assimp/code/PostProcessing/TriangulateProcess.cpp

592 lines
22 KiB
C++

/*
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/** @file TriangulateProcess.cpp
* @brief Implementation of the post processing step to split up
* all faces with more than three indices into triangles.
*
*
* The triangulation algorithm will handle concave or convex polygons.
* Self-intersecting or non-planar polygons are not rejected, but
* they're probably not triangulated correctly.
*
* DEBUG SWITCHES - do not enable any of them in release builds:
*
* AI_BUILD_TRIANGULATE_COLOR_FACE_WINDING
* - generates vertex colors to represent the face winding order.
* the first vertex of a polygon becomes red, the last blue.
* AI_BUILD_TRIANGULATE_DEBUG_POLYS
* - dump all polygons and their triangulation sequences to
* a file
*/
#ifndef ASSIMP_BUILD_NO_TRIANGULATE_PROCESS
#include "PostProcessing/TriangulateProcess.h"
#include "PostProcessing/ProcessHelper.h"
#include "Common/PolyTools.h"
#include <memory>
#include <cstdint>
//#define AI_BUILD_TRIANGULATE_COLOR_FACE_WINDING
//#define AI_BUILD_TRIANGULATE_DEBUG_POLYS
#define POLY_GRID_Y 40
#define POLY_GRID_X 70
#define POLY_GRID_XPAD 20
#define POLY_OUTPUT_FILE "assimp_polygons_debug.txt"
using namespace Assimp;
namespace {
/**
* @brief Helper struct used to simplify NGON encoding functions.
*/
struct NGONEncoder {
NGONEncoder() : mLastNGONFirstIndex((unsigned int)-1) {}
/**
* @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 Triangle to encode.
*/
void ngonEncodeTriangle(aiFace * tri) {
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]);
}
mLastNGONFirstIndex = tri->mIndices[0];
}
/**
* @brief Encode a quad (2 triangles) in ngon encoding, and make sure they are seen as a single ngon.
*
* @param tri1 First quad triangle
* @param tri2 Second quad triangle
*
* @pre Triangles must be properly fanned from the most appropriate vertex.
*/
void ngonEncodeQuad(aiFace *tri1, aiFace *tri2) {
ai_assert(tri1->mNumIndices == 3);
ai_assert(tri2->mNumIndices == 3);
ai_assert(tri1->mIndices[0] == tri2->mIndices[0]);
// 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
if (isConsideredSameAsLastNgon(tri1)) {
// Right-rotate indices for tri1 (index 2 becomes the new fanning vertex)
std::swap(tri1->mIndices[0], tri1->mIndices[2]);
std::swap(tri1->mIndices[1], tri1->mIndices[2]);
// Left-rotate indices for tri2 (index 2 becomes the new fanning vertex)
std::swap(tri2->mIndices[1], tri2->mIndices[2]);
std::swap(tri2->mIndices[0], tri2->mIndices[2]);
ai_assert(tri1->mIndices[0] == tri2->mIndices[0]);
}
mLastNGONFirstIndex = tri1->mIndices[0];
}
/**
* @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->mNumIndices == 3);
return tri->mIndices[0] == mLastNGONFirstIndex;
}
private:
unsigned int mLastNGONFirstIndex;
};
}
// ------------------------------------------------------------------------------------------------
// Returns whether the processing step is present in the given flag field.
bool TriangulateProcess::IsActive( unsigned int pFlags) const {
return (pFlags & aiProcess_Triangulate) != 0;
}
// ------------------------------------------------------------------------------------------------
// Executes the post processing step on the given imported data.
void TriangulateProcess::Execute( aiScene* pScene) {
ASSIMP_LOG_DEBUG("TriangulateProcess begin");
bool bHas = false;
for( unsigned int a = 0; a < pScene->mNumMeshes; a++)
{
if (pScene->mMeshes[ a ]) {
if ( TriangulateMesh( pScene->mMeshes[ a ] ) ) {
bHas = true;
}
}
}
if ( bHas ) {
ASSIMP_LOG_INFO( "TriangulateProcess finished. All polygons have been triangulated." );
} else {
ASSIMP_LOG_DEBUG( "TriangulateProcess finished. There was nothing to be done." );
}
}
// ------------------------------------------------------------------------------------------------
// Triangulates the given mesh.
bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh) {
// Now we have aiMesh::mPrimitiveTypes, so this is only here for test cases
if (!pMesh->mPrimitiveTypes) {
bool bNeed = false;
for( unsigned int a = 0; a < pMesh->mNumFaces; a++) {
const aiFace& face = pMesh->mFaces[a];
if( face.mNumIndices != 3) {
bNeed = true;
}
}
if (!bNeed)
return false;
}
else if (!(pMesh->mPrimitiveTypes & aiPrimitiveType_POLYGON)) {
return false;
}
// Find out how many output faces we'll get
uint32_t numOut = 0, max_out = 0;
bool get_normals = true;
for( unsigned int a = 0; a < pMesh->mNumFaces; a++) {
aiFace& face = pMesh->mFaces[a];
if (face.mNumIndices <= 4) {
get_normals = false;
}
if( face.mNumIndices <= 3) {
numOut++;
} else {
numOut += face.mNumIndices-2;
max_out = std::max(max_out,face.mNumIndices);
}
}
// Just another check whether aiMesh::mPrimitiveTypes is correct
ai_assert(numOut != pMesh->mNumFaces);
aiVector3D *nor_out = nullptr;
// if we don't have normals yet, but expect them to be a cheap side
// product of triangulation anyway, allocate storage for them.
if (!pMesh->mNormals && get_normals) {
// XXX need a mechanism to inform the GenVertexNormals process to treat these normals as preprocessed per-face normals
// nor_out = pMesh->mNormals = new aiVector3D[pMesh->mNumVertices];
}
// the output mesh will contain triangles, but no polys anymore
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);
NGONEncoder ngonEncoder;
// Apply vertex colors to represent the face winding?
#ifdef AI_BUILD_TRIANGULATE_COLOR_FACE_WINDING
if (!pMesh->mColors[0])
pMesh->mColors[0] = new aiColor4D[pMesh->mNumVertices];
else
new(pMesh->mColors[0]) aiColor4D[pMesh->mNumVertices];
aiColor4D* clr = pMesh->mColors[0];
#endif
#ifdef AI_BUILD_TRIANGULATE_DEBUG_POLYS
FILE* fout = fopen(POLY_OUTPUT_FILE,"a");
#endif
const aiVector3D* verts = pMesh->mVertices;
// 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++) {
aiFace& face = pMesh->mFaces[a];
unsigned int* idx = face.mIndices;
int num = (int)face.mNumIndices, ear = 0, tmp, prev = num-1, next = 0, max = num;
// Apply vertex colors to represent the face winding?
#ifdef AI_BUILD_TRIANGULATE_COLOR_FACE_WINDING
for (unsigned int i = 0; i < face.mNumIndices; ++i) {
aiColor4D& c = clr[idx[i]];
c.r = (i+1) / (float)max;
c.b = 1.f - c.r;
}
#endif
aiFace* const last_face = curOut;
// if it's a simple point,line or triangle: just copy it
if( face.mNumIndices <= 3)
{
aiFace& nface = *curOut++;
nface.mNumIndices = face.mNumIndices;
nface.mIndices = face.mIndices;
face.mIndices = nullptr;
// points and lines don't require ngon encoding (and are not supported either!)
if (nface.mNumIndices == 3) ngonEncoder.ngonEncodeTriangle(&nface);
continue;
}
// optimized code for quadrilaterals
else if ( face.mNumIndices == 4) {
// quads can have at maximum one concave vertex. Determine
// this vertex (if it exists) and start tri-fanning from
// it.
unsigned int start_vertex = 0;
for (unsigned int i = 0; i < 4; ++i) {
const aiVector3D& v0 = verts[face.mIndices[(i+3) % 4]];
const aiVector3D& v1 = verts[face.mIndices[(i+2) % 4]];
const aiVector3D& v2 = verts[face.mIndices[(i+1) % 4]];
const aiVector3D& v = verts[face.mIndices[i]];
aiVector3D left = (v0-v);
aiVector3D diag = (v1-v);
aiVector3D right = (v2-v);
left.Normalize();
diag.Normalize();
right.Normalize();
const float angle = std::acos(left*diag) + std::acos(right*diag);
if (angle > AI_MATH_PI_F) {
// this is the concave point
start_vertex = i;
break;
}
}
const unsigned int temp[] = {face.mIndices[0], face.mIndices[1], face.mIndices[2], face.mIndices[3]};
aiFace& nface = *curOut++;
nface.mNumIndices = 3;
nface.mIndices = face.mIndices;
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];
sface.mIndices[0] = temp[start_vertex];
sface.mIndices[1] = temp[(start_vertex + 2) % 4];
sface.mIndices[2] = temp[(start_vertex + 3) % 4];
// prevent double deletion of the indices field
face.mIndices = nullptr;
ngonEncoder.ngonEncodeQuad(&nface, &sface);
continue;
}
else
{
// A polygon with more than 3 vertices can be either concave or convex.
