How to use the Solid Particle System

Solid Particle System (SPS)


The SPS is a single updatable mesh. The solid particles are simply separate parts or faces fo this big mesh.
As it is just a mesh, the SPS has all the same properties than any other BJS mesh : not more, not less. It can be scaled, rotated, translated, enlighted, textured, moved, etc.

The SPS is also a particle system. It provides some methods to manage the particles.
However it is behavior agnostic. This means it has no emitter, no particle physics, no particle recycler. You have to implement your own behavior.

The particles can be built from any BJS existing mesh as a model. Actually, each particle is a copy of some BJS mesh geometry : vertices, indices, uvs.

The expected usage if this one :

  • First, create your SPS with new SolidParticleSystem().
  • Then, add particles in the SPS from a mesh model with addShape(model, number).
  • Redo this as many times as needed with any model.
  • When done, build the SPS mesh with buildMesh().

Your SPS is then ready to manage particles. So now :

  • Init all your particles : set their positions, colors, uvs, age, etc with initParticles()
  • Call setParticles() to update the SPS mesh and to draw it.
  • If you particles have to be animated, define their individual behavior in updateParticle(particle) and just call setParticles() within the render loop.

Basic Usage

SPS Creation

First you create an empty SPS and you add particles to it with the addShape(mesh, nb) method as many times you need.
Its underlying mesh name will be the SPS name.

Then you build the mesh.
Example :

var SPS = new SolidParticleSystem("SPS", scene);
var sphere = BABYLON.MeshBuilder.CreateSphere("s", {}, scene);
var poly = BABYLON.MeshBuilder.CreatePolyhedron("p", {type: 2}, scene);
SPS.addShape(sphere, 20);      // 20 spheres
SPS.addShape(poly, 120);       // 120 polyhedrons 
SPS.addShape(sphere, 80);      // 80 other spheres

var mesh = SPS.buildMesh();  // finally builds and displays the real mesh

Now your SPS is visible as it is just built.
If just want to create immutable things (not moving, not rotating, not changing their colors, etc) set somewhere in your scene, you would probably stop here.

However the SPS is ready to get a behavior.
Once the behavior will be given (or not), you actually display the particles at their current updated positions with current properties with :

SPS.billboard = true; // or false by default

SPS.billboard is a boolean (default false). If set to true, all the particles will face the cam and their x and y rotation values will be ignored.
This is rather useful if you display only plane particles. However, if you deal only with 2D particles you should consider to use the BJS Particle System or the Sprite Manager which are more performant in 2D computation.
In order to display the SPS in billboard mode, you need to call SPS.setParticles() within the scene.registerBeforeRender() function.

Here is an example with plane particles in billboard mode : http://www.babylonjs-playground.com/#WCDZS#7 -

The same but with plane particle rotations and no billboard mode : http://www.babylonjs-playground.com/#WCDZS#1 -

The same with solid particles, boxes and tetrahedrons : http://www.babylonjs-playground.com/#WCDZS#2 -

Another one with colors and rotations : http://www.babylonjs-playground.com/#2FPT1A#9 -

Particle Management

The setParticles() function can be used in the BJS render loop.
It is mandatory to use this function to update and display the mesh.

You can give your SPS a behavior by setting some custom functions :

  • initParticles() : lets you set all the initial particle properties. You must iterate over all the particles by using the SPS.nbParticles property. The usage of this function is not mandatory.
  • recycleParticle(particle) : lets you set a particle to be recycled. It is called per particle. The usage of this function is not mandatory.
  • updateParticle(particle) : lets you set the particle properties. This function is called per particle by SPS.setParticles(). The usage of this function is not mandatory.
  • beforeUpdateParticles() : lets you make things within the call to SPS.setParticles() just before iterating over all the particles. The usage of this function is not mandatory.
  • afterUpdateParticles() : lets you make things within the call to SPS.setParticles() just after the iteration over all the particles is done. The usage of this function is not mandatory.
  • So to better understand how it works, here is a pseudo-code schema :
    var particles: SolidParticles[] = [array of SolidParticle objects];
    function setParticles() {
      beforeUpdateParticles();                 // custom function
      for (var p = 0; p < nbParticles; p++) {
          updateParticles(particles[p]);         // custom function
      updateTheWholeMesh();                   // does the WebGL work
      afterUpdateParticles();                 // custom function
    So you could call recycleParticle(particle) in your own `updateParticle(particle)รจ function for instance :
    SPS.updateParticle = function(particle) {
      if (particle.velocity < 0) {
        particle.alive = false;
        SPS.recycleParticle(particle);    // call to your own recycle function

The particle properties that can be set are :

  • position : Vector3 default = (0, 0, 0)
  • rotation : Vector3 default = (0, 0, 0)
  • rotationQuaternion : Vector3 default = undefined
  • velocity : Vector3 default = (0, 0, 0)
  • color : Vector4 default = (1, 1, 1, 1)
  • scaling : Vector3 default = (1, 1, 1)
  • uvs : Vector(4) default = (0,0, 1,1)
  • isVisible : boolean default = true
  • alive : boolean default = true

If you set a particle rotation quaternion, its rotation property will then be ignored.
If you set your SPS in billboard mode, you should only set a rotation.z value.

Please note that all positions are expressed in the mesh local space and not in the World space.
Please note also that, even a particle is invisible (isVisible set to false), its other property values can be updated and updateParticle() is called for every particle whatever it is visible or not.

You can obviously also create your own properties like acceleration: Vector3 or age, in initParticles() for instance.

SPS.initParticles = function() {
    for (var p = 0; p < SPS.nbParticles; p++) {
      particles[p].age = Math.random() * 20;

You may also access to some read-only properties :

  • idx : particle index
  • shapeId : shape model ID

Actually each time you call the SPS.addShape() method, the related newly created particle set shapeID is returned.

var plane = BABYLON.MeshBuilder.CreatePlane("", {}, scene);
var quadsID = SPS.addShape(plane, 20);

This is usefull if you want to apply a given behavior to some particle types only.

SPS Management

You have access to some SPS properties :

  • SPS.particles : this is the array containing all the particles. You should iterate over this array in initParticles() function for instance.
  • SPS.nbParticles : this is number of particles in the SPS.
  • SPS.counter : this is a counter for your own usage. It's not set by any SPS default functions.

Here again, you can add your own properties like capacity or rate if needed.

If you don't need some given features (ex : particle colors), you can disable/enable them at any time (disabling a feature will improve the performance) :

SPS.computeParticleRotation = false;       // prevents from computing particle.rotation
SPS.computeParticleTexture = false;        // prevents from computing particle.uvs
SPS.computeParticleColor = false;          // prevents from computing particle.color
SPS.computeParticleVertex = false;         // prevents from calling the custom updateParticleVertex() function

All these properties, except SPS.computeParticleVertex, are enabled set to true by default. These affect the SPS.setParticles() process only.
If these properties are set to false, they don't prevent from using the related feature (ie : the particles can still have a color even if SPS.computeParticleColor is set to false), they just prevent from updating the value of the particle property on the next setParticle() call.
Example : if your particles have colors, you can set their colors wihtin the initParticles() call and you can call then once the setParticles() method to set these colors. If you need to animate them later on and these colors don't change, just set then SPS.computeParticleColor to false once before runing the render loop which will call setParticles() each frame.
If you are familiar with how BJS works, you could compare the SPS and its mesh creation to some classical BJS mesh creation (vertex and indice settings) and the particle management to the World Matrix computation (rotation, scaling, positioning).

Note you can also use the standard BJS mesh freezeXXX() methods if the SPS mesh is immobile or if the normals aren't needed :

SPS.mesh.freezeWorldMatrix();       // prevents from re-computing the World Matrix each frame
SPS.mesh.freezeNormals();           // prevents from re-computing the normals each frame

If you don't need your SPS any longer, you can dispose it to free the memory

SPS = null    // tells the GC the reference can be cleaned up also


The SPS is behavior-agnostic. This means it doesn't know how the particles will move, rotate, if they have a mass, if there are forces, etc. You have to see it like a big mesh that you can create (buildMesh) from many shape models, some BJS existing meshes, (addShape) that will be its solid particles. It provides some methods to access then and to manage these solid particles.

So the initial stuff to do is to create the SPS, then to add as many shapes you need and at last to build the SPS mesh.

Once you've done it, you need to manage your solid particles.

The way to fix the particle status and then to display them at this status is to call SPS.setParticles().
Each time you call setParticles() the particles are rendered at their current status. Easy, isn't it ?

So three steps :

  • SPS and its mesh creation
  • compute your particle behavior
  • call setParticles() to update the mesh and draw it

Just remember that, once the mesh is build, only setParticles() does then the job : updates the mesh VBO and draws it.

To help you to update each particle status, setParticle() will call for each particle SPS.updateParticle(particle).
This function doesn't do anything by default, so it is the place were you can implement your particle behavior, with physics if you want. It is passed each particle object in turn, which you can set its initial properties (position, rotation, rotationQuaternion, scaling, color, uvs, velocity) or add and set your own if your logic needs it (age ? mass ? etc).
So updateParticle() just changes the particle data, not the mesh itself. The setParticles() process updates the mesh. Fortunately, setParticles() calls updateParticle(particle) for you.

