How to draw/cut lines on a mesh

[lslzl3000]

The meshinspector cloud easily add lines and cut a mesh by lines, but the lines goes by Geodesic, like the white lines:

Can I draw lines just follow the red lines as a triangle on screen space view?

We want to select the area on mesh from a screen space view, e.g. a rectangle on the middle of screen, how to select the projection area of the rectangle from screen viewport, by code, prefer python, not just in meshinspector?

[lslzl3000]

if I need to project a screen-space letter 'A' onto a mesh, e.g.

how to do this?

[Fedr]

We do not have mesh cutting in screen space out of the box right now. We will probably include such functionality later.

However we have selection of mesh primitives in screen space. You can

1. Subdivide the cube to fill it with very small triangles:

2. Use Select Primitives tool in Polygon mode:

3. to select mesh faces inside any triangle drawn on screen:

[lslzl3000]

In blender, there is a function called knife project could do the right thing.

but this function requires UI interactive and window gl context, unable to do by API in background mode
It would be nice for Meshlib to add features like knife project, and it could be done by API only in background

[Grantim]

Hello!

I think it can be implemented like this:

    auto cube = makeCube( Vector3f::diagonal( 1 ), Vector3f() );

    // subdivide can prevent some cutMesh issues
    auto ss = SubdivideSettings();
    ss.maxEdgeSplits = 1000;
    ss.maxAngleChangeAfterFlip = 0.0f;
    ss.maxDeviationAfterFlip = 1e-6f;
    ss.maxEdgeLen = 0.1f;
    subdivideMesh( cube, ss );

    // create text contours at zero coord
    auto smp = SymbolMeshParams();
    smp.pathToFontFile = "C:\\Windows\\Fonts\\arial.ttf";
    smp.text = "Text";
    auto conts3f = convertContoursTo3f( *createSymbolContours( smp ) ); // better check error before resolving expected

    // calc text transformation for ray-casting along look direction
    Box3f box3f;
    for ( const auto& cont : conts3f )
        for ( const auto& p : cont )
            box3f.include( p );

    auto lookDir = Vector3f::diagonal( -1 ).normalized();
    auto scale = Matrix3f::scale( 0.5f / box3f.size().x ); // x length of text to be equal to 0.5
    auto zDir = -lookDir;
    auto yDir = Vector3f( -1, -1, 1 ).normalized();
    auto xDir = cross( yDir, zDir );
    auto rotate = Matrix3f::fromColumns( xDir, yDir, zDir );
    auto translation = zDir * 3 - xDir * 0.25f;
    auto textXf = AffineXf3f::translation( translation ) * AffineXf3f::linear( rotate * scale );

    // ray-cast contours points to mesh
    auto rayPrecomp = IntersectionPrecomputes<float>( lookDir );
    auto cutContours = OneMeshContours( conts3f.size() );
    ParallelFor( cutContours, [&] ( size_t i )
    {
        auto mtps = std::vector<MeshTriPoint>( conts3f[i].size() );
        ParallelFor( mtps, [&] ( size_t j )
        {
            auto intRes = rayMeshIntersect( cube, Line3f( textXf( conts3f[i][j] ), lookDir ), 0, FLT_MAX, &rayPrecomp );
            assert( intRes );
            if ( !intRes )
                return;
            mtps[j] = intRes.mtp;
        } );
        mtps.back() = mtps.front();
        auto convRes = convertMeshTriPointsToMeshContour( cube, mtps, [&] ( const MeshTriPoint& start, const MeshTriPoint& end, int, int )->Expected<SurfacePath>
        {
            auto plPoint = cube.triPoint( start ) - lookDir;
            auto ccwPath = trackSection( cube, start, end, plPoint, true );
            auto cwPath = trackSection( cube, start, end, plPoint, false );
            if ( ccwPath.has_value() && cwPath.has_value() )
            {
                auto ccwL = surfacePathLength( cube, *ccwPath );
                auto cwL = surfacePathLength( cube, *cwPath );
                if ( ccwL < cwL )
                    return ccwPath;
                else
                    return cwPath;
            }
            else if ( ccwPath.has_value() )
                return ccwPath;
            else if ( cwPath.has_value() )
                return cwPath;
            else
            {
                auto locRes = computeGeodesicPath( cube, start, end );
                if ( !locRes.has_value() )
                    return unexpected( toString( locRes.error() ) );
                return *locRes;
            }
        } );
        if ( !convRes.has_value() )
            return;
        cutContours[i] = std::move( *convRes );
    } );
    cutMesh( cube, cutContours );

    ( void ) MeshSave::toAnySupportedFormat( cube, "C:\\Path\\To\\cutMesh.obj" );

We will probably make a separate function for it later