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

On Mon, Nov 16, 2015 at 1:40 PM, Rik Cabanier <cabanier@gmail.com> wrote:

>
>
> 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();
>
>
To be clear, my problem with the wording is that "(possibly implied)"
implies that the moveTo may also be explicit. In the case where there is an
explicit (non-implied) moveTo, does that make closePath essentially a no-op?

>
>
>
>
>> 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 18:54:58 UTC