If you want to set an initial status, different from the live behavior that you would implement in SPS.updateParticle(particle), you can use SPS.initParticles().
This function doesn't do anything, you have to implement it.
It doesn't draw the mesh, it just changes the particle initial status that will be taken in account by the next SPS.setParticle() call. The same thing with SPS.recycleParticle(particle) what is not called automatically and that you have to implement by your own and to call when you need.

Remember finally that there are also some other means to deal with particles in BJS like the Particle System or the Sprite Manager :

The Particle System is the most performant in terms of speed and of particle quantity.
The particles are 2D quads, have all the same texture and colour. They ever face the screen, so they can have only a z-axis rotation. They aren't z-sorted and aren't pickable.
This sytem provides a behavior : emitter, recycler, updater and many particle properties to manage their individual status (position, rotation, lifetime, size, etc).

The Sprite Manager is intended to manage sprites. They can be used as particles if needed as the Manager is also very performant.
The sprites are 2D quads always facing the screen, so they have only a z-axis rotation also. You can adjust from a given texture a different image per sprite and update it at will (sprite atlas). The sprites are z-sorted and are pickable.
The Manager doesn't provide a behavior but you can access to many properties to set each sprite status.

The SPS is a BJS mesh.
Its particles are just parts of this big mesh. They can be planar, from a simple triangle to any planar polygon, or/and 3D solid. They face the screen only in billboard mode (maybe you should use the SPS in billboard mode only if the two previous means don't yet fit your needs as they are more performant for 2D). You can merge 2D and 3D particles in the same SPS and give them rotation in the space. Each particle can have its own color (vertex color) and own image from a single common texture. They are z-sorted and alpha-blended like any other BJS mesh. They are also pickable. They even have normals and reflect the light. Actually, all the features accessible to a mesh are accessible to the SPS.
The SPS provides no behavior but only methods to access and to set each particle.

About transparency and mesh rendering, you could read this documentation.

In order to have only one draw call to the GPU, these three systems use only one material/texture for all their particles.

Advanced Features

Create an immutable SPS

You may have to create many similar objects in your scene that won't change afterwards : buildings in the distance, asteroids, scraps, etc. It may thus be useful to use the SPS to set only one mesh in your scene, so one draw call for the rendering.
Example : http://www.babylonjs-playground.com/#2FPT1A#5 -

You can achieve this by two different ways.

  • You can just build your SPS as explained before and then call just once setParticles(), before and outside the render loop, to set your particles where and how you need.
    This method is quite simple. Though, in order to allow you to set the final particle locations, the SPS mesh is built as updatable by default. This means its vertex buffer isn't passed once for all to the GPU, but is cached, waiting for a hypothetical further change.
    So this is a simple solution if you don't have many draw calls to handle for the other really moving or changing meshes of your scene.
    Remember also that, if you need to display your SPS in billboard mode, this is the only way to do it and you'll have to call setParticles() in the render loop also even if the particles don't move.

  • Else you can build your mesh as non updatable.
    Actually the SPS contructor expects a parameter updatable what is true by default.
    So, to build a non-updatable mesh, just call explicitly :

    var SPS = new SolidParticleSystem(name, scene, {updatable: false});

    As the mesh can't be updated now, setParticles() won't have any effect any longer : don't call it, you'll spare some CPU. Actually the particles array is not even populated !
    No particle management function called after SPS.buildMesh() will then have any effect.
    Note that the particles won't move but you can still move, scale or rotate the whole mesh.

So how to set the initial particle positions, colors, uvs, scales, and so on if the mesh can't be updated ?

To achieve this, you need to change the mesh at construction time, when adding the shapes.
You will have to define your own function to set these particle (what don't exist at this time) properties by modifying the way the shapes are added.
Actually, you can pass to SPS.addShape() an exra parameter which is your particle setting function.
This parameter is an object with the property positionFunction to what you will assign your custom function.

