInterface, EdgeAC);
    }    
}



void tri_stripper::LinkNeighboursTri(const triangle_edges & TriInterface, const triangle_edge Edge)
{
    typedef triangle_edges::const_iterator edge_const_it;

    // Find the first edge equal to Edge
    edge_const_it It = std::lower_bound(TriInterface.begin(), TriInterface.end(), Edge, _cmp_tri_interface_lt());

    // See if there are any other edges that are equal
    // (if so, it means that more than 2 triangles are sharing the same edge,
    //  which is unlikely but not impossible)
    for (; (It != TriInterface.end()) && ((It->A() == Edge.A()) && (It->B() == Edge.B())); ++It)
        m_Triangles.insert(Edge.TriPos(), It->TriPos());

    // Note: degenerated triangles will also point themselves as neighbour triangles
}



void tri_stripper::InitTriHeap()
{
    m_TriHeap.clear();
    m_TriHeap.reserve(m_Triangles.size());

    // Set up the triangles priority queue
    // The lower the number of available neighbour triangles, the higher the priority.
    for (size_t i = 0; i < m_Triangles.size(); ++i)
        m_TriHeap.push(triangle_degree(i, m_Triangles[i].number_of_out_arcs()));

    // Remove useless triangles
    // (Note: we had to put all of them into the heap before to ensure coherency of the heap_array object)
    while ((! m_TriHeap.empty()) && (m_TriHeap.top().Degree() == 0))
        m_TriHeap.pop();
}



void tri_stripper::InitCache()
{
    m_IndicesCache.clear();

    if (m_CacheSize > 0)
        m_IndicesCache.resize(m_CacheSize, static_cast<indice>(-1));
}



void tri_stripper::Stripify()
{
    // Reset the triangle strip id selector
    m_StripID = 0;

    // Reset the candidate list
    m_NextCandidates.clear();

    // Loop untill there is no available candidate triangle left
    while (! m_TriHeap.empty()) {

        // There is no triangle in the candidates list, refill it with the loneliest triangle
        const size_t HeapTop = m_TriHeap.top().TriPos();
        m_NextCandidates.push_back(HeapTop);

        // Loop while BuildStrip can find good candidates for us
        while (! m_NextCandidates.empty()) {

            // Choose the best strip containing that triangle
            // Note: FindBestStrip empties m_NextCandidates
            const triangle_strip TriStrip = FindBestStrip();

            // Build it if it's long enough, otherwise discard it
            // Note: BuildStrip refills m_NextCandidates
            if (TriStrip.Size() >= m_MinStripSize)
                BuildStrip(TriStrip);
        }

        // We must discard the triangle we inserted in the candidate list from the heap
        // if it led to nothing. (We simply removed it if it hasn't been removed by BuildStrip() yet)
        if (! m_TriHeap.removed(HeapTop))
            m_TriHeap.erase(HeapTop);


        // Eliminate all the triangles that have now become useless
        while ((! m_TriHeap.empty()) && (m_TriHeap.top().Degree() == 0))
            m_TriHeap.pop();
    }
}



triangle_strip tri_stripper::FindBestStrip()
{
    triangle_strip BestStrip;
    size_t BestStripDegree = 0;
    size_t BestStripCacheHits = 0;

    // Backup the cache, because it'll be erased during the simulations
    indices_cache CacheBackup = m_IndicesCache;

    while (! m_NextCandidates.empty()) {

        // Discard useless triangles from the candidates list
        if ((m_Triangles[m_NextCandidates.back()].marked()) || (m_TriHeap[m_NextCandidates.back()].Degree() == 0)) {
            m_NextCandidates.pop_back();

            // "continue" is evil! But it really makes things easier here.
            // The useless triangle is discarded, and the "while" just rebegins again
            continue;
        }

        // Invariant: (CandidateTri's Degree() >= 1) && (CandidateTri is not marked).
        // So it can directly be used.
        const size_t CandidateTri = m_NextCandidates.back();
        m_NextCandidates.pop_back();

        // Try to extend the triangle in the 3 possible directions
        for (size_t i = 0; i < 3; ++i) {