// Usually everything we're getting is convex and we could easily
// triangulate by tri-fanning. However, LightWave is probably the only
// modeling suite to make extensive use of highly concave, monster polygons ...
// so we need to apply the full 'ear cutting' algorithm to get it right.
// REQUIREMENT: polygon is expected to be simple and *nearly* planar.
// We project it onto a plane to get a 2d triangle.
// Collect all vertices of of the polygon.
for (tmp = 0; tmp < max; ++tmp) {
temp_verts3d[tmp] = verts[idx[tmp]];
}
// Get newell normal of the polygon. Store it for future use if it's a polygon-only mesh
aiVector3D n;
NewellNormal<3,3,3>(n,max,&temp_verts3d.front().x,&temp_verts3d.front().y,&temp_verts3d.front().z);
if (nor_out) {
for (tmp = 0; tmp < max; ++tmp)
nor_out[idx[tmp]] = n;
}
// Select largest normal coordinate to ignore for projection
const float ax = (n.x>0 ? n.x : -n.x);
const float ay = (n.y>0 ? n.y : -n.y);
const float az = (n.z>0 ? n.z : -n.z);
unsigned int ac = 0, bc = 1; /* no z coord. projection to xy */
float inv = n.z;
if (ax > ay) {
if (ax > az) { /* no x coord. projection to yz */
ac = 1; bc = 2;
inv = n.x;
}
}
else if (ay > az) { /* no y coord. projection to zy */
ac = 2; bc = 0;
inv = n.y;
}
// Swap projection axes to take the negated projection vector into account
if (inv < 0.f) {
std::swap(ac,bc);
}
for (tmp =0; tmp < max; ++tmp) {
temp_verts[tmp].x = verts[idx[tmp]][ac];
temp_verts[tmp].y = verts[idx[tmp]][bc];
done[tmp] = false;
}
#ifdef AI_BUILD_TRIANGULATE_DEBUG_POLYS
// plot the plane onto which we mapped the polygon to a 2D ASCII pic
aiVector2D bmin,bmax;
ArrayBounds(&temp_verts[0],max,bmin,bmax);
char grid[POLY_GRID_Y][POLY_GRID_X+POLY_GRID_XPAD];
std::fill_n((char*)grid,POLY_GRID_Y*(POLY_GRID_X+POLY_GRID_XPAD),' ');
for (int i =0; i < max; ++i) {
const aiVector2D& v = (temp_verts[i] - bmin) / (bmax-bmin);
const size_t x = static_cast<size_t>(v.x*(POLY_GRID_X-1)), y = static_cast<size_t>(v.y*(POLY_GRID_Y-1));
char* loc = grid[y]+x;
if (grid[y][x] != ' ') {
for(;*loc != ' '; ++loc);
*loc++ = '_';
}
*(loc+::ai_snprintf(loc, POLY_GRID_XPAD,"%i",i)) = ' ';
}
for(size_t y = 0; y < POLY_GRID_Y; ++y) {
grid[y][POLY_GRID_X+POLY_GRID_XPAD-1] = '\0';
fprintf(fout,"%s\n",grid[y]);
}
fprintf(fout,"\ntriangulation sequence: ");
#endif
//
// FIXME: currently this is the slow O(kn) variant with a worst case
// complexity of O(n^2) (I think). Can be done in O(n).
while (num > 3) {
// Find the next ear of the polygon
int num_found = 0;
for (ear = next;;prev = ear,ear = next) {
// break after we looped two times without a positive match
for (next=ear+1;done[(next>=max?next=0:next)];++next);
if (next < ear) {
if (++num_found == 2) {
break;
}
}
const aiVector2D* pnt1 = &temp_verts[ear],
*pnt0 = &temp_verts[prev],
*pnt2 = &temp_verts[next];
// Must be a convex point. Assuming ccw winding, it must be on the right of the line between p-1 and p+1.