SPS.addShape(mesh, nb, {positionFunction: myCustomFunction});

Your own function will be called, for a given shape, as many times as the wanted number of particles for this shape. It will be passed two parameters : a particle object and its current position in the total number wanted for this shape.
So your function must have this kind of signature :

var myBuilder = function(particle, i, s) {
  // particle is the current copy of the shape, the i-th one in the SPS and the s-th one in its shape

This particle object has the following properties :

property type default
position Vector3 (0,0,0)
rotation Vector3 (0,0,0)
rotationQuaternion Quaternion null, if rotationQuaternion is set, rotation is ignored
scaling Vector3 (1,1,1)
color Color4 null
uvs Vector4 (0,0,1,1)

The expected usage is thus for instance:

var myBuilder = function(particle, i, s) {
  // particle is the current particle
  // i is its global index in the SPS
  // s is its index in its shape, so here from 0 to 149
  particle.rotation.y = s / 150;
  particle.position.x = s - 150;
  particle.uvs = new BABYLON.Vector4(0, 0, 0.33, 0.33); // first image from an atlas
  particle.scaling.y = Math.random() + 1;
var box = BABYLON.MeshBuilder.CreateBox('b', {}, scene);
var SPS = new BABYLON.SolidParticleSystem('SPS', scene);
SPS.addShape(box, 150, {positionFunction: myBuilder)}; // myBuilder will be called for each of the 150 boxes
var mesh = SPS.buildMesh();

In this former example, each box particle will have its own rotation, position, scaling and uvs set once for all at construction time. As the mesh is not updatable, the particles are then not manageable with setParticles().
You've got here a real immutable mesh. You can still translate it, rotate it, scale it globally as any other mesh until you freeze its World Matrix.
Example : a town with 80 000 buildings http://www.babylonjs-playground.com/#2FPT1A#36 -

Note that this feature (modifying the mesh at construction time) is not directly related to the mesh updatable parameter. This means you can use it even with a default updatable mesh although it is easier to set the particles the classical war with setParticles().

Going further in immutable SPS
You've just seen how to modify for ever the SPS mesh at creation time in order to set the particles to your own initial positions, rotations, colors, etc by using the positionFunction property with your custom function.
You can also modify the shape of each particle in the SPS mesh at creation time the same way.
You will then to use the vertexPosition property, just like you used the positionFunction property, by defining your own function to set each vertex of each particle from its original value.
Your function will be then be called once by SPS.buildMesh() for each vertex of each particle object as defined in the former part.

var myVertexFunction = function(particle, vertex, i) {
    // particle : the current particle
    // vertex : the current vertex, a Vector3
    // i : index of the vertex in the particle shape
    vertex.x *= Math.random() + 1;
SPS.addShape(box, 150, {vertexFunction: myVertexFunction}); // the 150 boxes will have their vertices moved randomly

Of course you can use the both properties together :

SPS.addShape(box, 150, {vertexFunction: myVertexFunction, positionFunction: myPositionFunction});

Example with asteroids : http://www.babylonjs-playground.com/#2FPT1A#2 -

Note that you can also create some immutable objects rendered with only one draw call by using either MergeMesh() (tutorial), etheir Instances.

Start and End indexes for setParticles()

If you manage a big SPS with dozens of thousands particles, you may want, for performance reasons, not to compute all the new status of all the particles each frame. setParticles() expects three optional parameters to help you to choose what to compute or not : start, end, update

parameter definition default value
start (number) the index from where to start to iterate in the particles array 0
stop (number) the index (included) where to stop to iterate in the particles array nbParticles - 1
update (boolean) to force the SPS mesh vertex buffer to be updated true

If you pass a end value greater than nbParticles - 1, the iteration will stop anyway at nbParticles - 1 to prevent you from trying to access to undefined elements.

Example 1 : you may want to update your 10K particle mesh only every three frames

  • frame 1 : setParticles(0, 3300, false) computes everything for particles from 0 to 3300 and doesn't update the mesh.
  • frame 2 : setParticles(3301, 6600, false) computes everything for particles from 3301 to 6600 and doesn't update the mesh.
  • frame 3 : setParticles(6601, 9999, true) computes everything for particles from 6601 to 9999 and finally updates the mesh.

Example 2 : you could keep, say, the first 5000 particles as unused ones and compute the particle behavior only for the 5000 lasts in your global pool.

Colors and UVs

In the SPS, you can set a color or/and a different image per particle.


