source: ps/trunk/source/maths/Brush.cpp@ 25269

/* Copyright (C) 2021 Wildfire Games.
 * This file is part of 0 A.D.
 *
 * 0 A.D. is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * 0 A.D. 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
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "precompiled.h"

#include "Brush.h"

#include "maths/BoundingBoxAligned.h"
#include "maths/Frustum.h"

CBrush::CBrush() = default;

///////////////////////////////////////////////////////////////////////////////
// Convert the given bounds into a brush
CBrush::CBrush(const CBoundingBoxAligned& bounds)
{
    m_Vertices.resize(8);

    for(size_t i = 0; i < 8; ++i)
    {
        m_Vertices[i][0] = bounds[(i & 1) ? 1 : 0][0]; // X
        m_Vertices[i][1] = bounds[(i & 2) ? 1 : 0][1]; // Y
        m_Vertices[i][2] = bounds[(i & 4) ? 1 : 0][2]; // Z
    }

    // construct cube face indices, 5 vertex indices per face (start vertex included twice)

    m_Faces.resize(30);

    m_Faces[0] = 0; m_Faces[1] = 1; m_Faces[2] = 3; m_Faces[3] = 2; m_Faces[4] = 0; // Z = min
    m_Faces[5] = 4; m_Faces[6] = 5; m_Faces[7] = 7; m_Faces[8] = 6; m_Faces[9] = 4; // Z = max

    m_Faces[10] = 0; m_Faces[11] = 2; m_Faces[12] = 6; m_Faces[13] = 4; m_Faces[14] = 0; // X = min
    m_Faces[15] = 1; m_Faces[16] = 3; m_Faces[17] = 7; m_Faces[18] = 5; m_Faces[19] = 1; // X = max

    m_Faces[20] = 0; m_Faces[21] = 1; m_Faces[22] = 5; m_Faces[23] = 4; m_Faces[24] = 0; // Y = min
    m_Faces[25] = 2; m_Faces[26] = 3; m_Faces[27] = 7; m_Faces[28] = 6; m_Faces[29] = 2; // Y = max
}


///////////////////////////////////////////////////////////////////////////////
// Calculate bounds of this brush
void CBrush::Bounds(CBoundingBoxAligned& result) const
{
    result.SetEmpty();

    for(size_t i = 0; i < m_Vertices.size(); ++i)
        result += m_Vertices[i];
}


///////////////////////////////////////////////////////////////////////////////
// Cut the brush according to a given plane

/// Holds information about what happens to a single vertex in a brush during a slicing operation.
struct SliceOpVertexInfo
{
    float planeDist;    ///< Signed distance from this vertex to the slicing plane.
    size_t resIdx;      ///< Index of this vertex in the resulting brush (or NO_VERTEX if cut away)
};

/// Holds information about a newly introduced vertex on an edge in a brush as the result of a slicing operation.
struct SliceOpNewVertexInfo
{
    /// Indices of adjacent edge vertices in original brush
    size_t edgeIdx1, edgeIdx2;
    /// Index of newly introduced vertex in resulting brush
    size_t resIdx;

    /**
     * Index into SliceOpInfo.nvInfo; hold the indices of this new vertex's direct neighbours in the slicing plane face,
     * with no consistent winding direction around the face for either field (e.g., the neighb1 of X can point back to
     * X with either its neighb1 or neighb2).
     */
    size_t neighbIdx1, neighbIdx2;
};

/// Holds support information during a CBrush/CPlane slicing operation.
struct SliceOpInfo
{
    CBrush* result;
    const CBrush* original;

    /**
     * Holds information about what happens to each vertex in the original brush after the slice operation.
     * Same size as m_Vertices of the brush getting sliced.
     */
    std::vector<SliceOpVertexInfo> ovInfo;

    /// Holds information about newly inserted vertices during a slice operation.
    std::vector<SliceOpNewVertexInfo> nvInfo;

    /**
     * Indices into nvInfo; during the execution of the slicing algorithm, holds the previously inserted new vertex on
     * one of the edges of the face that's currently being evaluated for slice points, or NO_VERTEX if no such vertex
     * exists.
     */
    size_t thisFaceNewVertexIdx;
};

struct CBrush::Helper
{
    /**
     * Creates a new vertex between the given two vertices (indexed into the original brush).
     * Returns the index of the new vertex in the resulting brush.
     */
    static size_t SliceNewVertex(SliceOpInfo& sliceInfo, size_t v1, size_t v2);
};

size_t CBrush::Helper::SliceNewVertex(SliceOpInfo& sliceOp, size_t edgeIdx1, size_t edgeIdx2)
{
    // check if a new vertex has already been inserted on this edge
    size_t idx;
    for(idx = 0; idx < sliceOp.nvInfo.size(); ++idx)
    {
        if ((sliceOp.nvInfo[idx].edgeIdx1 == edgeIdx1 && sliceOp.nvInfo[idx].edgeIdx2 == edgeIdx2) ||
            (sliceOp.nvInfo[idx].edgeIdx1 == edgeIdx2 && sliceOp.nvInfo[idx].edgeIdx2 == edgeIdx1))
            break;
    }

    if (idx >= sliceOp.nvInfo.size())
    {
        // no previously inserted new vertex found on this edge; insert a new one
        SliceOpNewVertexInfo nvi;
        CVector3D newPos;

        // interpolate between the two vertices based on their distance from the plane
        float inv = 1.0 / (sliceOp.ovInfo[edgeIdx1].planeDist - sliceOp.ovInfo[edgeIdx2].planeDist);
        newPos = sliceOp.original->m_Vertices[edgeIdx2] * ( sliceOp.ovInfo[edgeIdx1].planeDist * inv) +
                 sliceOp.original->m_Vertices[edgeIdx1] * (-sliceOp.ovInfo[edgeIdx2].planeDist * inv);

        nvi.edgeIdx1 = edgeIdx1;
        nvi.edgeIdx2 = edgeIdx2;
        nvi.resIdx = sliceOp.result->m_Vertices.size();
        nvi.neighbIdx1 = NO_VERTEX;
        nvi.neighbIdx2 = NO_VERTEX;

        sliceOp.result->m_Vertices.push_back(newPos);
        sliceOp.nvInfo.push_back(nvi);
    }

    // at this point, 'idx' is the index into nvInfo of the vertex inserted onto the edge

    if (sliceOp.thisFaceNewVertexIdx != NO_VERTEX)
    {
        // a vertex has been previously inserted onto another edge of this face; link them together as neighbours
        // (using whichever one of the neighbIdx1 or -2 links is still available)

        if (sliceOp.nvInfo[sliceOp.thisFaceNewVertexIdx].neighbIdx1 == NO_VERTEX)
            sliceOp.nvInfo[sliceOp.thisFaceNewVertexIdx].neighbIdx1 = idx;
        else
            sliceOp.nvInfo[sliceOp.thisFaceNewVertexIdx].neighbIdx2 = idx;

        if (sliceOp.nvInfo[idx].neighbIdx1 == NO_VERTEX)
            sliceOp.nvInfo[idx].neighbIdx1 = sliceOp.thisFaceNewVertexIdx;
        else
            sliceOp.nvInfo[idx].neighbIdx2 = sliceOp.thisFaceNewVertexIdx;

        // a plane should slice a face only in two locations, so reset for the next face
        sliceOp.thisFaceNewVertexIdx = NO_VERTEX;
    }
    else
    {
        // store the index of the inserted vertex on this edge, so that we can retrieve it when the plane slices
        // this face again in another edge
        sliceOp.thisFaceNewVertexIdx = idx;
    }

    return sliceOp.nvInfo[idx].resIdx;
}

void CBrush::Slice(const CPlane& plane, CBrush& result) const
{
    ENSURE(&result != this);

    SliceOpInfo sliceOp;

    sliceOp.original = this;
    sliceOp.result = &result;
    sliceOp.thisFaceNewVertexIdx = NO_VERTEX;
    sliceOp.ovInfo.resize(m_Vertices.size());
    sliceOp.nvInfo.reserve(m_Vertices.size() / 2);

    result.m_Vertices.resize(0); // clear any left-overs
    result.m_Faces.resize(0);
    result.m_Vertices.reserve(m_Vertices.size() + 2);
    result.m_Faces.reserve(m_Faces.size() + 5);