            // Reset the cache hit count
            m_CacheHits = 0;

            // Try a new strip with that triangle in a particular direction
            const triangle_strip TempStrip = ExtendTriToStrip(CandidateTri, triangle_strip::start_order(i));

            // Restore the cache (modified by ExtendTriToStrip)
            m_IndicesCache = CacheBackup;

            // We want to keep the best strip
            // Discard strips that don't match the minimum required size
            if (TempStrip.Size() >= m_MinStripSize) {

                // Cache simulator disabled?
                if (m_CacheSize == 0) {

                    // Cache is disabled, take the longest strip
                    if (TempStrip.Size() > BestStrip.Size())
                        BestStrip = TempStrip;

                // Cache simulator enabled
                // Use other criteria to find the "best" strip
                } else {

                    // Priority 1: Keep the strip with the best cache hit count
                    if (m_CacheHits > BestStripCacheHits) {
                        BestStrip = TempStrip;
                        BestStripDegree = m_TriHeap[TempStrip.StartTriPos()].Degree();
                        BestStripCacheHits = m_CacheHits;

                    } else if (m_CacheHits == BestStripCacheHits) {

                        // Priority 2: Keep the strip with the loneliest start triangle
                        if ((BestStrip.Size() != 0) && (m_TriHeap[TempStrip.StartTriPos()].Degree() < BestStripDegree)) {
                            BestStrip = TempStrip;
                            BestStripDegree = m_TriHeap[TempStrip.StartTriPos()].Degree();

                        // Priority 3: Keep the longest strip 
                        } else if (TempStrip.Size() > BestStrip.Size()) {
                            BestStrip = TempStrip;
                            BestStripDegree = m_TriHeap[TempStrip.StartTriPos()].Degree();
                        }
                    }
                }
            }

        }

    }

    return BestStrip;
}



triangle_strip tri_stripper::ExtendTriToStrip(const size_t StartTriPos, const triangle_strip::start_order StartOrder)
{
    typedef triangles_graph::const_out_arc_iterator const_tri_link_iter;
    typedef triangles_graph::node_iterator tri_node_iter;

    size_t Size = 1;
    bool ClockWise = false;
    triangle_strip::start_order Order = StartOrder;

    // Begin a new strip
    ++m_StripID;

    // Mark the first triangle as used for this strip
    m_Triangles[StartTriPos]->SetStripID(m_StripID);

    // Update the indice cache
    AddTriToCache((* m_Triangles[StartTriPos]), Order);


    // Loop while we can further extend the strip
    for (tri_node_iter TriNodeIt = (m_Triangles.begin() + StartTriPos); 
        (TriNodeIt != m_Triangles.end()) && ((m_CacheSize <= 0) || ((Size + 2) < m_CacheSize)); 
        ++Size) {

        // Get the triangle edge that would lead to the next triangle
        const triangle_edge Edge = GetLatestEdge(** TriNodeIt, Order);

        // Link to a neighbour triangle
        const_tri_link_iter LinkIt;
        for (LinkIt = TriNodeIt->out_begin(); LinkIt != TriNodeIt->out_end(); ++LinkIt) {

            // Get the reference to the possible next triangle
            const triangle & Tri = (** (LinkIt->terminal()));

            // Check whether it's already been used
            if ((Tri.StripID() != m_StripID) && (! (LinkIt->terminal()->marked()))) {

                // Does the current candidate triangle match the required for the strip?

                if ((Edge.B() == Tri.A()) && (Edge.A() == Tri.B())) {
                    Order = (ClockWise) ? triangle_strip::ABC : triangle_strip::BCA;
                    AddIndiceToCache(Tri.C(), true);
                    break;
                }

                else if ((Edge.B() == Tri.B()) && (Edge.A() == Tri.C())) {
                    Order = (ClockWise) ? triangle_strip::BCA : triangle_strip::CAB;
                    AddIndiceToCache(Tri.A(), true);
                    break;
                }

                else if ((Edge.B() == Tri.C()) && (Edge.A() == Tri.A())) {
                    Order = (ClockWise) ? triangle_strip::CAB : triangle_strip::ABC;
                    AddIndiceToCache(Tri.B(), true);
                    break;
                }
            }
        }

