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Re: Adding winding rules to Canvas

From: Rik Cabanier <cabanier@gmail.com>
Date: Tue, 15 Jan 2013 13:49:37 -0800
Message-ID: <CAGN7qDDqB1JeOUsQWPKBDbkJv-dzMam4ftPrudeXGa+eHOdAEQ@mail.gmail.com>
To: Simon Sarris <simon.sarris@gmail.com>
Cc: public-canvas-api@w3.org
Hi Simon,

I completely agree with you.
As specified today, hit regions don't have support for winding. In fact, it
is even worse: if you have a set of drawing commands, you can't get their
The reason for this is that the path object (as currently defined) simply
accumulates path segments. This will wreak havoc with shapes that touch or
that have strokes.

I have stated this a couple of times already on whatwg...
What I would like to see is more like this:

class PathSink implements CanvasPathMethods {
  PathSink(DOMString); //takes SVG path syntax

class Path {
  Path(PathSink, CanvasWindingRule = "nonzero");
  Path(DomString text, CanvasDrawingSTyles...); // to get text outline

  Path Transform(matrix Transformation);
  Path Stroke(...);

  Path Add(Path); // <----

The 'Add' method would not simply aggregate path segments. Instead, the
area of resulting path will be a union.

So, for example, if you want to create a region with a stroke rectangle:

var h = new PathSink();
h.rect(100, 100, 200, 200);
var P = new Path(h);
P = P.Add(P.Stroke({'lineWidth': 10}));

On Tue, Jan 15, 2013 at 1:23 PM, Simon Sarris <simon.sarris@gmail.com>wrote:

> Before we comment on your proposal I have some notes I'd like to share
> because the current *fillRule *rules in the specification seem incomplete
> or at least ill-defined.
> The new hit regions use a *Path* in the *HitRegionOptions* (the argument
> to *addHitRegion*) in order to function, but its not clear what fill rule
> these *Path* objects are using for hit-testing. Even-odd and winding fill
> rules create different holes in a path, so it matters a good deal for hit
> testing.
> There seem to be three possibilities as implemented:
>    1. HitRegions only ever use winding paths. This seems like a bad idea.
>    2. Whichever *fillRule* is defined when *context.addHitRegion* is
>    called determines the *fillRule *for that hit region's *Path *permanently.
>    This seems acceptable but confusing and should be clarified if it is
>    currently the case.
>    3. The fillRule of a hit region changes dynamically as *
>    context.fillRule* changes. This would be a nightmare.
> I hope it is #2, but the specification makes no mention of this.
> Actually, I'd prefer that either *HitRegionOptions *or the *Path *object
> would need to have a *fillRule *attribute.
> If the specification does adopt something similar to Cabanier's
> suggestions then that would need to be done anyway.
> In short, if *isPointInPath *is changed to specify *fillRule*, *
> HitRegionOptions* (the argument to *addHitRegion*) or the *Path* given in
> the options needs to change too.
> Simon Sarris
> On Tue, Jan 15, 2013 at 3:42 PM, Rik Cabanier <cabanier@gmail.com> wrote:
>> All,
>> there was a recent discussion on adding winding rules to canvas. As you
>> may know until now, canvas only supported even-odd winding.
>> Maybe graphics libraries and SVG also support non-zero winding.[1][2]
>> Mozilla exposes this currently with 'mozFillRule'. Making this part of
>> the graphics state has several drawbacks.
>> The biggest is that fill/clip will now have to check the state every
>> time, or set/reset it. Winding is also part of path geometry.
>> I have the following proposal:
>> enum CanvasWindingRule { "nonzero", "evenodd" };
>> void fill(optional CanvasWindingRule w = "nonzero");
>> void clip(optional CanvasWindingRule w = "nonzero");
>> boolean isPointInPath(unrestricted double x, unrestricted double y,
>> optional CanvasWindingRule w = "nonzero");
>> proposed patches for this API can be found here:
>> https://bugs.webkit.org/show_bug.cgi?id=105508
>> https://bugzilla.mozilla.org/show_bug.cgi?id=827053
>> What do people think?
>> 1: http://www.w3.org/TR/SVG/painting.html#FillRuleProperty
>> 2: http://en.wikipedia.org/wiki/Nonzero-rule
Received on Tuesday, 15 January 2013 21:50:04 UTC

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