    // Copy vertices that weren't sliced away by the plane to the resulting brush.
    for(size_t i = 0; i < m_Vertices.size(); ++i)
    {
        const CVector3D& vtx = m_Vertices[i];            // current vertex
        SliceOpVertexInfo& vtxInfo = sliceOp.ovInfo[i];  // slicing operation info about current vertex

        vtxInfo.planeDist = plane.DistanceToPlane(vtx);
        if (vtxInfo.planeDist >= 0.0)
        {
            // positive side of the plane; not sliced away
            vtxInfo.resIdx = result.m_Vertices.size();
            result.m_Vertices.push_back(vtx);
        }
        else
        {
            // other side of the plane; sliced away
            vtxInfo.resIdx = NO_VERTEX;
        }
    }

    // Transfer faces. (Recall how faces are specified; see CBrush::m_Faces). The idea is to examine each face separately,
    // and see where its edges cross the slicing plane (meaning that exactly one of the vertices of that edge was cut away).
    // On those edges, new vertices are introduced where the edge intersects the plane, and the resulting brush's m_Faces
    // array is updated to refer to the newly inserted vertices instead of the original one that got cut away.

    size_t currentFaceStartIdx = NO_VERTEX; // index of the first vertex of the current face in the original brush
    size_t resultFaceStartIdx = NO_VERTEX;  // index of the first vertex of the current face in the resulting brush

    for(size_t i = 0; i < m_Faces.size(); ++i)
    {
        if (currentFaceStartIdx == NO_VERTEX)
        {
            // starting a new face
            ENSURE(sliceOp.thisFaceNewVertexIdx == NO_VERTEX);

            currentFaceStartIdx = m_Faces[i];
            resultFaceStartIdx = result.m_Faces.size();
            continue;
        }

        size_t prevIdx = m_Faces[i-1];  // index of previous vertex in this face list
        size_t curIdx = m_Faces[i];     // index of current vertex in this face list

        if (sliceOp.ovInfo[prevIdx].resIdx == NO_VERTEX)
        {
            // previous face vertex got sliced away by the plane; see if the edge (prev,current) crosses the slicing plane
            if (sliceOp.ovInfo[curIdx].resIdx != NO_VERTEX)
            {
                // re-entering the front side of the plane; insert vertex on intersection of plane and (prev,current) edge
                result.m_Faces.push_back(Helper::SliceNewVertex(sliceOp, prevIdx, curIdx));
                result.m_Faces.push_back(sliceOp.ovInfo[curIdx].resIdx);
            }
        }
        else
        {
            // previous face vertex didn't get sliced away; see if the edge (prev,current) crosses the slicing plane
            if (sliceOp.ovInfo[curIdx].resIdx != NO_VERTEX)
            {
                // perfectly normal edge; doesn't cross the plane
                result.m_Faces.push_back(sliceOp.ovInfo[curIdx].resIdx);
            }
            else
            {
                // leaving the front side of the plane; insert vertex on intersection of plane and edge (prev, current)
                result.m_Faces.push_back(Helper::SliceNewVertex(sliceOp, prevIdx, curIdx));
            }
        }

        // if we're back at the first vertex of the current face, then we've completed the face
        if (curIdx == currentFaceStartIdx)
        {
            // close the index loop
            if (result.m_Faces.size() > resultFaceStartIdx)
                result.m_Faces.push_back(result.m_Faces[resultFaceStartIdx]);

            currentFaceStartIdx = NO_VERTEX; // start a new face
        }
    }

    ENSURE(currentFaceStartIdx == NO_VERTEX);

    // Create the face that lies in the slicing plane. Remember, all the intersections of the slicing plane with face
    // edges of the brush have been stored in sliceOp.nvInfo by the SliceNewVertex function, and refer to their direct
    // neighbours in the slicing plane face using the neighbIdx1 and neighbIdx2 fields (in no consistent winding order).

    if (sliceOp.nvInfo.size())
    {
        // push the starting vertex
        result.m_Faces.push_back(sliceOp.nvInfo[0].resIdx);

