off-topic question (was: [whatwg] Splitting CanvasRenderingContext2D)

On Mon, Nov 30, 2015 at 3:40 PM, Ian Kilpatrick <ikilpatrick@chromium.org>
wrote:

> Hi whatwg,
>
> (public-houdini@w3.org cc'd)
>
> For the CSS Paint API[1] work in Houdini we would like to split up the
> current CanvasRenderingContext2D to remove references to the DOM and pixel
> readback.
>

Why are you disallowing pixel readback? Is this an optimization so you
don't have to create an image unless needed?


> Additionally OffscreenCanvas[2] needs a similar reduced subset of the
> CanvasRenderingContext2D api (remove references to the DOM on Workers).
>
> There are a couple of ways to split up the CanvasRenderingContext2D API,
> namely adding inheritance + [NoInterfaceObject] to superclasses (as to not
> change prototype chain, maybe not a large web compat concern), or to create
> additional interfaces and get *RenderingContext2D to implement.
>
> The following gist has a "split into interfaces" version:
> https://gist.github.com/bfgeek/72c2b26c2ccc8f728daf [see below for TL;DR
> version]
>
> Thoughts on this as a PR to the HTML spec?
> If so any preferences for how this is done?
>
> Additionally, if this is too premature to add to the HTML spec people have
> suggested moving this to the WICG.
>
> Thanks,
> Ian
>
> [1] https://drafts.css-houdini.org/css-paint-api/
> [2] https://wiki.whatwg.org/wiki/OffscreenCanvas
>
> TL;DR version (could be made more granular):
> interface CanvasRenderingContext2D {
>
>   // back-reference to the canvas
>   readonly attribute HTMLCanvasElement canvas;
>
>   void drawFocusIfNeeded(Element element);
>
>   // hit regions
>   void addHitRegion(HitRegionOptions options);
>   void removeHitRegion(DOMString id);
>   void clearHitRegions();
> };
> CanvasRenderingContext2D implements CanvasDrawingStyles;
> CanvasRenderingContext2D implements CanvasImageDataMethods;
> CanvasRenderingContext2D implements CanvasMethods;
> CanvasRenderingContext2D implements CanvasPathMethods;
>
> // TODO bikeshed name.
> interface OffscreenCanvasRenderingContext2D {
> };
> OffscreenCanvasRenderingContext2D implements CanvasDrawingStyles;
> OffscreenCanvasRenderingContext2D implements CanvasImageDataMethods;
> OffscreenCanvasRenderingContext2D implements CanvasMethods;
> OffscreenCanvasRenderingContext2D implements CanvasPathMethods;
>
> // TODO bikeshed name (would go in CSS Paint API spec).
> interface HoudiniCanvasRenderingContext2D {
> };
> HoudiniCanvasRenderingContext2D implements CanvasDrawingStyles;
> HoudiniCanvasRenderingContext2D implements CanvasMethods;
> HoudiniCanvasRenderingContext2D implements CanvasPathMethods;
>
> [NoInterfaceObject]
> interface CanvasImageDataMethods {
>   ImageData createImageData(unrestricted double sw, unrestricted double
> sh);
>   ImageData createImageData(ImageData imagedata);
>   ImageData getImageData(double sx, double sy, double sw, double sh); //
> NOTE arguably only getImageData should be here.
>   void putImageData(ImageData imagedata, double dx, double dy);
>   void putImageData(ImageData imagedata, double dx, double dy, double
> dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight);
> };
>
> // "write-only" methods should go here, i.e. no methods which would force a
> raster.
> [NoInterfaceObject]
> interface CanvasMethods {
>   // etc;
> };
>