webgl – Pal's Blog

Federated Events API

PixiJS 6.1.0 will ship with an experimental UI event infrastructure that provides a much more robust and DOM-compatible solution than the incumbent interaction plugin. This change made it through PixiJS’ RFC 7071 and merged in PR 7213.

I named it the “Federated Events API.” It’s federated because you can create multiple event boundaries and override their logic for parts of your scene. Each event boundary only controls events for the scene below them – not unlike a federation.


I developed the Federated Events API to overcome the two significant limitations in the Interaction API –

  • DOM Incompatibility
  • Extensibility

Apart from these API-facing issues, we also needed to refactor the implementation to make it more maintainable.

DOM Incompatibility

The Interaction API had a synthetic InteractionEvent that didn’t overlap with DOM’s PointerEvent well enough. If your UI shared DOM elements, then event handlers had to still be specific to PixiJS or the DOM.

The Federated Events API brings multiple events that inherit their DOM counterparts. This means your event handlers are agnostic to whether they’re looking at a DOM or PixiJS event. DisplayObject now also has addEventListener and removeEventListener methods.

The semantics of some interaction events diverged from those of the Pointer Events API.

  • pointerover and pointerout events didn’t bubble up to their common ancestor.
  • pointerenter and pointerleave events were missing.
  • pointermove events would fire throughout the scene graph, instead of just on hovered display object.

This gets corrected in this new API!

Another important addition is the capture phase for event propagation. The new API’s event propagation matches that of the DOM.


The Interaction API’s implementation was very brittle, and overriding any detail meant hell. The rigid architecture also means that customizing interaction for a part of your scene was impossible.

This new API lets you override specific details of the event infrastructure. That includes:

  • optimizing hit testing (spatial hash acceleration?)
  • adding custom events (focus, anyone?)
  • modifying event coordinates (handy if you’re using projections)

The API also lets you mount event boundaries at specific parts of your scene graph to override events for display objects underneath it.

Other improvements


The EventSystem is the main point of contact for federated events. Adding it to your renderer will register the system’s event listeners, and once it renders – the API will propagate FederatedEvents to your scene. The EventSystem‘s job is to normalize native DOM events into FederatedEvents and pass them to the rootBoundary. It’s just a thin wrapper with a bit of configuration & cursor handling on top.

The EventBoundary object holds the API’s core functionality – taking an upstream event, translating it into a downstream event, and then propagating it. The translation is implemented as an “event mapping” – listeners are registered for handling specific upstream event types and are responsible for translating and propagating the corresponding downstream events. This mapping isn’t always one-to-one; the default mappings are as follows:

  • pointerdownpointerdown
  • pointermovepointerout, pointerleave, pointermove, pointerover, pointerenter
  • pointeruppointerup
  • pointeroutpointerout, pointerleave
  • pointeroverpointerover, pointerenter
  • wheelwheel

This list doesn’t include the mouse- and touch-specific events that are emitted too.


An event boundary can search through and propagate events throughout a connected scene graph, which would be connected by the parent-child relationships.

In certain cases, however, you may want to “hide” the implementation scene for an object. @pixi-essentials/svg does this to prevent your scene from being dominated by SVG rendering nodes. Instead of holding the nodes below as children, you place them in a root container and render it separately.

// Crude anatomy of a disconnected scene
class HiddenScene {
  root: Container;
  render(renderer) {

This poses a problem when you want interactivity to still flow through this “point of disconnection”. Here, an additional event boundary that accepts upstream events and propagating them through root can fix this! See the nested boundary example at the end for how.


Basic usage

Since the Federated Events API won’t be production-ready until PixiJS 7, it’s not enabled by default. To use it, you’ll have to delete the interaction plugin and install the EventSystem manually. If you’re using a custom bundle, you can remove the @pixi/interaction module too.

import { EventSystem } from '@pixi/events';
import { Renderer } from '@pixi/core';// or pixi.js

delete Renderer.__plugins.interaction;

// Assuming your renderer is at "app.renderer"
if (!('events' in app.renderer)) {
    app.renderer.addSystem(EventSystem, 'events');


Let’s start with this barebones example – handling clicks on a display object. Just like the Interaction API, you need to mark it interactive and add a listener.

// Enable interactivity for this specific object. This
// means that an event can be fired with this as a target.
object.interactive = true;

// Listen to clicks on this object!
object.addEventListener('click', function onClick() {
    // Make the object bigger each time it's clicked!
    object.scale.set(object.scale.x * 1.1);

A handy tool for checking handling “double” or even “triple” clicks is the event’s detail. The event boundary keeps track of how many clicks have been done each within 200ms of each other. For double clicks, it’ll be set to 2. The following example scales the bunny based on this property – you have to click fast to make the bunny larger!


Dragging is done slightly differently with the new API – you have to register the pointermove handler on the stage, not the dragged object. Otherwise, if the pointer moves out of the selected DisplayObject, it’ll stop getting pointermove events (to emulate the InteractionManager’s behavior – enable moveOnAll in the root boundary).

The upside is much better performance and mirroring of the DOM’s semantics.

function onDragStart(e) {
    selectedTarget = e.target;

    // Start listening to dragging on the stage
    app.stage.addEventListener('pointermove', onDragMove);

function onDragMove(e) {
    // Don't use e.target because the pointer might
    // move out of the bunny if the user drags fast,
    // which would make e.target become the stage.
        e.global, null, selectedTarget.position);


The wheel event is available to use just like any other! You can move your display object by the event’s deltaY to implement scrolling. This example does that for a slider’s handle.

Right now, wheel events are implemented as “passive” listeners. That means you can’t do preventDefault() to block the browser from scrolling other content; this means you should only use it on fullscreen canvas apps.

slider.addEventListener('wheel', onWheel);

Advanced use-cases

Manual hit-testing

To override a specific part of event handling, you can inherit from EventBoundary and set the event system’s rootBoundary!

Here’s an example that uses a SpatialHash to accelerate hit-testing. A special HashedContainer holds a spatial hash for its children, and that is used to search through them instead of a brute force loop.

This technique is particularly useful for horizontal scene graphs, where a few containers hold most of the display objects as children.

Nested boundaries

The ultimate example: how you can use a nested EventBoundary in your scene graph. As mentioned before, this is useful when you have a disconnected scene graph and you want events to propagate over points of disconnection.

To forward events from upstream, you make the “subscene” interactive, listen to all the relevant events, and map them into the event boundary below. The event boundary should be attached to the content of your scene. It’s like implementing a stripped down version of the EventSystem.

// Override copyMouseData to apply inverse worldTransform 
// on global coords
this.boundary.copyMouseData = (from, to) => {
    // Apply default implementation first
        .call(this.boundary, from, to);

    // Then bring global coords into content's world
        to.global, to.global);

// Propagate these events down into the content's
// scene graph!
].forEach((event) => {
        (e) => this.boundary.mapEvent(e),

To make the cursor on internal objects work too, you should expose the event boundary’s cursor property on the subscene.

get cursor() {
    return this.boundary.cursor;

PixiJS Tilemap Kit 3

In my effort to bring tighter integration to the PixiJS ecosystem, I’m upgrading external PixiJS packages and working towards lifting them to the standard of the main project. @pixi/tilemap 3 is the first package in this process. Yes, I’ve republished pixi-tilemap as @pixi/tilemap.

Here, I want to cover the new, leaner API that @pixi/tilemap 3 brings to the table. This package by Ivan Popleyshev gives you an optimized rectangular tilemap implementation you can use to render a background for your game or app composed of tile textures. The documentation is available at https://api.pixijs.io/@pixi/tilemap.html.


A tileset is the set of tile textures used to build the scene. Generally, you’d want the tileset to be in one big base-texture to reduce the number of network requests and improve rendering batch efficiency.

To use @pixi/tilemap, you’ll first need to export a tileset atlas as a sprite sheet. PixiJS’ spritesheet loader populates your tile textures from the sheet’s manifest. If you don’t have one at hand, you can create a sample tileset as follows:

  • Download this freebie tileset from CraftPix.net here: https://craftpix.net/download/24818/. You’ll need to signup, however.
  • Download and install TexturePacker: https://www.codeandweb.com/texturepacker
  • Drag the “PNG” folder of the downloaded tileset into TexturePacker. It will automatically pack all the tiles into one big atlas image.
  • Then click on “Publish sprite sheet” and save the manifest.
The generate tileset should look like this!


The Tilemap class renders a tilemap from a predefined set of base-textures containing the tile textures. Each rendered tile references its base-texture by an index into the tileset array. This tileset array is first passed when the tilemap is created; however, you can still append base-textures without changing previously added tiles after it is instantiated.

The following example paints a static tilemap from a CraftPix tileset.

The texture passed to tile() must be one of the atlases in the tilemap’s tileset. Otherwise, the tilemap will silently drop the tile. As we’ll discuss later on, CompositeTilemap can be used to get around this limitation.

Animated Tiles

The options passed to tile let you animate the rendered tile between different tile textures stored in the same base-texture. The different frames must be located uniformly in a table (or a single row/column).

The texture you pass to tile will be the first frame. Then the following parameters specify how Tilemap will find other frames:

  • animX: The x-offset between frame textures.
  • animY: The y-offset between frames.
  • animCountX: The number of frame textures per row of the table. This is 1 by default.
  • animCountY: The number of frames per column of the table. This is 1 by default.

If your frames are all in a row, you don’t need to specify animY and animCountY.

The animation frame vector (tileAnim) specifies which frame to use for all tiles in the tilemap. tileAnim[0] specifies the column modulo, and tileAnim[1] specifies the row modulo. Since it wraps around when the column/row is fully animated, you don’t have to do it yourself.

The above example takes advantage of the fact that some regular doors and wide doors are placed in a row in the sample atlas. animX, animCountX are used to animate between them every 500ms.

Tileset limitations

Tilemap renders all of the tilemap in one draw call. It doesn’t intermediate batches of geometry like PixiJS’ Graphics. All the tileset base-textures are bound to the GPU together.

This means that there’s a limit to how many tile sprite sheets you can use in each tilemap. WebGL 1 guarantees that at least 8 base-textures can be used together; however, most devices support 16. You can check this limit by evaluating


If your tileset contains more base-textures than this limit, Tilemap will silently fail to render its scene.

If you’re using only one sprite sheet like the examples above, you don’t need to worry about hitting this limit. Otherwise, CompositeTilemap is here to help.


A “tilemap composite” will layer tilesets into multiple tilemaps. You don’t need to predefine the base-textures you’re going to use. Instead, it will try to find a tilemap with the same base-texture in its tileset when you add a tile; if none exists, the base-texture is added into a layered tilemap’s tileset. New tilemaps are automatically created when existing ones fill up.

In most cases, you can trivially swap usage of Tilemap with CompositeTilemap. However, you have to be careful about z-ordering. The tiles using textures in later tilemaps will always render above. This may become a problem with overlapping tiles in some cases.

The following example uses a CompositeTilemap to render one of the previous examples. Instead of using a separate Sprite for the background, it adds the background itself as a tile too.

Tilemap rendering


Tilemap internally stores tiles in a geometry buffer, which contains interleaved data for each vertex.

  • Position (in local space)
  • Texture coordinates
  • Texture frame of the tile
  • Animation parameters (specially encoded into a 32-bit 2-vector)
  • Texture index (into the tileset)

This buffer is mostly static and is lazily updated whenever the tiles are modified between rendering ticks. If the tilemap is left unchanged, the geometry is used directly from graphics memory.


The TileRenderer plugin holds the shared tilemap shader.

The vertex program decodes the animation parameters, calculates and passes the texture frame, texture coordinates, and texture index to the fragment program. The animation frame vector is passed as a uniform. 

Then, the fragment program samples the texel from the appropriate texture and outputs the pixel.


@pixi/tilemap’s settings (which is discussed further on) contains a property called TEXTILE_UNITS. This is the number of tile base-texture that are “sewn” together when uploaded to the GPU. You can use this to increase the tileset limit per texture.

The “combined” texture is called a textile. The textile is divided into a 2-column table of square slots. Each slot is a square of size TEXTILE_DIMEN. Your tileset base-textures must be smaller than this dimension for the textile to work.

The following demonstration shows what a textile looks like when uploaded. The textile-tile dimension was set to 256 so that images aren’t spread out too far (it is 1024 by default). 


@pixi/tilemap exports a “settings” object that you should configure before a tilemap is created.

  • TEXTURES_PER_TILEMAP: This is the limit of tile base-textures kept in each layer tilemap of a composite. Once the last tilemap is filled to this limit, the next texture will go into a new tilemap.
  • TEXTILE_DIMEN, TEXTILE_UNITS: Used to configure textiles. If TEXTILE_UNITS is set to 1 (the default), textiles are not used.
  • TEXTILE_SCALE_MODE: Used to set the scaling mode of the resulting textile textures.
  • use32bitIndex: This option enables tilemaps’ rendering with more than 16K tiles (64K vertices).
  • DO_CLEAR: This configures whether textile slots are cleared before the tile textures are uploaded. You can disable this if tile textures “fully” cover TEXTILE_DIMEN and leave no space for a garbage background to develop.

Canvas support

@pixi/tilemap has a canvas fallback, although it is significantly slower. In the future, I might spin out a @pixi/canvas-tilemap to make this fallback optional.