The colors are the Vertex colors, the color related to the vertices themselves. This means that, if you also use a colored material, the vertex colors and the material colors will mix nicely.
Unless you want to change the particle color, the particle will be given at creation the vertex color of their model if any. If the model has different vertex color per face (example : a box with different face colors), these colors are saved and all the particles built with this model will look like the model.
However, if you change the color of particle, the particle is then given this lone color. In other words, you can only set one single color for each particle (no more face color).
The particle colors are BJS Color4 object.
You can set them with the particle.color property. Please note that if you want to set this property at SPS creation time with the positionFunction parameter (new SolidParticleSystem("name", {positionFunction: myColorSettings}), the particle colors are initially null, since if you want to set it within the updateParticle(particle) method the particle colors are either the model colors if any, either white Color4(1, 1, 1, 1).
So, in positionFunction() :

particle.color = new BABYLON.Color4(red, green, blue, alpha);

and in updateParticle() :

// in order to not allocate new objects per particle each call
particle.color.r = red;
particle.color.g = green;
particle.color.b = blue;
particle.color.a = alpha;

If you want to set an alpha value, don't forget to enable the alpha channel for vertex colors :

SPS.mesh.hasVertexAlpha = true;


The SPS uses only one material, so only one texture.
However you can choose, per particle, which part of the texture you want to apply to this particle with the uvs particle property.
This property is a Vector4 and is initially set to (0, 0, 1, 1) for each particle (or to initial UVs values if the model had UVs per face), meaning the whole texture image, from its left lower corner (0, 0) to its right upper corner, is to be applied to each particle.
If you want apply just a portion of the texture, located at, say, 20% from the image width, 10% from its height for the left lower corner, and 60% from its width and 30% from this height for the right upper corner, you just set the uvs property like this :

particle.uvs.x = 0.2;   // left lower corner : 20% image width
particle.uvs.y = 0.1;   // left lower corner : 10% image height
particle.uvs.z = 0.6;   // right upper corner : 60% image width
particle.uvs.w = 0.3;   // right upper corner : 30% image width

This can be used as well either in the positionFunction call at SPS creation time, either in updateParticle().

Like for the colors, there can be only a UVs value per particle even if the particle model had initially different UVs per face. If you don't set the particle uvs property and if the model had UVs per face, they are saved.

Like for any other mesh, you can also enable the texture transparency with :

SPS.mesh.material.diffuseTexture.hasAlpha = true;

Or even use the alpha channel of the texture image :

SPS.mesh.material.useAlphaFromDiffuseTexture = true;

Please read this documentation for transparency concerns.

Color and UVs example : http://www.babylonjs-playground.com/#WCDZS#8 -

Texture with alpha : http://www.babylonjs-playground.com/#WCDZS#9 -

Update Each Particle Shape

  • SPS.updateParticleVertex() usage :
    It happens before particle scaling, rotation and translation and it allows to update the vertex coordinates of each particle.
    This function will be called for each vertex of each particle and it will be passed the current particlen the current vertex and its current index in the particle shape.
    SPS.computeParticleVertex = true; // false by default for performance reason
    SPS.updateParticleVertex = function(particle, vertex, v) {
      // particle : the current particle object
      // vertex : the current vertex, a Vector3
      // the index of the current vertex in the particle shape
      // example :
      if (particle.shapeID == 1) {
        vertex.x *= Math.random() + 1;
        vertex.y *= Math.random() + 1;
        vertex.z *= Math.random() + 1;
    Note well that this vertex update is not stored (the particle shape isn't modified) but just computed in the next call to setParticles(). So there is no value accumulation : the vertex coordinates are always the initial ones when entering this function.
    Note also that the shape reference for each particle is the original shape of the mesh model you passed in addShape(), even if you had passed also a custom vertexFunction (see in the part : "Going furhter in immutable SPS").
    The good news is that the very same function can be use for SPS.updateParticleVertex and for the custom vertexFunction expected by addShape().
    So to better understand how it works, here is another global pseudo-code schema :
    var particles: SolidParticles[] = [array of SolidParticle objects];
    function setParticles() {
      beforeUpdateParticles();                 // your custom function
      for (var p = 0; p < nbParticles; p++) {
        var particle = particles[p];
        updateParticles(particle);             // your custom position function
        for(var v = 0; particle.vertices.length; v++) {
          var vertex = particle.vertices[v];
          updateParticleVertex(particle, vertex, v);   // your ustom vertex function
      updateTheWholeMesh();                   // does the WebGL work
      afterUpdateParticles();                 // your ustom function
    Example : http://www.babylonjs-playground.com/#1X7SUN#5 -

    or dancing worms : http://www.babylonjs-playground.com/#1X7SUN#7 -

Pickable Particles

You can set your particles as pickable with the parameter isPickable (default false) when creating your SPS :

var SPS = new BABYLON.SolidParticleSystem('SPS', scene, {isPickable: true});

This will set the underlying mesh as pickable and populate an array called SPS.pickedParticles. So, don't set your SPS as pickable if you don't need it to be, this will save much memory.
This array has as many elements as the SPS mesh has many faces and each element is an object with these properties :

  • idx : the picked particle idx
  • faceId : the face index of the picked particle (counted within this particle)