        // Is it the end of the strip?
        if (LinkIt == TriNodeIt->out_end()) {
            TriNodeIt = m_Triangles.end();
            --Size;
        } else {
            TriNodeIt = LinkIt->terminal();
    
            // Setup for the next triangle
            (* TriNodeIt)->SetStripID(m_StripID);
            ClockWise = ! ClockWise;
        }
    }


    return triangle_strip(StartTriPos, StartOrder, Size);
}



triangle_edge tri_stripper::GetLatestEdge(const triangle & Triangle, const triangle_strip::start_order Order) const
{
    switch (Order) {
    case triangle_strip::ABC:
        return triangle_edge(Triangle.B(), Triangle.C(), 0);
    case triangle_strip::BCA:
        return triangle_edge(Triangle.C(), Triangle.A(), 0);
    case triangle_strip::CAB:
        return triangle_edge(Triangle.A(), Triangle.B(), 0);
    default:
        return triangle_edge(0, 0, 0);
    }
}



void tri_stripper::BuildStrip(const triangle_strip TriStrip)
{
    typedef triangles_graph::const_out_arc_iterator const_tri_link_iter;
    typedef triangles_graph::node_iterator tri_node_iter;

    const size_t StartTriPos = TriStrip.StartTriPos();

    bool ClockWise = false;
    triangle_strip::start_order Order = TriStrip.StartOrder();

    // Create a new strip
    m_PrimitivesVector.push_back(primitives());
    m_PrimitivesVector.back().m_Type = PT_Triangle_Strip;

    // Put the first triangle into the strip
    AddTriToIndices((* m_Triangles[StartTriPos]), Order);

    // Mark the first triangle as used
    MarkTriAsTaken(StartTriPos);


    // Loop while we can further extend the strip
    tri_node_iter TriNodeIt = (m_Triangles.begin() + StartTriPos);

    for (size_t Size = 1; Size < TriStrip.Size(); ++Size) {

        // Get the triangle edge that would lead to the next triangle
        const triangle_edge Edge = GetLatestEdge(** TriNodeIt, Order);

        // Link to a neighbour triangle
        const_tri_link_iter LinkIt;
        for (LinkIt = TriNodeIt->out_begin(); LinkIt != TriNodeIt->out_end(); ++LinkIt) {

            // Get the reference to the possible next triangle
            const triangle & Tri = (** (LinkIt->terminal()));

            // Check whether it's already been used
            if (! (LinkIt->terminal()->marked())) {

                // Does the current candidate triangle match the required for the strip?
                // If it does, then add it to the Indices
                if ((Edge.B() == Tri.A()) && (Edge.A() == Tri.B())) {
                    Order = (ClockWise) ? triangle_strip::ABC : triangle_strip::BCA;
                    AddIndice(Tri.C());
                    break;
                }

                else if ((Edge.B() == Tri.B()) && (Edge.A() == Tri.C())) {
                    Order = (ClockWise) ? triangle_strip::BCA : triangle_strip::CAB;
                    AddIndice(Tri.A());
                    break;
                }

                else if ((Edge.B() == Tri.C()) && (Edge.A() == Tri.A())) {
                    Order = (ClockWise) ? triangle_strip::CAB : triangle_strip::ABC;
                    AddIndice(Tri.B());
                    break;
                }
            }
        }

        // Debug check: we must have found the next triangle
        //assert(LinkIt != TriNodeIt->out_end());        
        if (LinkIt == TriNodeIt->out_end()) throw "tri_stripper::BuildStrip(,) error, next triangle not found";


        // Go to the next triangle
        TriNodeIt = LinkIt->terminal();
        MarkTriAsTaken(TriNodeIt - m_Triangles.begin());
        
        // Setup for the next triangle
        ClockWise = ! ClockWise;
    }
}



void tri_stripper::MarkTriAsTaken(const size_t i)
{
    typedef triangles_graph::node_iterator tri_node_iter;
    typedef triangles_graph::out_arc_iterator tri_link_iter;

