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Re: Handling of orient="auto" markers

From: Paul LeBeau <paul.lebeau@gmail.com>
Date: Sun, 26 May 2013 22:04:39 +1200
Message-ID: <CACfsppARn_EpRE3=XAZ1pOPAMzJmMbT8ucz25t5jge40Pcb3hg@mail.gmail.com>
To: www-svg@w3.org
I've put up a document showing the current problems with orient=auto
markers here:

http://paullebeau.com/svg2/markers/

At the bottom of that page is a copy of my proposed clarifications to the
SVG 2 spec. It is a slightly expanded version of what I had in my first
email.

Paul




On 26 May 2013 15:57, Paul LeBeau <paul.lebeau@gmail.com> wrote:

> Sure. I'll put together something to show what I mean.
>
> P
>
>
>
> On 26 May 2013 10:03, Dirk Schulze <dschulze@adobe.com> wrote:
>
>> Hi Paul,
>>
>> You said that the implementations are very quirky and inconsistent. It
>> would be extremely helpful if you have tests that you can share with the
>> community. Quite often there edge cases not respected by implementations
>> and specifications because of a lack of tests. Do you have some test cases?
>>
>> I did not have the time to review your analysis but will do it before the
>> next F2F meeting.
>>
>> Greetings,
>> Dirk
>>
>> On May 25, 2013, at 10:25 AM, Paul LeBeau <paul.lebeau@gmail.com> wrote:
>>
>> > Hi all
>> >
>> > When I was implementing my SVG renderer, the Marker section was one of
>> the head-scratchiest parts of the spec to try and understand and get right.
>>  It did not help that pretty much all SVG renderers have bugs (or "quirks")
>> in their marker handling so there wasn't any consensus to go on.
>> >
>> > I notice the SVG 2 spec is a bit better, but I think the section on
>> "auto" oriented markers is still a bit confusing.
>> >
>> > 'auto'
>> > A value of 'auto' indicates that the marker is oriented such that its
>> positive x-axis is pointing in the direction of the path at the point it is
>> placed.
>> >
>> > If the marker is a segment marker, then the direction the marker is
>> oriented is, if considering the incoming and outgoing directions as unit
>> vectors, in the direction of the sum of these two vectors. If this sum is
>> zero, then the marker is oriented in the incoming direction.
>> >
>> > Is this a typo? Seems like this should be referring to a vertex marker.
>> >
>> > If the marker is on the first or last vertex of a closed subpath,
>> [..snip..]
>> >
>> > I think it is not obvious in all cases what the "first" and "last"
>> vertex is.  If the last point in the subpath does not coincide with the
>> first point, we are told to draw a line to the start point. In this case,
>> which point is the "last" point?  Also should we be drawing a start marker,
>> an end marker, or both?
>> >
>> > My feeling that in a closed subpath, there is conceptually no real
>> start or end point. All vertexes should be considered mid points and only
>> midpoint markers should be rendered.  So my proposed replacement for this
>> section would be as follows:
>> > ------------------------------------------
>> > 'auto'
>> > A value of 'auto' indicates that the marker is oriented such that its
>> positive x-axis is pointing in the direction of the path at the point it is
>> placed.
>> >
>> > If the marker is a vertex marker, then the direction the marker is
>> oriented is, if considering the incoming and outgoing directions as unit
>> vectors, in the direction of the sum of these two vectors. If this sum is
>> zero, then the marker is oriented in the incoming direction.
>> >
>> > If there are two or more consecutive points in a path that coincide
>> exactly (both their x and y values are the same) then all the consecutive
>> coinciding points should be treated as if they were a single vertex.
>> >
>> > If the marker is on the first vertex of a closed subpath, then the
>> outgoing direction is taken from the first path segment and the incoming
>> direction is taken from:
>> >        the last path segment of the subpath, if the last point
>> coincides with the first point, or
>> >        the line drawn from the last point to the start point, if they
>> do not coincide.
>> > For closed subpaths, the marker definition to be rendered for all
>> vertexes is the marker specified by marker-mid. No start or end markers
>> should be applied.
>> > ------------------------------------------
>> >
>> > That last paragraph might be a bit controversial, but for closed
>> subpaths I think it makes sense.  As I said, from my testing, renderers
>> rarely agree on how markers are positioned at the start/end of closed
>> subpaths, so I don't think making this change would affect too many people.
>> >
>> > I also have a suggestion for an enhancement:
>> > ------------------------------------------
>> > 'auto-start'
>> > A value of 'auto-start' behaves exactly the same as 'auto' except for
>> the treatment of the start marker.  If a subpath is not closed, then the
>> start marker is positioned so that it points in the exact opposite
>> direction (inverse vector) to the first path segment.
>> > ------------------------------------------
>> >
>> > The idea is that would allow for the easy creation of double-ended
>> arrows, for example, without having to define two markers.
>> >
>> > This is my first contribution to this list, so if I am an idiot, go
>> easy on me! :)
>> >
>> > Paul
>>
>>
>
>
> --
> Phone: +64 3 9423700 / Mobile: +64 21 1666127
> Skype: paul.lebeau / XMPP: paul.lebeau@gmail.com
> Twitter: @paullebeau
>



-- 
Phone: +64 3 9423700 / Mobile: +64 21 1666127
Skype: paul.lebeau / XMPP: paul.lebeau@gmail.com
Twitter: @paullebeau
Received on Sunday, 26 May 2013 10:05:31 UTC

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