Example :

var SPS = new BABYLON.SolidParticleSystem('SPS', scene, {isPickable: true});
// add shapes, build the mesh, init particles, etc
SPS.setParticles();                                 // initial SPS draw
SPS.refreshVisibleSize();                           // force the BBox recomputation
scene.onPointerDown = function(evt, pickResult) {
    var meshFaceId = pickResult.faceId;             // get the mesh picked face
    if (faceId == -1) { return; }                   // return if nothing picked
    var idx = SPS.pickedParticles[meshFaceId].idx;  // get the picked particle idx from the pickedParticles array
    var p = SPS.particles[idx];                     // get the picked particle
    p.color.r = 1;                                  // turn it red    
    p.color.b = 0;
    p.color.g = 0;
    p.velocity.y = -1;                              // drop it

The SPS pickability is directly related to the size of its bounding box (please read 'SPS Visibility' part). So, in order to make sure your particles will be pickable, don't forget to force, at least once, the bounding box size recomputation once the particles are set in the space with setParticles().
Pickable particle example (no SPS update in the render loop) : http://www.babylonjs-playground.com/#2FPT1A#41 -

Pickable particle example (particle rotation) : http://www.babylonjs-playground.com/#2FPT1A#14 -

Digest a Mesh

There is another way than adding shapes of meshes used as models to populate the SPS : you can directly "digest" a mesh.
To digest a mesh means that the SPS will decompose this mesh geometry and use all its facets to generate the particles. So, by default, a digested mesh generates as many particles as the mesh number of facets.

var model = BABYLON.MeshBuilder.CreateTorusKnot('s', {radius: 20, tube: 6, tubularSegments: 64, radialSegments: 128}, scene);

Note that in this case, all the generated particles have their property "position" set with some values and no more to (0, 0, 0).
This method is obviously compatible with addShape() and you can even call it several times with the same model, or different models, in the same SPS.

var model = BABYLON.MeshBuilder.CreateTorusKnot('s', {radius: 20, tube: 6, tubularSegments: 64, radialSegments: 128}, scene);
SPS.addShape(boxModel, 50);
SPS.addShape(sphereModel, 20);
SPS.digest(model, {number: 10});

This method accepts three optional parameters : facetNb, delta and number

  • facetNb is the number of the mesh facets required to build each particle. By default, the value is set to 1, this means each particle will just be a triangle (a mesh facet). Set to 2 and you'll probably get quads instead.
    The number of generated particles depends then on the mesh initial number of facets and on the faceNb value.
    This parameter is overriden if the parameter number is set.
  • delta (default 0), used with facetNb, allows to generate each particle with a random size between facetNb and facetNb + delta facets.
  • number is the wanted number of particles. digest() divides then the mesh into number particles of the same size in term of the number of facets used per particle.
    If number is greater than the total number of mesh facets, then this total number is used for the value of number.
    var model = BABYLON.MeshBuilder.CreateTorusKnot('s', {radius: 20, tube: 6, tubularSegments: 64, radialSegments: 128}, scene);
    SPS.digest(model, {facetNb: 10});   // 10 facets per particle whatever their final number
    SPS.digest(model, {number: 200});   // 200 particles whatever their final size
    SPS.digest(model, {facetNb: 10, delta: 30});   // between 10 and 40 facets per particle, randomly, whatever their final number
    Example (click on the torus knot) : http://www.babylonjs-playground.com/#HDHQN -

SPS Visibility

To render the meshes on the screen, BJS uses their bounding box (BBox) : if the BBox is in the frustum, then the mesh is selected to be rendered on the screen. This method is really performant as it avoids to make the GPU compute things that wouldn't be visible. The BBox of each mesh is recomputed when its World Martix is updated.
When you create a SPS, unless you use the positionFunction at creation time, all its particles are set by default at the position (0, 0, 0). So the size of the SPS mesh is initially the size of its biggest particle, so it is for its BBox.
If you animate your particles without updating the SPS mesh World Matrix (ex : the whole SPS doesn't move, rotate or scale), its BBox may keep far more little than the current space occupied by the moving particles. So, if this little BBox gets out of the screen (cam rotation for instance), the whole SPS can then disappear at once !

In order to manage the SPS visibility, you have some ways : the methods SPS.refreshVisibleSize() or SPS.setVisibilityBox(size) and the properties SPS.isAlwaysVisible (default false),SPS.computeBoundingBox (default false) or SPS.isVisibilityBoxLocked (default false)

  • SPS.refreshVisibleSize() : updates the SPS mesh BBox size on demand. This is an intensive computation, so it's better not to use it in the render loop each frame. You could call it once the mesh has reached its maximum size for instance. This the method to use if you have a SPS located its in own space somewhere in your scene, like a particle explosion, a fountain, etc. Remember when using this method that it iterates over each mesh vertices. So if your mesh has 20K vertices what is usual with a SPS, it needs 20K iterations.

  • SPS.isAlwaysVisible : if true, forces the SPS mesh to be computed by the GPU even if its BBox is not visible. This property is to use when the player evolves inside the SPS (maze, asteroid field) or if the SPS is always bigger than the visible part on the screen. Note that setting it to true doesn't recompute the BBox size, so if you need for some reason (pickability, collisions, etc) to update the BBox, you have to call also at least once SPS.refreshVisibleSize().

  • SPS.computeBoundingBox (default false) : if set to true, the next calls to setParticles() will compute the mesh bounding bow within the same loop than the particle computations. This means this is much more faster than calling refreshVisibleSize() and you can use it in the render loop.
    The reason refreshVisibleSize() and SPS.computeBoundingBox exist together is that refreshVisibleSize() can be called at any time and doesn't require to call setParticles() whereas SPS.computeBoundingBox is taken in account for the BBox computation only from a call of setParticles().
    Note that SPS.computeBoundingBox can be set to true or false at any time and will affect only the next calls of setParticles().

  • SPS.setVisibilityBox(size) : sets a fixed size to the SPS Mesh BBox whatever its own real size. This may be useful when you know in advance the limits that the visible particles won't overrange. Note that setting a value doesn't prevent any further BBox recomputation.

  • SPS.isVisibilityBoxLocked : if true, the SPS mesh BBox won't be computed any longer until it is reset to false.

So what method to use ?
It depends on your needs.
If your SPS is ever everywhere around the camera environment like an asteroid field, you may use SPS.isAlwaysVisible.
If you need a variable visibility or the pickability, you'll need to set at least once the bounding box.
So if your SPS stays within some fixed bounds that you don't know the values, you may use SPS.refreshVisibleSize() at least once when the SPS has reached these limits and then lock the visibility box.
If the SPS keeps within some known limits, then it is better to use SPS.setVisibilityBox(size) with the right value and then to lock the visibility box.
At last, if you still need pickability or variable visibility, and don't know how your SPS will evolve, then you might set SPS.computeBoundingBox to true.

Particle Intersections

The SPS is physics agnostic. This means you need to implement your own particle behavior if you want to animate them.
For this, you may need to check if the solid particles intersect or not other ones or other meshes in the scene. Example : you would know if your particles collide against an obstacle and then make them bounce back.
The SPS provides a simple way to deal with particle intersections. As this feature consumes more memory and CPU, it's disabled by default.
Enable it explicitly with the parameter particleIntersection when creating your SPS :

var SPS = new SolidParticleSystem("sps", scene, {particleIntersection: true});

Then you can simply call the method intersectsMesh(<SolidParticle | AbstractMesh>target) of any solid particle to check if this particle intersects the target.
It just will return true or false if the particle intersects or not the target.

// for instance, in your SPS.updateParticle(p) function : particle / particle
if (p.intersectsMesh(otherParticle)) { // change p velocity vector }

You can pass the method intersectsMesh() either a solid particle object, either a standard BJS mesh. Both work the same.

// for instance, in your SPS.updateParticle(p) function : particle / mesh
if (p.intersectsMesh(anyMesh)) { // change p velocity vector }

Beware that invisible solid particles can still intersect, exactly like meshes do. So it's up to you to test the particle visibility (isVisible), if you want to exclude it from an intersection computation.

If you prefer, you can even use the AbstractMesh method intersectsMesh() and pass it a solid particle.

// for instance, in your SPS.updateParticle(p) function : mesh / particle
if (someMesh.intersectsMesh(p)) { // change p velocity vector }

Under the hood, when creating a SPS with particleIntersection, a bounding box and a bouding sphere are given to each solid particle.
For performance reasons, the particle intersections are always computed the fastest way, it is to say with Axis Aligned Bounding Boxes (AABB). Please read this : http://doc.babylonjs.com/tutorials/Intersect_Collisions_-_mesh
If you use the AbstractMesh intersectsMesh() method, what allows to force OBB computation (precise mode), only the mesh bounding box will be rotated, not the particle one, so the intersection detection will be just a bit better than in AABB mode.
The precise mode has a CPU significant cost, so it's not recommended to use it with solid particles.

