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Open Asset Import Library (assimp)
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*/

/** @file Implementation of the post processing step to generate face
* normals for all imported faces.
*/

#include "GenFaceNormalsProcess.h"
#include <assimp/Exceptional.h>
#include <assimp/postprocess.h>
#include <assimp/qnan.h>
#include <assimp/scene.h>
#include <assimp/DefaultLogger.hpp>

using namespace Assimp;

// ------------------------------------------------------------------------------------------------
// Returns whether the processing step is present in the given flag field.
bool GenFaceNormalsProcess::IsActive(unsigned int pFlags) const {
    force_ = (pFlags & aiProcess_ForceGenNormals) != 0;
    flippedWindingOrder_ = (pFlags & aiProcess_FlipWindingOrder) != 0;
    leftHanded_ = (pFlags & aiProcess_MakeLeftHanded) != 0;
    return (pFlags & aiProcess_GenNormals) != 0;
}

// ------------------------------------------------------------------------------------------------
// Executes the post processing step on the given imported data.
void GenFaceNormalsProcess::Execute(aiScene *pScene) {
    ASSIMP_LOG_DEBUG("GenFaceNormalsProcess begin");

    if (pScene->mFlags & AI_SCENE_FLAGS_NON_VERBOSE_FORMAT) {
        throw DeadlyImportError("Post-processing order mismatch: expecting pseudo-indexed (\"verbose\") vertices here");
    }

    bool bHas = false;
    for (unsigned int a = 0; a < pScene->mNumMeshes; a++) {
        if (this->GenMeshFaceNormals(pScene->mMeshes[a])) {
            bHas = true;
        }
    }
    if (bHas) {
        ASSIMP_LOG_INFO("GenFaceNormalsProcess finished. "
                        "Face normals have been calculated");
    } else {
        ASSIMP_LOG_DEBUG("GenFaceNormalsProcess finished. "
                         "Normals are already there");
    }
}

// ------------------------------------------------------------------------------------------------
// Executes the post processing step on the given imported data.
bool GenFaceNormalsProcess::GenMeshFaceNormals(aiMesh *pMesh) {
    if (nullptr != pMesh->mNormals) {
        if (force_) {
            delete[] pMesh->mNormals;
        } else {
            return false;
        }
    }

    // If the mesh consists of lines and/or points but not of
    // triangles or higher-order polygons the normal vectors
    // are undefined.
    if (!(pMesh->mPrimitiveTypes & (aiPrimitiveType_TRIANGLE | aiPrimitiveType_POLYGON))) {
        ASSIMP_LOG_INFO("Normal vectors are undefined for line and point meshes");
        return false;
    }

    // allocate an array to hold the output normals
    pMesh->mNormals = new aiVector3D[pMesh->mNumVertices];
    const float qnan = get_qnan();

    // iterate through all faces and compute per-face normals but store them per-vertex.
    for (unsigned int a = 0; a < pMesh->mNumFaces; a++) {
        const aiFace &face = pMesh->mFaces[a];
        if (face.mNumIndices < 3) {
            // either a point or a line -> no well-defined normal vector
            for (unsigned int i = 0; i < face.mNumIndices; ++i) {
                pMesh->mNormals[face.mIndices[i]] = aiVector3D(qnan);
            }
            continue;
        }

        const aiVector3D *pV1 = &pMesh->mVertices[face.mIndices[0]];
        const aiVector3D *pV2 = &pMesh->mVertices[face.mIndices[1]];
        const aiVector3D *pV3 = &pMesh->mVertices[face.mIndices[face.mNumIndices - 1]];
        // Boolean XOR - if either but not both of these flags is set, then the winding order has
        // changed and the cross product to calculate the normal needs to be reversed
        if (flippedWindingOrder_ != leftHanded_) 
            std::swap(pV2, pV3);
        const aiVector3D vNor = ((*pV2 - *pV1) ^ (*pV3 - *pV1)).NormalizeSafe();

        for (unsigned int i = 0; i < face.mNumIndices; ++i) {
            pMesh->mNormals[face.mIndices[i]] = vNor;
        }
    }
    return true;
}
