Graphics Context

Why Graphics?

ExcaliburGraphicsContext is the abstraction over the underlying drawing mechanism used to displaying images and graphics to the screen. It is recommended to use ex.Graphics objects with Actors/Entities but the ExcaliburGraphicsContext can be directly drawn to.

The default implementation in Excalibur uses WebGL, however Excalibur can fallback to the 2D Canvas implementation if WebGL isn't supported or there isn't browser hardware acceleration. Read more

Canvas support

If you have need to switch to 2D canvas based rendering turn on the flag before engine construction.

Some features like

custom renderers

and

post processors

do not work with the Canvas 2D implementation!

ex.Flags.useCanvasGraphicsContext();
const game = new ex.Engine(...);

Drawing to the Context

The graphics context automatically batches draw calls and flushes them to the screen at the end of every frame. It is therefore recommended you draw in one of the supported drawing lifecycle events.

Either directly by extending the Excalibur Scene

const game = new ex.Engine({...});
class MyScene extends ex.Scene {
    onPreDraw(ctx: ExcaliburGraphicsContext) {
        ctx.save();
        ctx.drawRectangle(...);
        ctx.restore();
    }
    onPostDraw(ctx: ExcaliburGraphicsContext) {
        ctx.save();
        ctx.drawRectangle(...);
        ctx.restore();
    }
}
game.addScene('myscene', new MyScene());
game.goToScene('myscene');
game.start();

Or as an event on the the Excalibur Scene

const game = new ex.Engine({...});
game.currentScene.on('predraw', (ctx: ExcaliburGraphicsContext) => {
    ctx.save();
    ctx.drawRectangle(...);
    ctx.restore();
});

Or as part of an Actor/Entity graphics component

const game = new ex.Engine({...});
const actor = new ex.Actor({pos: ex.vec(100, 100)});
game.currentScene.add(actor);
game.start();

// Draw before graphics component but after the transform for actor pos/rotation/scale
actor.graphics.onPreDraw = (ctx: ExcaliburGraphicsContext) => {

    ctx.save();
    ctx.drawRectangle(...);
    ctx.restore();
}

// Draw after graphics component but after the transform for actor pos/rotation/scale
actor.graphics.onPostDraw = (ctx: ExcaliburGraphicsContext) => {

    ctx.save();
    ctx.drawRectangle(...);
    ctx.restore();
}

Save and Restore

The ExcaliburGraphicsContext emulates other graphics context APIs by providing a save/restore feature. These are used to save the state of the context before you modify its global state, and then restore it back after you are finished drawing. If you do not, modifications to ExcaliburGraphicsContext will affect other draw calls.

onPostDraw(ctx: ExcaliburGraphicsContext) {
    ctx.save();
    ctx.translate(50, 50);
    ctx.rotate(Math.PI / 2);
    ctx.drawRectangle(...);
    ctx.restore();
}

Setting z-index

Excalibur allows you to set the z-index of any draw call by setting the ExcaliburGraphicsContext.z property on the context.

onPostDraw(ctx: ExcaliburGraphicsContext) {
    ctx.save();
    ctx.z = -1;
    ctx.drawRectangle(...);
    ctx.restore();
}

Custom Renderer

Custom renderers are a way to extend what the ExcaliburGraphicsContextWebGL can draw by default. In fact all of the things that the graphics context can draw are implemented this way internally.


const MyRenderer implements ex.RendererPlugin {
    ...
}

const graphicsContextWebGL = game.graphicsContext as ExcaliburGraphicsContextWebGL;
graphicsContextWebGL.register(new MyRenderer());

graphicsContextWebGL.draw<MyRenderer>(...);

Read a more in-depth example here

Performance

Excalibur's performance fallback behavior can be configured by developers to help players experiencing poor performance in non-standard browser configurations

This will fallback to the Canvas2D rendering graphics context which usually performs better on non hardware accelerated browsers, currently postprocessing effects are unavailable in this fallback.

By default if a game is running at 20fps or lower for 100 frames or more after the game has started it will be triggered, the developer can optionally show a player message that is off by default.

var game = new ex.Engine({
    ...
    configurePerformanceCanvas2DFallback: {
        // opt-out of the fallback
        allow: true,
        // opt-in to a player pop-up message
        showPlayerMessage: true,
        // configure the threshold to trigger the fallback
        threshold: { fps: 20, numberOfFrames: 100 }
    }
});