Three.js How to get position of a mesh?

问题

With the below code, position of a mesh is returned as (0, 0, 0) but it is not. So is the positioın vector calculated after render process?

me.scene.add(objMesh); //me is a project class
objMesh.updateMatrixWorld(true);
alert(objMesh.position.x + ',' + objMesh.position.y + ',' + objMesh.position.z);

objMesh is created from objfile, it is added to the scene correctly and centroid is approx (-8, 3, 0) but position vector of objMesh is (0, 0, 0) do we have to auto calculate something first or should i calculate it manually from geometry vertices of the mesh ?

http://81.214.75.32:8181/admin is the url

the site is in Turkish so i will translate the UI items

in the site there is "Dosya" menu item oppen the menu item and select "Proje Aç" a dialog appears in that dialog select MUTFAK_1 scene will appear in that scene, every meshes position is (0, 0, 0) is that possible :)

回答1:

object.position is always local to the object. If you want to get the position in world space you need to get it from object.matrixWorld.

Try with this:

scene.add(objMesh);
scene.updateMatrixWorld(true);
var position = new THREE.Vector3();
position.getPositionFromMatrix( objMesh.matrixWorld );
alert(position.x + ',' + position.y + ',' + position.z);

r58

Update:

The function getPositionFromMatrix() has been renamed to setFromMatrixPosition().

回答2:

For finding where in world space is the geometry centroid, try this:

objMesh.geometry.computeBoundingBox();

var boundingBox = objMesh.geometry.boundingBox;

var position = new THREE.Vector3();
position.subVectors( boundingBox.max, boundingBox.min );
position.multiplyScalar( 0.5 );
position.add( boundingBox.min );

position.applyMatrix4( objMesh.matrixWorld );

alert(position.x + ',' + position.y + ',' + position.z);

r58

回答3:

Yeah. after some talk with mrdoob, i realized that .position of objects are local to theirselves. My situation was to find the center point of my mesh considering the vertices. Below is the code to get the centroid which came from an answer #447 ( https://github.com/mrdoob/three.js/issues/447 )

geom.centroid = new THREE.Vector3();
for (var i = 0, l = geom.vertices.length; i < l; i++) {
    geom.centroid.addSelf(geom.vertices[i]);
}
geom.centroid.divideScalar(geom.vertices.length);

Now we have centroid of geometry...

Update according to https://github.com/mrdoob/three.js/wiki/Migration, the .addSelf had been renamed to .add after r55

回答4:

alert(objMesh.matrixWorld.getPosition().x + ',' + objMesh.matrixWorld.getPosition().y + ',' + objMesh.matrixWorld.getPosition().z);

回答5:

According to this post the center of gravity C for a mesh can be found by

C = [sum of all (A*R)] / [sum of all A]
A = (area of a face * 2)
R = face centroid = average of vertices making the face

and here is the code in three.js

function calculateCenterOfMass( mesh ){

    var centroid = new THREE.Vector3();

    // centroid = centroidNominator / centroidDenominator;
    var centroidNominator = new THREE.Vector3(); 
    var centroidDenominator = 0;

    for(var i = 0; i < mesh.geometry.faces.length; i++){

        var Pi = mesh.geometry.faces[i].a;
        var Qi = mesh.geometry.faces[i].b;
        var Ri = mesh.geometry.faces[i].c;

        var a = new THREE.Vector3(mesh.geometry.vertices[Pi].x, mesh.geometry.vertices[Pi].y, mesh.geometry.vertices[Pi].z);
        var b = new THREE.Vector3(mesh.geometry.vertices[Qi].x, mesh.geometry.vertices[Qi].y, mesh.geometry.vertices[Qi].z);
        var c = new THREE.Vector3(mesh.geometry.vertices[Ri].x, mesh.geometry.vertices[Ri].y, mesh.geometry.vertices[Ri].z);

        var ab = b.clone().sub(a);
        var ac = c.clone().sub(a);

        var cross = new THREE.Vector3();
        cross.crossVectors( ab, ac );

        var faceArea = cross.lengthSq() / 2;
        var faceCentroid = new THREE.Vector3( (a.x + b.x + c.x)/3, (a.y + b.y + c.y)/3, (a.z + b.z + c.z)/3 );

        if (!isNaN(faceArea)){
            centroidNominator.add(faceCentroid.multiplyScalar(faceArea));
            centroidDenominator += faceArea;
        }
    }


    centroid = centroidNominator.divideScalar(centroidDenominator);

    return centroid;
}

来源：https://stackoverflow.com/questions/14211627/three-js-how-to-get-position-of-a-mesh

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javascript

three.js