Function loadMtlLibrary

source/MRMesh/MRMeshLoadObj.cpp:430–509

using MtlLibrary = HashMap<std::string, MtlMaterial>;

Expected<MtlLibrary> loadMtlLibrary( const std::filesystem::path& path )
{
    std::ifstream mtlIn( path, std::ios::binary );
    if ( !mtlIn.is_open() )
        return unexpected( "unable to open MTL file" );

    const auto mtlContent = readCharBuffer( mtlIn );
    if ( !mtlContent )
        return unexpected( "Unable to open MTL file: " + mtlContent.error() );
    auto data = mtlContent->data();
    auto mtlSize = mtlContent->size();
    if ( hasBom( data ) )
    {
        data += 3;
        mtlSize -= 3;
    }

    if ( mtlSize == 0 )
        return unexpected( "empty MTL file" );

    const auto newlines = splitByLines( data, mtlSize );

    const auto groups = groupLines<MtlElement>( data, mtlSize, newlines );

    MtlLibrary result;
    std::string currentMaterialName;
    MtlMaterial currentMaterial;

    for ( const auto& group : groups )
    {
        std::string parseError;
        switch ( group.element )
        {
        case MtlElement::Unknown:
            break;
        case MtlElement::MaterialName:
        {
            const auto li = group.end - 1;
            std::string_view line( data + newlines[li], newlines[li + 1] - newlines[li + 0] );

            if ( !currentMaterialName.empty() )
            {
                result.emplace( std::move( currentMaterialName ), std::move( currentMaterial ) );
                currentMaterial = {};
            }
            currentMaterialName = line.substr( 6, std::string_view::npos );
            boost::trim( currentMaterialName );
        }
        break;
        case MtlElement::DiffuseColor:
        {
            const auto li = group.end - 1;
            std::string_view line( data + newlines[li], newlines[li + 1] - newlines[li + 0] );

            auto res = parseMtlColor( line, currentMaterial.diffuseColor );
            if ( !res.has_value() )
                parseError = std::move( res.error() );
        }