Efficient way to update some portion of geometry

Questions
1

At the moment I am working on building web based viewer (ofcourse using three.js) for incremental 3D reconstruction framework. I have VoxelBlocks which are then “meshed” into meshes (let’s call them m0, m1 … mx). At the moment, I am merging them as follows:

        var mesh1 = getMineCraftMesh(MCMesher,dataField,dataDims,[1,0,0]);
        var mesh2 = getMineCraftMesh(MCMesher,dataField,dataDims,[0,3,0]);
        var mesh3 = getMineCraftMesh(MCMesher,dataField,dataDims,[2,2,0]);

        var singleGeometry = new THREE.Geometry();	

        mesh1.updateMatrix(); 
        singleGeometry.merge(mesh1.geometry, mesh1.matrix);

        mesh2.updateMatrix(); 
        singleGeometry.merge(mesh2.geometry, mesh2.matrix);

        mesh3.updateMatrix(); 
        singleGeometry.merge(mesh3.geometry, mesh3.matrix);
        
        var material = new THREE.MeshPhongMaterial({color: 0x777777});
        surfacemesh = new THREE.Mesh(singleGeometry, material);
        surfacemesh.geometry.computeBoundingBox();
        surfacemesh.geometry.computeBoundingSphere();

        scene.add(surfacemesh);

Considering if some mesh (let’s say m2) gets updated, how can I efficiently update the merged mesh?

PS. I have already tried to consider a big bufferedGeometry with some predefined number of vertices, colors, normals and faces but that strategy wasted a lot of memory as meshes comes in various shape and sizes.

2

Merging meshes is only recommended if the meshes are static. If the world transformation of mesh vary, it’s better to give instanced rendering a shot. Have you considered to use THREE.InstancedMesh?

Of course this only works if the objects share the geometry. But in context of voxels, this premise should be true.

3

Hi Mugen87, does InstancedMesh support different meshes? and any pointers (three.js example etc) on how to dynamically change some part of mesh on run-time?

4

This is the basic example for InstancedMesh: https://threejs.org/examples/webgl_instancing_dynamic

You can have multiple, distinct meshes but they have to share the same geometry.

5

Thanks again, but that’s exactly what was confusing me. Since different (VoxelBlock) meshes will have different number of vertices, colors and faces etc. will that be a problem?

I have already modified dynamic instancing example (for some other project) but in this example every mesh share identical geometry (i.e. same number of vertices, colors and faces)

6

This is definitely a problem, yes.

7

Ahh thanks, will different number of faces will also be problem? (provided if I keep the number of vertices same)

8

Yes, on BufferGeometry level, there is no separate face data structure. Just vertex data. If the number of primitives are different, you have a different geometry.

9

Thanks again, this may be stupid question. Can I create copies of same geometry (i.e. same no. of vertices, colors, normals and faces) but different values of these parameters in instancedMesh?

And can I forcefully make some faces uninitialized (i.e. the ones not used at the moment)?

10

looking at this line there must be a way. and it is even in the docs.

2 Likes
11

I’m not sure I understand what you mean here (sorry). InstancedMesh supports by default different transformations per instance (meaning position, rotation and scale). If you need more instanced attributes (like a different color per instance), you have to define these data by yourself. However, this also makes it necessary to modify the build-in shaders so they are able to work with additional attributes. The following example demonstrates this approach:

https://threejs.org/examples/webgl_instancing_modified

@makc3d I don’t think BufferAttribute.updateRange is relevant for this use case.

1 Like
12

Dear @Mugen87 @makc3d thanks again for your reply and explanation. I have already asked the question about vertex color in InstanceMesh at PerVertexColor which works for a same geometry but with different color, but I do not posses enough experience in OpenGL/WebGL to come up with a shader which is needed to accomodate attributes such as different faces/vertices. :frowning: