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Re: [whatwg] Proposal: Add CanvasRenderingContext2D.fillRule with "nonzero" (default) and "evenodd" options

From: Rik Cabanier <cabanier@gmail.com>
Date: Tue, 8 Jan 2013 09:35:51 -0800
Message-ID: <CAGN7qDD3BAkGzpS_70QtQefow2Ujg75NVqad+3bCheX0YjYY4g@mail.gmail.com>
To: Ian Hickson <ian@hixie.ch>
Cc: Dirk Schulze <dschulze@adobe.com>, James Ascroft-Leigh <jwal@jwal.me.uk>, whatwg@whatwg.org, Chris Jones <cjones@mozilla.com>
I looked at pdf2js which is using this fillRule property. As I expected,
introduction of the property results in code like this:

    eoFill: function CanvasGraphics_eoFill() {
      var savedFillRule = this.setEOFillRule();
      this.fill();
      this.restoreFillRule(savedFillRule);
    },

...
    // We generally keep the canvas context set for
    // nonzero-winding, and just set evenodd for the operations
    // that need them.
    setEOFillRule: function CanvasGraphics_setEOFillRule() {
      var savedFillRule = this.ctx.mozFillRule;
      this.ctx.mozFillRule = 'evenodd';
      return savedFillRule;
    },
    restoreFillRule: function CanvasGraphics_restoreFillRule(rule) {
      this.ctx.mozFillRule = rule;
    },


So, for even odd winding, all this code needs to run. With my proposal,
this gets much simpler:

   eoFill: function CanvasGraphics_eoFill() {
      this.eoFill();
    },


You can find a pull request with the needed changes to pdf2js here:
https://github.com/cabanier/pdf.js/commit/8e80b086376013a5438087714a4d2abb6fe67de1

I also created patches (with test files) for WebKit and mozilla:
https://bugs.webkit.org/show_bug.cgi?id=106188
https://bugzilla.mozilla.org/show_bug.cgi?id=827053

On Thu, Jan 3, 2013 at 3:38 PM, Ian Hickson <ian@hixie.ch> wrote:

> On Fri, 10 Jun 2011, Chris Jones wrote:
> >
> > In 2D canvas, determining whether a point is "inside" a path is
> > currently always done using the non-zero winding rule.  I propose
> > extending 2D canvas to allow determining inside-ness using the even-odd
> > rule.
>
> I've added this to the spec.
>
>
> On Wed, 2 Jan 2013, Dirk Schulze wrote:
> >
> > There was a complain on the webkit bug report if fillRule should be part
> > of the graphics state or not. Did you investigate what current 2d
> > graphics libraries do (qt, Cairo, CG, ...)? Is it part of the graphics
> > state there?
>
> I have made it be part of the graphics state in the spec; it would be
> unusual in the API for it not to be. However, if this doesn't match
> implementations, please let me know.
>
>
> On Wed, 2 Jan 2013, Rik Cabanier wrote:
> >
> > this features is not a trivial as it seems. Adding this will necessitate
> > updates to the algorithms that deal with paths and the outlining of
> > strokes and text.
>
> Can you elaborate on what updates are needed? I couldn't see any that
> actually needed to be changed.
>
>
> > As Dirk mentioned, instead of making it part of the graphics state, it's
> > more likely better to make it part of the fill or clip operator like
> > SVG, PDF and PostScript.
>
> That seems like it would be inconsistent with the rest of the canvas API.
>
>
> > In addition, the path object will need to be extended so it can deal
> > with this idiom.
>
> Can you elaborate on how this affects the Path object? It seems like it
> would be somewhat orthogonal.
>
>
> > The easiest way to implement this, would be to leave the core interface
> of
> > canvas alone and just extend the path object with winding rules and a
> > method to 'simplify' a path so it can be drawn with any winding rule.
>
> This doesn't seem like it would be easier... in particular, fillRule is
> now implemented in two browsers, so the implementation cost for them would
> be zero, and they don't yet implement Path at all, so the implementation
> cost for Path would be quite high, even without "simplify". :-)
>
>
> On Wed, 2 Jan 2013, Rik Cabanier wrote:
> >
> > However, just look at how stroke is implemented in the Canvas 2d spec or
> > how you can create paths by stroking or stroked text. They're all
> > affected by the winding rules.
>
> How so?
>
>
> > (The description on how to do strokes in the spec is very wrong, but
> > that can be addressed later)
>
> Can you elaborate on this? If there's a mistake obviously I'd like to fix
> it...
>
>
> > Dirk and I did a bit more research and found that SVG, PDF, Flash,
> > PostScript, Skia, Core Graphics and Direct2D all have the winding rules
> > as part of the fill operator. It seems strange that canvas would choose
> > to have a different interface...
>
> People using the canvas API are more likely to know the canvas API, and
> thus want extensions to be consistent with the canvas API, than they are
> to be familiar with PDF, Flash, PostScript, Skia, Core Graphics, or
> Direct2D. Incidentally, of those, I'm only familiar with SVG, and SVG is
> similar to what I specced (indeed I don't see how it could be part of the
> operator since it's a declarative language).
>
> --
> Ian Hickson               U+1047E                )\._.,--....,'``.    fL
> http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
> Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
>
Received on Tuesday, 8 January 2013 17:36:16 GMT

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