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Re: [whatwg] Interpretation of CanvasRenderingContext2D.closePath()

From: Rik Cabanier <cabanier@gmail.com>
Date: Mon, 16 Nov 2015 10:40:32 -0800
Message-ID: <CAGN7qDBrCLD5_py8s9iY_9f8PCifxr151fAgV0kGA3i7n9v4nA@mail.gmail.com>
To: Justin Novosad <junov@google.com>
Cc: WHAT Working Group <whatwg@whatwg.org>
On Mon, Nov 16, 2015 at 9:41 AM, Justin Novosad <junov@google.com> wrote:

> Also, the part about "repeating the last (possibly implied) moveTo() call"
> doesn't make much sense if we assume that closePath() applies to the new
> sub path that was started by the last moveTo() call.
>

It *is* super confusing. I complained about this in the past but it didn't
go anywhere.

For the implied moveTo case, take the following code::

ctx.lineTo(0,0); // no moveTo, so moveTo(0,0) is implied -> create new
subpath with points (0,0), (0,0)
ctx.lineTo(100,100); -> subpath (0,0), (0,0), (100,100)
ctx.closePath(); // draw line to (0,0) -> subpath (0,0), (0,0), (100,100),
(0,0) then create new subpath with point (0,0)
ctx.stroke();




> On Mon, Nov 16, 2015 at 12:38 PM, Justin Novosad <junov@google.com> wrote:
>
>> That makes sense, but the text for closePath() talks about "the last
>> subpath", which I guess is a bit unclear.
>>
>> On Mon, Nov 16, 2015 at 12:30 PM, Rik Cabanier <cabanier@gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Mon, Nov 16, 2015 at 9:02 AM, Justin Novosad <junov@google.com>
>>> wrote:
>>>
>>>> Hi All,
>>>>
>>>> The text in the spec:
>>>>
>>>> <snip>
>>>>
>>>> The closePath() method must do nothing if the object's path has no
>>>> subpaths. Otherwise, it must mark the last subpath as closed, create a
>>>> new
>>>> subpath whose first point is the same as the previous subpath's first
>>>> point, and finally add this new subpath to the path.
>>>>
>>>> Note: If the last subpath had more than one point in its list of points,
>>>> then this is equivalent to adding a straight line connecting the last
>>>> point
>>>> back to the first point, thus "closing" the shape, and then repeating
>>>> the
>>>> last (possibly implied) moveTo() call.
>>>>
>>>> </snip>
>>>>
>>>> Problematic use case:
>>>>
>>>> ctx.moveTo(9.8255,71.1829);
>>>> ctx.lineTo(103,25);
>>>> ctx.lineTo(118,25);
>>>> ctx.moveTo(9.8255,71.1829);
>>>> ctx.closePath();
>>>> ctx.stroke();
>>>>
>>>> Should this draw a closed triangle or two connected line segments?
>>>> According to the "Note" (or at least my interpretation of it), this
>>>> should
>>>> draw a closed triangle. But it appears that this is not what many
>>>> browsers
>>>> have implemented.  Chrome recently became compliant (or what I think is
>>>> compliant), and the change in behavior was reported as a regression.
>>>>
>>>> Thoughts?
>>>>
>>>
>>> moveTo creates a new subpath. This means the closePath is going to do
>>> nothing because the subpath is empty.
>>> So according to the spec, this should create 2 connected lines.
>>>
>>
>>
>
Received on Monday, 16 November 2015 18:41:05 UTC

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