How fill a loaded STL mesh ( NOT SIMPLE SHAPES LIKE CUBE ETC) with random particles and animate with this geometry bound in three.js - three.js

Question

How I can fill a loaded STL mesh ( like suzane NOT SIMPLE SHAPES LIKE CUBE etc) with random particles and animate it inside this geometry bounds with three.js ?
I see many examples but all of it for simple shapes with geometrical bounds like cube or sphere with limit by coordinates around center
https://threejs.org/examples/?q=points#webgl_custom_attributes_points3
TNX

Answer 1

A concept, using a ray, that counts intersections of the ray with faces of a mesh, and if the number is odd, it means that the point is inside of the mesh:
Codepen
function fillWithPoints(geometry, count) {
var ray = new THREE.Ray()
var size = new THREE.Vector3();
geometry.computeBoundingBox();
let bbox = geometry.boundingBox;
let points = [];
var dir = new THREE.Vector3(1, 1, 1).normalize();
for (let i = 0; i < count; i++) {
let p = setRandomVector(bbox.min, bbox.max);
points.push(p);
}
function setRandomVector(min, max){
let v = new THREE.Vector3(
THREE.Math.randFloat(min.x, max.x),
THREE.Math.randFloat(min.y, max.y),
THREE.Math.randFloat(min.z, max.z)
);
if (!isInside(v)){return setRandomVector(min, max);}
return v;
}
function isInside(v){
ray.set(v, dir);
let counter = 0;
let pos = geometry.attributes.position;
let faces = pos.count / 3;
let vA = new THREE.Vector3(), vB = new THREE.Vector3(), vC = new THREE.Vector3();
for(let i = 0; i < faces; i++){
vA.fromBufferAttribute(pos, i * 3 + 0);
vB.fromBufferAttribute(pos, i * 3 + 1);
vC.fromBufferAttribute(pos, i * 3 + 2);
if (ray.intersectTriangle(vA, vB, vC)) counter++;
}
return counter % 2 == 1;
}
return new THREE.BufferGeometry().setFromPoints(points);
}

Answer 2

The concepts from the previous answer is very good, but it has some performance limitations:
the whole geometry is tested with every ray
the recursion on points outside can lead to stack overflow
Moreover, it's incompatible with indexed geometry.
It can be improved by creating a spatial hashmap storing the geometry triangles and limiting the intersection test to only some part of the mesh.
Demonstration