CSC517 OffScreenCanvas Servo

About Servo:

Servo is a parallel web-engine project under Mozilla, which aims to develop a web engine to better utilize the potential of multiple processing units to parse and display web-pages faster than conventional browser engines. Servo is implemented using Rust programming language, which is similar to C, except the fact that it is specially tuned for better memory safety and concurrency features. Hence, servo seeks to create a highly parallel environment, in which many components are handled by fine-grained and isolated tasks.

Servo makes web-page rendering faster by using parallel layout, styling, web-renders and constellation. Basically, these help to sandbox processes, threads and tasks based on their privilege levels. Sandboxing not only makes way for more responsive web-pages, but it also makes then more secure and robust. Since, when a thread or javascript of a particular section of the webpage fails, it doesn't affects other tabs, browser or eve other elements of the same web-page. So, in a way, Rust lets the browser to "fail better" than other browsers.

CSS3 and HTML5 parser of servo engine have been successfully implemented using Rust, and it successfully passes the acid2 test. Although it is still in early phase of development, it renders web pages significantly more quickly and smoothly, compared to Mozilla's other rendering engines.

Why use rust ?

Algorithms which deal with multiple threads running in parallel are inherently difficult, due to synchronization, data sharing and other typical issues which arise while designing parallel algorithms. C++ makes this process even harder as it does not have efficient mechanisms to deal with this. So, the idea behind rust and servo is to rewrite C++ (and create rust) and use that to rewrite a browser.

Off Screen Canvas:

Canvas is a popular way of drawing all kinds of graphics on the screen and an entry point to the world of WebGL. It can be used to draw shapes, images, run animations, or even display and process video content. It is often used to create beautiful user experiences in media-rich web applications and online games. Because canvas logic and rendering happens on the same thread as user interaction, the (sometimes heavy) computations involved in animations can harm the app’s real and perceived performance.

Until now, canvas drawing capabilities were tied to the <canvas> element, which meant it was directly depending on the DOM. OffscreenCanvas, as the name suggests, decouples the DOM and the Canvas API by moving it off-screen. Due to this decoupling, rendering of OffscreenCanvas is fully detached from the DOM and therefore offers some speed improvements over the regular canvas as there is no synchronization between the two.

Web workers and Offscreen Canvas

Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application. The advantage of this is that laborious processing can be performed in a separate thread, allowing the main (usually the UI) thread to run without being blocked/slowed down. So, JavaScript could run in the background without affecting the performance of the webpage, independent of other scripts. But, web-workers are treated as external files, which makes it impossible for them to access window object, document object and the parent object. Hence, web-workers can only communicate with the rendering-engine by passing messages. But, how is OffscreenCanvas rendered using web-workers?: Suppose the task is to render an image in the canvas. The rendering engine creates a web-worker, and sends it the image data that it needs to process. The web-worker does the required processing, and returns the resultant image back to the rendering-engine. In this way, an image is rendered using an entirely independent process from the webpage.

Scope:

The scope of the project included:

• Creating the OffscreenCanvas and OffscreenCanvasRenderingContext2d interfaces with stub method implementations.

• Implementing the OffscreenCanvas Constructor [1] that creates a new canvas.

Web IDL

Web IDL is an interface definition language that can be used to describe interfaces that are intended to be implemented in web browsers. It is an IDL variant with:

• A number of features that allow one to more easily describe the behavior of common script objects in a web context.

• A mapping of how interfaces described with Web IDL correspond to language constructs within an ECMAScript execution environment.

Code Flow

• First, IDL files namely OffScreenCanvas.webidl and OffScreenCanvasRenderingContext2D.webidl were added at the locations components/script/dom/webidls/OffScreenCanvas.webidl and components/script/dom/webidls/OffScreenCanvasRenderingContext2D.webidl respectively.

• The corresponding .rs files were added which would implement the DOM interface described in the .webidl files.

Code Snippet

Here are the .webIDL files we added:

OffScreenCanvas.webidl

typedef (OffscreenCanvasRenderingContext2D or WebGLRenderingContext or WebGL2RenderingContext) OffscreenRenderingContext;

dictionary ImageEncodeOptions {
  DOMString type = "image/png";
  unrestricted double quality = 1.0;
};

enum OffscreenRenderingContextId { "2d", "webgl", "webgl2" };

[Constructor([EnforceRange] unsigned long long width, [EnforceRange] unsigned long long height), Exposed=(Window,Worker), Transferable]
interface OffscreenCanvas : EventTarget {
  attribute [EnforceRange] unsigned long long width;
  attribute [EnforceRange] unsigned long long height;

  OffscreenRenderingContext? getContext(OffscreenRenderingContextId contextId, optional any options = null);
  ImageBitmap transferToImageBitmap();
  Promise<Blob> convertToBlob(optional ImageEncodeOptions options);
};

OffScreenCanvasRenderingContext2D.webidl

[Exposed=Window]
interface CanvasRenderingContext2D {
  // back-reference to the canvas
  readonly attribute HTMLCanvasElement canvas;
};
CanvasRenderingContext2D implements CanvasState;
CanvasRenderingContext2D implements CanvasTransform;
CanvasRenderingContext2D implements CanvasCompositing;
CanvasRenderingContext2D implements CanvasImageSmoothing;
CanvasRenderingContext2D implements CanvasFillStrokeStyles;
CanvasRenderingContext2D implements CanvasShadowStyles;
CanvasRenderingContext2D implements CanvasFilters;
CanvasRenderingContext2D implements CanvasRect;
CanvasRenderingContext2D implements CanvasDrawPath;
CanvasRenderingContext2D implements CanvasUserInterface;
CanvasRenderingContext2D implements CanvasText;
CanvasRenderingContext2D implements CanvasDrawImage;
CanvasRenderingContext2D implements CanvasImageData;
CanvasRenderingContext2D implements CanvasPathDrawingStyles;
CanvasRenderingContext2D implements CanvasTextDrawingStyles;
CanvasRenderingContext2D implements CanvasPath;

References:

1. https://research.mozilla.org/servo-engines/
2. https://research.mozilla.org/rust/
3.