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RE: Allowing <mask>, <clipPath> etc. to target their parents

From: David Dailey <ddailey@zoominternet.net>
Date: Tue, 21 Feb 2012 07:42:45 -0500
To: "'Brian Birtles'" <birtles@gmail.com>, "'www-svg'" <www-svg@w3.org>
Message-ID: <000801ccf096$4f757480$ee605d80$@net>
Hi Brian,

I'd suggest that as you consider this (which seems quite reasonable, btw), you also look at some of the proposed syntax associated with the replicate tag: http://srufaculty.sru.edu/david.dailey/svg/SVGOpen2010/replicate.htm 

In that proposal, each of the things that modifies an element: patterns, gradients, clipPaths, masks, animates, filters, transforms, replicates are considered to be "modifiers." As modifiers of an element to be replicated (or animated) these objects can be cloned and customized appropriate to time or space-based interpolations. This conceptual model unifies its handling of all modifiers in ways that make a lot of sense and which seem not to have been considered during the early design phases of SVG 1.  The ability to modify attributes of an element's gradient, filter, pattern or other modifiers as that element is interpolated/extrapolated over time and space is quite powerful. I would recommend that working toward a common syntax for all of these (including how they are referred to (i.e., #id vs url(#id) ) will be in everyone's best interest.


-----Original Message-----
From: Brian Birtles [mailto:birtles@gmail.com] 
Sent: Tuesday, February 21, 2012 12:19 AM
To: 'www-svg'
Subject: Allowing <mask>, <clipPath> etc. to target their parents

Dear all,

I have been assigned ACTION-3223, to investigate ISSUE-2431, "Consider allowing <mask>, <clipPath>, etc. as direct children of elements they'd apply to, to avoid IDs."[1]

The issue:

Some elements like <mask> and <clipPath> can only be used by referring to them by id, e.g. mask="url(#batsignal)".

This is sub-optimal for two reasons:

1. When generating content via script, the script must generate a unique ID. Note that in many applications it's not sufficient to use a counter to generate IDs such as mask0, mask1, mask2 etc. since the generated content may be mashed-up with other content leading to ID collisions. 
Instead, scripts need to generate a globally unique ID which is (a) more complicated to generate, (b) longer, obscuring the content and increasing it's file size, parse time etc.

I suspect that for many authors, one of the first steps when generating content is to create a globally unique ID generator which seems like unnecessary overhead.

2. When a mask, clipPath etc. is only intended to be used once, IDs obscure the relationship between elements in the document. The best you can do is make the mask and its target, siblings.


I've identified the following combinations of properties/attributes and target elements where ID references are currently required but where it might be preferable if they were not.

clip-path -> <clipPath>
cursor -> <cursor>
fill, stroke -> <linearGradient>, <radialGradient>, <pattern> filter -> <filter> font -> <font> ?
marker, marker-start, marker-mid, marker-end -> <marker> mask -> <mask> <feImage xlink:href> -> SVG content <font-face-uri xlink:href> -> <font> <textPath xlink:href> -> <path>

Possible solution:

It would be really nice if a mask etc. could be applied to an element simply by making it a child of that element. This is currently how animations work. An <animate> element can either refer to its target by xlink:href="#id" or, if no such attribute exists, it will default to targeting its parent element.

Using that approach, instead of:

   <filter id="myFilter">
   <path filter="url(#myFilter)"/>

You could have:


But, of course, that breaks existing content such as:

     <filter id="MyFilter">
     <path filter="url(#MyFilter)"/>

I think our next best option is to extend the syntax.

For properties we could have:

   clip-path: <funciri> | none | inherit | child

If it's 'child', it means "find the first child element that is a <clipPath> and use that."

Judging by the minutes of the previous discussion I'm pretty sure credit for this syntax goes to Cameron McCormack.[1]

(On that note, does CSS already have syntax for this sort of thing? And is it too short-sighted to limit this to the children?)

For 'xlink:href' attributes it's a bit more difficult since really all 'xlink:href' attributes should take the same values. However, I wonder if for the three examples above you could get away with just allowing the target element to be child content in the absence of an 'xlink:href' 
attribute. feImage is probably easy, I don't really know if font-face-uri makes much sense, and textPath looks a little complicated:

       <path d="..."/>
       Text along path

For reference, feImage might look like:

     <feImage x="500" y="5">
       <path ... />

In either case, the absence of xlink:href means, "but what about the children?"

As well as extending the grammar for various CSS properties, this would also mean changes to the permitted child content of various elements.

What do you think?

Best regards,

Brian Birtles

[1] Original discussion here: http://www.w3.org/2012/01/12-svg-minutes.html
Received on Tuesday, 21 February 2012 12:43:24 GMT

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