    // Mark the triangle node
    m_Triangles[i].mark();

    // Remove triangle from priority queue if it isn't yet
    if (! m_TriHeap.removed(i))
        m_TriHeap.erase(i);

    // Adjust the degree of available neighbour triangles
    for (tri_link_iter LinkIt = m_Triangles[i].out_begin(); LinkIt != m_Triangles[i].out_end(); ++LinkIt) {

        const size_t j = LinkIt->terminal() - m_Triangles.begin();

        if ((! m_Triangles[j].marked()) && (! m_TriHeap.removed(j))) {
            triangle_degree NewDegree = m_TriHeap.peek(j);
            NewDegree.SetDegree(NewDegree.Degree() - 1);
            m_TriHeap.update(j, NewDegree);

            // Update the candidate list if cache is enabled
            if ((m_CacheSize > 0) && (NewDegree.Degree() > 0))
                m_NextCandidates.push_back(j);
        }
    }
}



void tri_stripper::AddIndiceToCache(const indice i, bool CacheHitCount)
{
    // Cache simulator enabled?
    if (m_CacheSize > 0) {

        // Should we simulate the cache hits and count them?
        if (CacheHitCount) {
            if (std::find(m_IndicesCache.begin(), m_IndicesCache.end(), i) != m_IndicesCache.end())
                ++m_CacheHits;
        }
        
        // Manage the indices cache as a FIFO structure
        m_IndicesCache.pop_back();
        m_IndicesCache.push_front(i);
    }
}



void tri_stripper::AddIndice(const indice i)
{
    // Add the indice to the current indices array
    m_PrimitivesVector.back().m_Indices.push_back(i);

    // Run cache simulator
    AddIndiceToCache(i);
}



void tri_stripper::AddTriToCache(const triangle & Tri, const triangle_strip::start_order Order)
{
    // Add Tri indices in the right order into the indices cache simulator.
    // And enable the cache hit count
    switch (Order) {
    case triangle_strip::ABC:
        AddIndiceToCache(Tri.A(), true);
        AddIndiceToCache(Tri.B(), true);
        AddIndiceToCache(Tri.C(), true);
        return;
    case triangle_strip::BCA:
        AddIndiceToCache(Tri.B(), true);
        AddIndiceToCache(Tri.C(), true);
        AddIndiceToCache(Tri.A(), true);
        return;
    case triangle_strip::CAB:
        AddIndiceToCache(Tri.C(), true);
        AddIndiceToCache(Tri.A(), true);
        AddIndiceToCache(Tri.B(), true);
        return;
    }
}



void tri_stripper::AddTriToIndices(const triangle & Tri, const triangle_strip::start_order Order)
{
    // Add Tri indices in the right order into the latest Indices vector.
    switch (Order) {
    case triangle_strip::ABC:
        AddIndice(Tri.A());
        AddIndice(Tri.B());
        AddIndice(Tri.C());
        return;
    case triangle_strip::BCA:
        AddIndice(Tri.B());
        AddIndice(Tri.C());
        AddIndice(Tri.A());
        return;
    case triangle_strip::CAB:
        AddIndice(Tri.C());
        AddIndice(Tri.A());
        AddIndice(Tri.B());
        return;
    }
}



void tri_stripper::AddLeftTriangles()
{
    // Create the latest indices array
    // and fill it with all the triangles that couldn't be stripped
    primitives Primitives;
    Primitives.m_Type = PT_Triangles;
    m_PrimitivesVector.push_back(Primitives);
    indices & Indices = m_PrimitivesVector.back().m_Indices;

    for (size_t i = 0; i < m_Triangles.size(); ++i)
        if (! m_Triangles[i].marked()) {
            Indices.push_back(m_Triangles[i]->A());
            Indices.push_back(m_Triangles[i]->B());
            Indices.push_back(m_Triangles[i]->C());
        }

    // Undo if useless
    if (Indices.size() == 0)
        m_PrimitivesVector.pop_back();
}




} // namespace triangle_stripper