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Re: marker-pattern syntax

From: Dirk Schulze <dschulze@adobe.com>
Date: Tue, 27 Nov 2012 11:15:10 -0800
To: "Tab Atkins Jr." <jackalmage@gmail.com>
CC: SVG public list <www-svg@w3.org>
Message-ID: <093DD68E-19B0-4481-83D3-3F1C300ECD1F@adobe.com>

On Nov 27, 2012, at 10:56 AM, Tab Atkins Jr. <jackalmage@gmail.com> wrote:

> On Tue, Nov 27, 2012 at 8:54 AM, Dirk Schulze <dschulze@adobe.com> wrote:
>> Tab, Cameron and I had a discussion about the syntax of 'marker', 'marker-pattern' and 'marker-segment' on IRC yesterday[1].
>> 
>> Tabs proposal is the following:
>> 
>>        marker-pattern: [ [ segment | path ] <distance>? <marker-instance> [ <distance> <marker-instance>? ]* ]#
> 
> Actually, that grammar is no longer quite right, based on some of the
> tweaks we made to my proposal as the chatroom discussion progressed.
> Here's an up-to-date version, formatted for readability:
> 
> marker-pattern: [
>    [ segment | path ]
>    <distance>?
>    [
>      <marker-instance> |
>      <marker-instance> <distance> [ <marker-instance>? <distance> ]*
>    ]
>  ]#
> 
> Now, the first distance is the offset.  It is followed by either a
> single marker instance, or a list of (marker-instance, distance)
> pairs.  If the former, it paints a single marker at the offset, and
> stops.  If the latter, it cycles through the list, painting the marker
> then advancing the distance, until the path ends.  (Pairs after the
> first may omit their marker-instance, which implies that they should
> reuse the last non-omitted one in the list.)  The "path" and "segment"
> keywords are as Dirk describes.  This constitutes one "marker track",
> and you may provide multiple independent marker tracks
> comma-separated.
> 
>> The first <distance> would be an optional offset of the marker-pattern (similar to the offset on 'stroke-dasharray').
>> 
>> This would combine marker-pattern and marker-segment and is easy to integrate into the current marker syntax (which is underspecified in SVG 1.1).
> 
> In particular, the 'marker' syntax would just become:
> 
> marker: <url>{0,3} <'marker-pattern'>
> 
> The required "segment" or "path" keyword at the start of
> 'marker-pattern' would disambiguate.  (Without it, a single "url()" is
> a valid 'marker-pattern' value, and thus is ambiguous in the
> shorthand.)

If we would decide to make marker a shorthand. Not sure if it is worth it.

> 
>> It allow the same syntax for marker-segments as for marker-patterns. The behavior is similar, with some exceptions: For segment, he marker repeat starts on every segment again; relative values are resolved according to the segment length, instead of the path length.
>> 
>> I don't really care about the new keywords and would also be fine to not make 'marker' a shorthand at all and keep 'marker-pattern' and 'marker-segment' separate.
>> 
>> Here some examples and my understanding of the syntax(- is the stroke; # is marker m1, O is marker m2):
>> 
>>        marker-pattern: path url(#m1) 20px;
>>        #--#--#--#.
>> 
>>        marker-pattern: path 10px url(#m1) 20px;
>>        -#--#--#--#.
>> 
>>        marker-pattern: path 10px url(#m1) 20px 50px;
>>        -#--#-----#--#-----#--#.
>> 
>>        marker-pattern: path 10px url(#m1) 20px url(#m2) 50px;
>>        -#--#-----O--#-----O--#.
>> 
>>        marker-pattern: path 10px url(#m1) 20px url(#m2) 50px 10px;
>>        -#--#-----O-O--#-----O-O--#.
> 
> These last two aren't right.  They should be, respectively:
> 
> -#--O-----#--O-----#--O---...
> 
> -#--O-----O-#--O-----O-#--O---

Would just be a different way to interpret the syntax, with the same strength. I don't car about the way to interpret it. It would still be the question: Do we allow negative values for <distance>? I actually don't see a problem with it. But I might be wrong. What would a distance of 150% mean? is it just (150 mod 100)%?

> 
> The point is that you should be able to just read across the
> declaration, drawing symbols and moving distances as they show up.  In
> other words, the first one means "starting at a 10px offset, draw #m1,
> move 20px, draw #m2, move 50px, etc.".  This is basically a simplified
> turtle syntax.  ^_^
> 
>> It would also be possible to draw patterns with a specific paint order, which will cause overlapping markers:
>> 
>>        marker-pattern: path 10px url(#m1) 20px, url(#m2) 30px;
>>        -#--#--#--#--#--#...
>>        O---O---O---O---O
>> m2 would always be drawn on top of m1 (or the other way around, this is up to the specification).
> 
> In other words, assuming that later tracks paint on top of earlier
> ones, this would render as:
> 
> O#--O--#O-#-O#--O...
> 
>> Special case, no repeat:
>> 
>>        marker-pattern: path 50px url(#m1);
>>        -----#----------------..
>> 
>>        marker-pattern: path 50px url(#m1), 60px url(#m2);
>>        -----#O---------------..
>> 
>> The syntax and behavior for segment is the same, just that it starts again on every segment.
> 
> We could potentially do some fanciness to simplify the common case
> that marker-segment is trying to address, too, and make it so that an
> omitted offset defaults to "50%" in "segment" mode, rather than the
> "0" of "path" mode.  Dunno if that's worthwhile, but it does mean that
> we're simply replacing "marker-segment: url()" with "marker-pattern:
> segment url()", rather than "marker-pattern: segment 50% url()".  (In
> addition to being more verbose, it's not immediately clear what the
> difference is between that last one and "marker-pattern: segment url()
> 50%;", which actually draws *three* markers, at the beginning, middle,
> and end of each segment.  Shortening the default case might make this
> easier to get right.)  However, defaults changing based on other parts
> of the declaration can also be confusing on their own, so I dunno.
> 
>> I would like to know if we can agree on the general idea?
> 
> +1.
> 
> ~TJ
Received on Tuesday, 27 November 2012 19:15:36 GMT

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