// for instance, in your SPS.updateParticle(p) function : precise mode, mesh / particle
if (someMesh.intersectsMesh(p, true)) { // change p velocity vector }

Example : http://www.babylonjs-playground.com/#10RCC9 -

For a SPS having thousands of particles, computing the bounding box for each particle each frame is still a heavy CPU operation. So, if you need more performance and if you don't mind about the intersection accurary, you may choose to limit the computation to the particle bounding sphere only (a bounding box requires 8 iterations per particle, one for each box vertex) by using the optional boolean parameter boundingSphereOnly (default false) at SPS creation.

var SPS = new SolidParticleSystem("sps", scene, {particleIntersection: true, boundingSphereOnly: true});

Example : http://www.babylonjs-playground.com/#2BXZC#2 -

As you may know, a mesh -so a solid particle- is inside its bounding box and its bounding box is inside its bounding sphere. So the bounding sphere is bigger than the bounding box, what is bigger than the mesh.
If your particles look like some some tiny spherical objects and if you use the boundingSphereOnly mode, you would probably like to tweak the bounding sphere to make it closer to the embedded particle.
You can then use the parameter bSphereRadiusFactor, a float number that is multiplied by the current bounding sphere radius.
Imagine that your particle is a spherical shape with a radius of R. Its bounding sphere radius is then by default : R * sqrt(3). So if you multiply the bounding sphere radius by 1 / sqrt(3), the bounding sphere will get the same radius than the particle one and both will exactly match.

var SPS = new SolidParticleSystem("sps", scene, {particleIntersection: true, boundingSphereOnly: true, bSphereRadiusFactor: 1 / Math.sqrt(3)});

Example : http://www.babylonjs-playground.com/#29F0EG#2 -

At last, in case you are using the boundingSphereOnly mode, just remember that the particle bounding box isn't computed, only its bouding sphere, so don't test the intersection from a mesh object :

// boundingSphereOnly case :
mesh.intersectsMesh(particle);  // won't give the right result
particle.intersectsMesh(mesh);  // will give the right bounding sphere intersection

Garbage Collector Concerns

In Javascript, the Garbage Collector is usually your friend : it takes care about cleaning up all the not any longer needed variables you could have declared and thus it sets the memory free.
However, it can sometimes become an awkward friend because it can start its cleaning just while you are displaying a very smooth animation, so it takes the CPU for itself and leaves to you only those nice lags on the screen.
In order to avoid unpredictable GC pauses, it's best to avoid allocating new objects in loops that execute often, where particles are created or updated.
For example, updateParticle() and updateParticleVertex() are often called each frame for each particle or each particle vertex. Now imagine that you have a SPS with 30 000 particles. Suppose you write code like this to add a particle acceleration :

SPS.updateParticle = function(particle) {
    var accel = new BABYLON.Vector3(0, 0.5, 0);
    particle.velocity = particle.velocity.add(accel);
    // ...

Code like the above would create two new Vector3 objects each call, or 60 000 new objects over the course of the update. And although modern JS engines are quite good at cleaning up short-lived objects, it's best not to unnecessarily strain the GC this way.
Instead, make your update loops reuse variables that were declared outside the loop, and don't call any methods inside the loop that create new objects:

var accel = new BABYLON.Vector3(0, 0.5, 0);
SPS.updateParticle = function(particle) {
    // ...

SPS also has a vars property, which is an object that you can use to store any variables you want to reuse. Any variables you store there will share the SPS' lifecycle, and get cleaned up when you dispose it:

SPS.vars.tempVector = new BABYLON.Vector3(0, 0, 0);
// ...
SPS.dispose();  // cleans explicitly all your SPS.vars !

A good JS practice for the compiler is to never change the variable type once it has been set :

SPS.vars.myFloat = 0.01;   // just keep setting float values to myFloat afterwards
SPS.vars.myInt = 5;        // just keep setting integer values to myInt afterwards
SPS.vars.myString = "foo"; // just keep setting string values to myString afterwards

Example : From this article, here is an implementation of a simple particle IA called "flocking" what a behavior of association, then cohesion and separation. This example uses SPS.vars to allocate the memory used for results only once instead of in-function temporary variables.
http://www.babylonjs-playground.com/#2FPT1A#35 -

Rebuild the mesh

if a mesh, changed at creation time with positionFunction or vertexFunction has been then modified with setParticles(), it can be rebuild by reapplying the internally stored positionFunction or vertexFunction functions.
Note that only the function are stored, not their results. This means that if one of your function produces different results each call (using Math.random() for instance), you won't get back the same SPS mesh shape but another computed shape.


Except in some very specific cases, you might not need to use this function.