        // At this point, there is no consistent winding order in the neighbX fields, so at each vertex we need to figure
        // out whether neighb1 or neighb2 points 'onwards' along the face, according to an initially chosen winding direction.
        // (or, equivalently, which one points back to the one we were just at). At each vertex, we then set neighb1 to be the
        // one to point onwards, deleting any pointers which we no longer need to complete the trace.

        size_t idx;
        size_t prev = 0;

        idx = sliceOp.nvInfo[0].neighbIdx2; // pick arbitrary starting direction
        sliceOp.nvInfo[0].neighbIdx2 = NO_VERTEX;

        while(idx != 0)
        {
            ENSURE(idx < sliceOp.nvInfo.size());
            if (idx >= sliceOp.nvInfo.size())
                break;

            if (sliceOp.nvInfo[idx].neighbIdx1 == prev)
            {
                // neighb1 is pointing the wrong way; we want to normalize it to point onwards in the direction
                // we initially chose, so swap it with neighb2 and delete neighb2 (no longer needed)
                sliceOp.nvInfo[idx].neighbIdx1 = sliceOp.nvInfo[idx].neighbIdx2;
                sliceOp.nvInfo[idx].neighbIdx2 = NO_VERTEX;
            }
            else
            {
                // neighb1 isn't pointing to the previous vertex, so neighb2 must be (otherwise a pair of vertices failed to
                // get paired properly during face/plane slicing).
                ENSURE(sliceOp.nvInfo[idx].neighbIdx2 == prev);
                sliceOp.nvInfo[idx].neighbIdx2 = NO_VERTEX;
            }

            result.m_Faces.push_back(sliceOp.nvInfo[idx].resIdx);

            // move to next vertex; neighb1 has been normalized to point onward
            prev = idx;
            idx = sliceOp.nvInfo[idx].neighbIdx1;
            sliceOp.nvInfo[prev].neighbIdx1 = NO_VERTEX; // no longer needed, we've moved on
        }

        // push starting vertex again to close the shape
        result.m_Faces.push_back(sliceOp.nvInfo[0].resIdx);
    }
}



///////////////////////////////////////////////////////////////////////////////
// Intersect with frustum by repeated slicing
void CBrush::Intersect(const CFrustum& frustum, CBrush& result) const
{
    ENSURE(&result != this);

    if (!frustum.GetNumPlanes())
    {
        result = *this;
        return;
    }

    CBrush buf;
    const CBrush* prev = this;
    CBrush* next;

    // Repeatedly slice this brush with each plane of the frustum, alternating between 'result' and 'buf' to
    // save intermediate results. Set up the starting brush so that the final version always ends up in 'result'.

    if (frustum.GetNumPlanes() & 1)
        next = &result;
    else
        next = &buf;

    for(size_t i = 0; i < frustum.GetNumPlanes(); ++i)
    {
        prev->Slice(frustum[i], *next);
        prev = next;
        if (prev == &buf)
            next = &result;
        else
            next = &buf;
    }

    ENSURE(prev == &result);
}

const std::vector<CVector3D>& CBrush::GetVertices() const
{
    return m_Vertices;
}

void CBrush::GetFaces(std::vector<std::vector<size_t>>& out) const
{
    // split the back-to-back faces into separate face vectors, so that they're in a
    // user-friendlier format than the back-to-back vertex index array
    // i.e. split 'x--xy------yz----z' into 'x--x', 'y-------y', 'z---z'

    size_t faceStartIdx = 0;
    while (faceStartIdx < m_Faces.size())
    {
        // start new face
        std::vector<size_t> singleFace;
        singleFace.push_back(m_Faces[faceStartIdx]);

        // step over all the values in the face until we hit the starting value again (which closes the face)
        size_t j = faceStartIdx + 1;
        while (j < m_Faces.size() && m_Faces[j] != m_Faces[faceStartIdx])
        {
            singleFace.push_back(m_Faces[j]);
            j++;
        }

        // each face must be closed by the same value that started it
        ENSURE(m_Faces[faceStartIdx] == m_Faces[j]);

        singleFace.push_back(m_Faces[j]);
        out.push_back(singleFace);

        faceStartIdx = j + 1;
    }
}