Re: stroke-miterlimit: Why not use partial miters when a stroke linejoin exceeds the limit?

Thanks Tav,

One detail I thought of after I sent out the previous message: what to do
when the two lines are completely coincident (e.g., theta = 0).  The draft
SVG2 implementation notes currently say to crop flush with the point (no
additional line join area).  In contrast, the Firefox/Windows
implementation maintains the trimmed miter as an extension to the lines.
I've updated the codepen to include this scenario.

To maintain smooth transitions while removing extension for 0-degree
vertices -- and to more accurately reflect the definition of
stroke-miterlimit as being proportional to the complete stroke width, not
just the outside half -- the miter cut-off point could be a fixed distance
from the inside corner, not from the actual path vertex.

In other words, the final implementation instructions for basic miter would
be:

14. Let P5 be the intersection of the two stroke edges on the inside of the
corner (opposite side of P versus P3). Construct a clipping rectangle,
*limit*, centered on the line between P5 and P3, that extends (d =
‘stroke-miterlimit’ * ‘stroke-width’) on either side perpendicular to that
line, and and the same distance along that line from P5.  In other words,
if P5 is (x,y) and P3 is (x1,y1), then the four vertexes of the clipping
polygon would be defined as (x,y) ± d*(y1-1, x-x1) and  (x,y) + d(x1-x,
y1-y) ± d*(y1-1, x-x1) .

15. Return the the union of *bevel* with (*extension* clipped to *limit*).



I also agree that the arc join needs to be finessed.  A simple rectangular
clip, on second consideration, wouldn't do: it could result in disconnected
pieces, with the tip of the talon curving back into the rectangle when the
main arc has been clipped.    I would really have to play around with a
working implementation of the arc joins to get the possibilities clear in
my mind.

ABR


On 19 October 2014 13:12, Tavmjong Bah <tav.w3c@gmail.com> wrote:

>
> Hi,
>
> The behavior you've described seems reasonable. I've added the topic of
> miter limit to the agenda requests for the TPAC meeting this month. The
> limit as applied to the arc join needs to be studied further; it's on my
> list of things to do.
>
> Tav
>
> On Sat, 2014-10-18 at 16:18 -0600, Amelia Bellamy-Royds wrote:
> > [ General comment: If it would preferable to submit these feature
> > request / implementation discussions through an issue tracker, please
> > let me know.  I'm not quite clear on the relationship between the two
> > issue trackers, one at http://www.w3.org/Graphics/SVG/WG/track/ and
> > one as part of the W3 Bugzilla.
> >
> >
> > Sorry for dumping a lot of ideas all at once, but as I mentioned
> > previously I'm starting to figure out what's new in SVG2, and I know
> > that the working group has got a big meeting coming up at the end of
> > the month, so might as well mention sooner rather than later. ]
> >
> >
> > ______________________
> >
> >
> > Background:
> > ------------------
> > The stroke-miterlimit prevents mitered strokes on acute linejoins from
> > extending far beyond the dimensions of the shape.
> >
> >
> > However, the behaviour was not clearly defined in SVG1.1 [1].  The
> > general description seems to imply that `stroke-miterlimit` is a
> > *limit* that the miter reaches but does not surpass.  However, the
> > specific rule is "When the limit is exceeded, the join is converted
> > from a miter to a bevel", which has been interpretted by most browsers
> > as meaning to convert the linejoin to `stroke-linejoin:bevel`, which
> > is a much tighter crop.
> >
> >
> > The detailed implementation notes in the current SVG2 draft would lock
> > in that interpretation. [2]
> >
> >
> >
> > The Problem:
> > ------------------
> > In my mind, this is contrary to the spirit of a "limit" attribute.
> > Furthermore, it results in unpleasant effects in animation.  See
> > http://codepen.io/AmeliaBR/pen/JACBu?editors=110
> >
> >
> > Current behaviour in all browsers tested except Firefox is as
> > described above.  The switch to the the extreme bevel when the miter
> > exceeds the limit results in sudden jumps during animation, and ends
> > up bevelling acute angles much more closely than the miter on more
> > oblique angles.
> >
> >
> > (Batik 1.7 seems to be cutting the difference, removing the bevelled
> > area but keeping the point!  Oops!)
> >
> >
> >
> > In contrast, in Firefox (v32/33 on Win7), the animation is smooth.  If
> > you had a shape with multiple mitered corners of similar angles on
> > either side of the limit, they would be rendered consistently.
> >
> >
> > The discrepancy has been noted on this list before in May of this
> > year [3], where it was determined that the Firefox behaviour was
> > operating-system dependant.  That thread ended with a link to the
> > corresponding Mozilla bug page about standardizing the rendering [5].
> >
> >
> > Standardizing is good, but why not standardize in the other direction,
> > to the more aesthetically pleasing result that allows for smooth
> > animations?  Yes, this would somewhat sacrifice
> > backwards-compatibility, but only for a feature that is currently
> > implemented in incompatible ways by different agents.
> >
> >
> >
> >
> > Request:
> > ------------
> > That SVG 2 define the behaviour of stroke-miterlimit so that it never
> > crops mitered corners shorter than the limit.
> >
> >
> > A similar rule could also be applied to the proposed arc/talon
> > linejoin, taking the shape defined in the talon and clipping it to the
> > miter limit.
> >
> >
> > In order to make this the defined behaviour, the final points in the
> > line join shape implementation notes (after calculating the basic
> > intersection points) would be amended as follows:
> >
> >
> > 8. Let *bevel* be the triangle formed from the three points P, P1 and
> > P2.
> >
> >
> > 9. If ‘stroke-linejoin’ is bevel, return *bevel*.
> >
> >
> > 10. If ‘stroke-linejoin’ is round, then return the union of *bevel*
> > and a circular sector of radius ‘stroke-width’, centered on P, and
> > which has P1 and P2 as the two endpoints of the arc.
> >
> >
> > 10. If ‘stroke-linejoin’ is arcs, then find the circles that are
> > tangent to the stroke edges at P1 and P2 with the same curvature as
> > the edges at those points (see "Computing the circles for the arcs
> > 'stroke-linejoin'"). If both curvatures are zero fall through to
> > miter. Extend the stroke edges using these circles (or a line, in the
> > case of zero curvature). If the two circles (or circle and line)
> > intersect, let *extension* be the area inside the region defined by
> > the lines that connect P with P1 and P2 and the arcs defined by the
> > circles (or arc and line) between the closest intersection point to P,
> > and P1 and P2. If the two circles (or circle and line) do not
> > intersect, fall through to miter. Note that the curvatures are
> > calculated in user-space before any transforms are applied.
> >
> >
> > 12. If ‘stroke-linejoin’ is miter, or if *extension* could not be
> > defined using the arcs method, let *extension* be the triangle formed
> > from the three points P1, P2 and P3.
> >
> >
> > 13. Let θ be the angle between A and B.  If 1 / sin(θ / 2) ≤
> > ‘stroke-miterlimit’, then return the union of *extension* and
> > *bevel*.
> >
> >
> > 14. Construct a clipping rectangle, *limit*, centered on P that
> > extends (‘stroke-miterlimit’ * ‘stroke-width’) from P in the
> > directions perpendicular to the line between P and P3, and half that
> > distance in the directions parallel to that line.  In other words, if
> > P is (x,y), P3 is (x1,y1), and the computed distance is d, then the
> > four vertexes of the clipping polygon would be defined as (x,y) ±
> > d/2*(x1-x, y1-y) ± d*(y1-1, x-x1).
> >
> >
> > 15. Return the the union of *bevel* with *extension*, clipped to
> > *limit*.
> >
> >
> > Depending on implementations, user agents might skip 13 and just apply
> > the clipping region; it depends which is faster, a null-effect
> > clipping operation or trigonometry.
> >
> >
> > I'm fairly certain this reflects the current Firefox/Windows
> > implementation for miters, and I think it would result in acceptable
> > appearance of "clipped talons" when applied to the arc line joins,
> > although I haven't explored it carefully.  The use of a clipping
> > rectangle centered on P is intended to encompass any degree of
> > curvature of the talons, including curving back towards the other side
> > of the join.  However, I'm not sure whether the use of 1 / sin(θ / 2)
> > will cover all cases of long talons; it might be better to directly
> > calculate the distance to the intersection point of the two arcs.
> >
> >
> >
> >
> > [1]: http://www.w3.org/TR/SVG11/painting.html#StrokeMiterlimitProperty
> > [2]:
> >
> http://www.w3.org/TR/2014/WD-SVG2-20140211/painting.html#TermLineJoinShape
> > [3]: http://lists.w3.org/Archives/Public/www-svg/2013May/0020.html
> > [4]: https://bugzilla.mozilla.org/show_bug.cgi?id=854296
>
>
>