if (OnLeftSideOfLine2D(*pnt0,*pnt2,*pnt1)) {
continue;
}
// Skip when three point is in a line
aiVector2D left = *pnt0 - *pnt1;
aiVector2D right = *pnt2 - *pnt1;
left.Normalize();
right.Normalize();
auto mul = left * right;
// if the angle is 0 or 180
if (std::abs(mul - 1.f) < ai_epsilon || std::abs(mul + 1.f) < ai_epsilon) {
// skip this ear
ASSIMP_LOG_WARN("Skip a ear, due to its angle is near 0 or 180.");
continue;
}
// and no other point may be contained in this triangle
for ( tmp = 0; tmp < max; ++tmp) {
// We need to compare the actual values because it's possible that multiple indexes in
// the polygon are referring to the same position. concave_polygon.obj is a sample
//
// FIXME: Use 'epsiloned' comparisons instead? Due to numeric inaccuracies in
// PointInTriangle() I'm guessing that it's actually possible to construct
// input data that would cause us to end up with no ears. The problem is,
// which epsilon? If we chose a too large value, we'd get wrong results
const aiVector2D& vtmp = temp_verts[tmp];
if ( vtmp != *pnt1 && vtmp != *pnt2 && vtmp != *pnt0 && PointInTriangle2D(*pnt0,*pnt1,*pnt2,vtmp)) {
break;
}
}
if (tmp != max) {
continue;
}
// this vertex is an ear
break;
}
if (num_found == 2) {
// Due to the 'two ear theorem', every simple polygon with more than three points must
// have 2 'ears'. Here's definitely something wrong ... but we don't give up yet.
//
// Instead we're continuing with the standard tri-fanning algorithm which we'd
// use if we had only convex polygons. That's life.
ASSIMP_LOG_ERROR("Failed to triangulate polygon (no ear found). Probably not a simple polygon?");
#ifdef AI_BUILD_TRIANGULATE_DEBUG_POLYS
fprintf(fout,"critical error here, no ear found! ");
#endif
num = 0;
break;
}
aiFace& nface = *curOut++;
nface.mNumIndices = 3;
if (!nface.mIndices) {
nface.mIndices = new unsigned int[3];
}
// setup indices for the new triangle ...
nface.mIndices[0] = prev;
nface.mIndices[1] = ear;
nface.mIndices[2] = next;
// exclude the ear from most further processing
done[ear] = true;
--num;
}
if (num > 0) {
// We have three indices forming the last 'ear' remaining. Collect them.
aiFace& nface = *curOut++;
nface.mNumIndices = 3;
if (!nface.mIndices) {
nface.mIndices = new unsigned int[3];
}
for (tmp = 0; done[tmp]; ++tmp);
nface.mIndices[0] = tmp;
for (++tmp; done[tmp]; ++tmp);
nface.mIndices[1] = tmp;
for (++tmp; done[tmp]; ++tmp);
nface.mIndices[2] = tmp;
}
}
#ifdef AI_BUILD_TRIANGULATE_DEBUG_POLYS
for(aiFace* f = last_face; f != curOut; ++f) {
unsigned int* i = f->mIndices;
fprintf(fout," (%i %i %i)",i[0],i[1],i[2]);
}
fprintf(fout,"\n*********************************************************************\n");
fflush(fout);
#endif
for(aiFace* f = last_face; f != curOut; ) {
unsigned int* i = f->mIndices;
i[0] = idx[i[0]];
i[1] = idx[i[1]];
i[2] = idx[i[2]];
// 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;
}
delete[] face.mIndices;
face.mIndices = nullptr;
}
#ifdef AI_BUILD_TRIANGULATE_DEBUG_POLYS
fclose(fout);
#endif
// kill the old faces
delete [] pMesh->mFaces;
// ... and store the new ones
pMesh->mFaces = out;
pMesh->mNumFaces = (unsigned int)(curOut-out); /* not necessarily equal to numOut */
return true;
}
#endif // !! ASSIMP_BUILD_NO_TRIANGULATE_PROCESS