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Refactoring and fixing remaining issues in triangulation process

pull/3695/head
Clement Jacob 2021-03-30 09:06:51 +02:00
parent 596001c89c
commit 2e90fed527
1 changed files with 87 additions and 28 deletions
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115
code/PostProcessing/TriangulateProcess.cpp
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@ -76,6 +76,69 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
using namespace Assimp;
namespace {
/**
* @brief Helper struct used to simplify NGON encoding functions.
*/
struct NGONEncoder {
NGONEncoder(const aiFace * outTriArray, size_t outTriArraySize)
: mOutTriArrayBeg(outTriArray), mOutTriArrayEnd(outTriArray + outTriArraySize) {}
/**
* @brief Encode the current triangle, and make sure it is recognized as a triangle.
*
* This method will rotate indices in tri if needed in order to avoid tri to be considered
* part of the previous ngon. This method is to be used whenever you want to emit a real triangle,
* and make sure it is seen as a triangle.
*
* @param tri Current triangle, must be in bounds of the outTriArray (= be a cell within the array).
*/
void ngonEncodeTriangle(aiFace * tri) const {
ai_assert(tri >= mOutTriArrayBeg && tri < mOutTriArrayEnd);
ai_assert(tri->mNumIndices == 3);
// Rotate indices in new triangle to avoid ngon encoding false ngons
// Otherwise, the new triangle would be considered part of the previous NGON.
if (isConsideredSameAsLastNgon(tri)) {
std::swap(tri->mIndices[0], tri->mIndices[2]);
std::swap(tri->mIndices[1], tri->mIndices[2]);
}
}
/**
* @brief Check whether this triangle would be considered part of the lastly emitted ngon or not.
*
* @param tri Current triangle.
* @return true If used as is, this triangle will be part of last ngon.
* @return false If used as is, this triangle is not considered part of the last ngon.
*/
bool isConsideredSameAsLastNgon(const aiFace * tri) const {
ai_assert(tri >= mOutTriArrayBeg && tri < mOutTriArrayEnd);
ai_assert(tri->mNumIndices == 3);
// First triangle to be emitted, so no problem here
if (tri == mOutTriArrayBeg) return false;
const aiFace * prevTri = tri - 1;
return tri->mIndices[0] == prevTri->mIndices[0];
}
private:
/**
* @brief Begining of triangulation process out triangles array
*/
const aiFace * mOutTriArrayBeg;
/**
* @brief End of triangulation process out triangles array (out of bounds, a la C++ iterators).
*/
const aiFace * mOutTriArrayEnd;
};
}
// ------------------------------------------------------------------------------------------------
// Constructor to be privately used by Importer
TriangulateProcess::TriangulateProcess()
@ -175,10 +238,15 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
pMesh->mPrimitiveTypes |= aiPrimitiveType_TRIANGLE;
pMesh->mPrimitiveTypes &= ~aiPrimitiveType_POLYGON;
// The mesh becomes NGON encoded now, during the triangulation process.
pMesh->mPrimitiveTypes |= aiPrimitiveType_NGONEncodingFlag;
aiFace* out = new aiFace[numOut](), *curOut = out;
std::vector<aiVector3D> temp_verts3d(max_out+2); /* temporary storage for vertices */
std::vector<aiVector2D> temp_verts(max_out+2);
const NGONEncoder ngonEncoder(out, numOut);
// Apply vertex colors to represent the face winding?
#ifdef AI_BUILD_TRIANGULATE_COLOR_FACE_WINDING
if (!pMesh->mColors[0])
@ -195,13 +263,6 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
const aiVector3D* verts = pMesh->mVertices;
// NGON encoding note: making sure that triangles are not recognized as false ngons.
// To do so, we make sure the first indice of the new emitted triangle is not the same as previous one.
unsigned int prev_first_indice = (unsigned int)-1;
// The mesh becomes NGON encoded now, during the triangulation process.
pMesh->mPrimitiveTypes |= aiPrimitiveType_NGONEncodingFlag;
// use std::unique_ptr to avoid slow std::vector<bool> specialiations
std::unique_ptr<bool[]> done(new bool[max_out]);
for( unsigned int a = 0; a < pMesh->mNumFaces; a++) {
@ -228,6 +289,7 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
nface.mNumIndices = face.mNumIndices;
nface.mIndices = face.mIndices;
face.mIndices = nullptr;
ngonEncoder.ngonEncodeTriangle(&nface);
continue;
}
// optimized code for quadrilaterals
@ -236,12 +298,6 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
// quads can have at maximum one concave vertex. Determine
// this vertex (if it exists) and start tri-fanning from
// it.
//
// Due to NGON encoding, if this concave vertex is the same as the previously
// emitted triangle, we use the opposite vertex which also happens to work
// for tri-fanning a concave quad.
// ref: https://github.com/assimp/assimp/pull/3695#issuecomment-805999760
unsigned int start_vertex = 0;
for (unsigned int i = 0; i < 4; ++i) {
const aiVector3D& v0 = verts[face.mIndices[(i+3) % 4]];
@ -266,11 +322,6 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
}
}
// ngon encoding: if vertex is same as last triangle first index,
// then we chose the opposite vertex (works for both concave & convex quad).
if (face.mIndices[start_vertex] == prev_first_indice)
start_vertex = (start_vertex+2) % 4;
const unsigned int temp[] = {face.mIndices[0], face.mIndices[1], face.mIndices[2], face.mIndices[3]};
aiFace& nface = *curOut++;
@ -281,6 +332,20 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
nface.mIndices[1] = temp[(start_vertex + 1) % 4];
nface.mIndices[2] = temp[(start_vertex + 2) % 4];
// Due to NGON encoding, if the selected fanning vertex is the same as the previously
// emitted ngon, we use the opposite vertex which also happens to work
// for tri-fanning a concave quad.
// ref: https://github.com/assimp/assimp/pull/3695#issuecomment-805999760
//
// @warning No need to call ngonEncoder.ngonEncodeTriangle() here. We want these 2 faces to be seen as
// a single quad, not 2 separate triangles. This is the whole purpose!
if (ngonEncoder.isConsideredSameAsLastNgon(&nface)) {
start_vertex = (start_vertex+2) % 4;
nface.mIndices[0] = temp[start_vertex];
nface.mIndices[1] = temp[(start_vertex + 1) % 4];
nface.mIndices[2] = temp[(start_vertex + 2) % 4];
}
aiFace& sface = *curOut++;
sface.mNumIndices = 3;
sface.mIndices = new unsigned int[3];
@ -526,16 +591,10 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
i[1] = idx[i[1]];
i[2] = idx[i[2]];
// NGON encoding: only quads are supported.
// For everything else, we make sure we don't emit 'false' ngons. We thus avoid having
// 2 consecutive triangles with their first index identical.
if (face.mNumIndices != 4 && i[0] == prev_first_indice) {
// rotate indices
std::swap(i[0], i[2]);
std::swap(i[1], i[2]);
}
prev_first_indice = i[0];
// IMPROVEMENT: Polygons are not supported yet by this ngon encoding + triangulation step.
// So we encode polygons as regular triangles. No way to reconstruct the original
// polygon in this case.
ngonEncoder.ngonEncodeTriangle(f);
++f;
}