- From: Jeremy Orlow <jorlow@chromium.org>
- Date: Mon, 22 Feb 2010 22:35:33 +0000
On Mon, Feb 22, 2010 at 7:57 PM, Drew Wilson <atwilson at google.com> wrote:
>
>
> On Mon, Feb 22, 2010 at 11:13 AM, David Levin <levin at google.com> wrote:
>
>> I've talked with some other folks on WebKit (Maciej and Oliver) about
>> having a canvas that is available to workers. They suggested some nice
>> modifications to make it an offscreen canvas, which may be used in the
>> Document or in a Worker.
>>
>> Proposal:
>> Introduce an OffscreenCanvas which may be created from a Document or a
>> Worker context.
>>
>> interface OffscreenCanvas {
>> attribute unsigned long width;
>> attribute unsigned long height;
>> DOMString toDataURL (in optional DOMString type, in any... args);
>> object getContext(in DOMString contextId);
>> };
>>
>>
>> When it is created in the Worker context, OffscreenCanvas.getContext("2d")
>> returns a CanvasWorkerContext2D. In the Document context, it returns a
>> CanvasRenderingContext2D.
>>
>> The base class for both CanvasWorkerContext2D and CanvasRenderingContext2D
>> is CanvasContext2D. CanvasContext2D is just like a CanvasRenderingContext2D
>> except for omitting the font methods and any method which uses HTML
>> elements. It does have some replacement methods for createPattern/drawImage
>> which take an OffscreenCanvas. The canvas object attribute is either a
>> HTMLCanvasElement or an OffscreenCanvas depending on what the canvas context
>> came from.
>>
>> interface CanvasContext2D {
>> readonly attribute object canvas;
>>
>> void save();
>> void restore();
>>
>> void scale(in float sx, in float sy);
>> void rotate(in float angle);
>> void translate(in float tx, in float ty);
>> void transform(in float m11, in float m12, in float m21, in float
>> m22, in float dx, in float dy);
>> void setTransform(in float m11, in float m12, in float m21, in
>> float m22, in float dx, in float dy);
>>
>> attribute float globalAlpha;
>> attribute [ConvertNullToNullString] DOMString
>> globalCompositeOperation;
>>
>> CanvasGradient createLinearGradient(in float x0, in float y0, in
>> float x1, in float y1)
>> raises (DOMException);
>> CanvasGradient createRadialGradient(in float x0, in float y0, in
>> float r0, in float x1, in float y1, in float r1)
>> raises (DOMException);
>> CanvasPattern createPattern(in OffscreenCanvas image, in
>> DOMString repetition);
>>
>> attribute float lineWidth;
>> attribute [ConvertNullToNullString] DOMString lineCap;
>> attribute [ConvertNullToNullString] DOMString lineJoin;
>> attribute float miterLimit;
>>
>> attribute float shadowOffsetX;
>> attribute float shadowOffsetY;
>> attribute float shadowBlur;
>> attribute [ConvertNullToNullString] DOMString
>> shadowColor;
>>
>> void clearRect(in float x, in float y, in float width, in float
>> height);
>> void fillRect(in float x, in float y, in float width, in float
>> height);
>> void strokeRect(in float x, in float y, in float w, in float h);
>>
>> void beginPath();
>> void closePath();
>> void moveTo(in float x, in float y);
>> void lineTo(in float x, in float y);
>> void quadraticCurveTo(in float cpx, in float cpy, in float x, in
>> float y);
>> void bezierCurveTo(in float cp1x, in float cp1y, in float cp2x, in
>> float cp2y, in float x, in float y);
>> void arcTo(in float x1, in float y1, in float x2, in float y2, in
>> float radius);
>> void rect(in float x, in float y, in float width, in float
>> height);
>> void arc(in float x, in float y, in float radius, in float
>> startAngle, in float endAngle, in boolean anticlockwise);
>> void fill();
>> void stroke();
>> void clip();
>> boolean isPointInPath(in float x, in float y);
>>
>> void drawImage(in OffscreenCanvas image, in float dx, in float
>> dy, in optional float dw, in optional float dh);
>> void drawImage(in OffscreenCanvas image, in float sx, in float
>> sy, in float sw, in float sh, in float dx, in float dy, in float dw, in
>> float dh);
>>
>> // pixel manipulation
>> ImageData createImageData(in float sw, in float sh)
>> raises (DOMException);
>> ImageData getImageData(in float sx, in float sy, in float sw, in
>> float sh)
>> raises(DOMException);
>> void putImageData(in ImageData imagedata, in float dx, in float
>> dy, in optional float dirtyX, in optional float dirtyY, in optional float
>> dirtyWidth, in optional float dirtyHeight]);
>> };
>>
>> interface CanvasWorkerContext2D : CanvasContext2D {
>> };
>>
>> interface CanvasRenderingContext2D : CanvasContext2D {
>> CanvasPattern createPattern(in HTMLImageElement image, in
>> DOMString repetition);
>> CanvasPattern createPattern(in HTMLCanvasElement image, in
>> DOMString repetition);
>> CanvasPattern createPattern(in HTMLVideoElement image, in
>> DOMString repetition);
>>
>> // focus management
>> boolean drawFocusRing(in Element element, in float xCaret, in
>> float yCaret, in optional boolean canDrawCustom);
>>
>> // text
>> attribute DOMString font;
>> attribute DOMString textAlign;
>> attribute DOMString textBaseline;
>> void fillText(in DOMString text, in float x, in float y, in
>> optional float maxWidth);
>> void strokeText(in DOMString text, in float x, in float y, in
>> optional float maxWidth);
>> TextMetrics measureText(in DOMString text);
>>
>> // drawing images
>> void drawImage(in HTMLImageElement image, in float dx, in float
>> dy, in optional float dw, in float dh);
>> void drawImage(in HTMLImageElement image, in float sx, in float
>> sy, in float sw, in float sh, in float dx, in float dy, in float dw, in
>> float dh);
>> void drawImage(in HTMLVideoElement image, in float dx, in float
>> dy, in optional float dw, in optional float dh);
>> void drawImage(in HTMLVideoElement image, in float sx, in float
>> sy, in float sw, in float sh, in float dx, in float dy, in float dw, in
>> float dh);
>>
>> void drawImage(in HTMLCanvasElement image, in float dx, in float
>> dy, in optional float dw, in float dh);
>> void drawImage(in HTMLCanvasElement image, in float sx, in float
>> sy, in float sw, in float sh, in float dx, in float dy, in float dw, in
>> float dh);
>>
>> };
>>
>> Questions and comments are welcome.
>>
>> Note that something similar did come up in December and this proposal
>> avoids doing any text related items in the offscreen canvas which was a
>> concern brought up by Robert O'Callahan at the time (
>> http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-December/024478.html
>> ).
>>
>
> Do we feel that text APIs are, in general, difficult to implement in a
> multi-thread safe manner? Or would we be limiting this API primarily because
> of the specifics of a single current implementation? Because if it's the
> latter, I would argue against omitting the text APIs, since they seem
> fundamental to many use cases. It seems like there are a range of solutions
> for retrofitting multi-thread support to non-multi-thread-safe operations,
> ranging from the use of global mutexes to changing the code itself to be
> safely re-entrant.
>
It does indeed seem pretty short sighted.
Why can't implementations that aren't able to render text (or whatever) on a
worker thread simply schedule such work on their main thread? I know it
defeats the parallelism, but it's better than _some_ implementations are
single threaded when doing things like rendering text rather than _all_
implementations (not to mention that it's easier from the web developer's
perspective).
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Received on Monday, 22 February 2010 14:35:33 UTC