Re: [cgmo-webcgm] XCF and "inherit" value

[Benoit Bezaire <benoit@itedo.com>]

Hi Lofton,

I'm catching up to emails... I've read the whole thread and I'm
replying only to this email. I don't think the problem is as
complicated as the thread seems to imply. See inline.

Monday, May 23, 2005, 9:04:58 PM, Lofton wrote:

LH> On 11-may, we resolved to add "inherit" value to 'interactivity' and
LH> 'visibility' APS Attributes.  The minutes say something about "...require a
LH> change to the XCF DTD".

LH> I'm editing now.
Thanks!

LH>   Just to be clear, "inherit" will be allowed in the WebCGM
LH> 2.0 metafile instance, in the applicable DOM places, and in the
LH> XCF, correct?
Yes.

LH> Next, while editing the XCF, this interesting question came up.  Is
LH> "inherit" a valid value for visibility and interactivity on the 'layer'
LH> element?
Yes. I don't know many specification (none actually) that restrict
an attribute value based on the element it's on. Usually, if that
happens, you instead define a new attribute. However, I don't think
this is the case here.

LH> Recall that the WebCGM content model requires layer (if present) is the
LH> top-level object within a picture.  So there is no APS above a 'layer',
LH> from which to inherit.  (As shown in figure 5.1b, above 'layer' is Picture
LH> and Metafile, neither of which carry these attributes, AFAIK.) So if the
LH> value (visibility and/or interactivity) are not specified for a layer, they
LH> would seem to take the Initial Value, "on".

LH> We could stop there, and make some such ad hoc specification to solve the
LH> layer question.  But the same question pops up if there are one or more
LH> top-level (just below Picture) 'grobject' in the picture.  Which leads to
LH> the real question, about the inheritance model...

LH> When I turned to the inheritance stuff of 5.4 to try to sort it out ...
LH> more questions than answers.  The inheritance discussion of 5.4.1.1 (et
LH> al), talks about going up the branch to look for values, if the node is
LH> "..not the root of the document tree".  Neither layers nor top-level APSs
LH> can be the "root" of the document tree -- there can be multiple sibling
LH> layers, and we need a single "root".   So aside from whether or not
LH> "inherit" is valid for these attributes on layer, what is the "root" for
LH> the purposes of the inheritance discussion?

LH> It seems to me that the parentNode attribute on a layer or a top-level APS
LH> would be the Picture, right?  So for the purposes of the inheritance model
LH> discussion, do we need to say that it is a "virtual APS" that has the
LH> Initial Value of the ApsAttrs in question?  (And if "yes", then ... well
LH> the Picture isn't the root yet, is it?  The Picture's parentNode attribute
LH> is the Metafile, right?  Same question there.)

LH> Thoughts?  How can we deal with this cleanly?
To me, the thing seems quite simple and I doubt any changes are
required. Here's why?

(i'm using markup, it's easier :)
<metafile>
  <picture>
    <grobject visibility="inherit"/>
  </picture>
</metafile>

What is the value of visibility on the <grobject>?
From section: 5.4.1.1 Specified values,
"1. If the style attribute is assigned a value, use it."
Ok, simple enough... so we go to section 5.4.1.2 Computed values,
"See the section on inheritance for the definition of computed values
when the specified value is 'inherit'."
Ok, to section 5.4.2.1 The 'inherit' value,
"the property takes the same computed value as the style attribute for
the Application Structure's parent."
Here it doesn't really matter if you think there is a parent or not,
you will end up that you have to use the initial value, which is "on".
In both cases you will end up with "3. Otherwise use the style
attributes's initial value." of section 5.4.1.1

BTW, this definition seems to work perfectly fine for HTML and SVG.
And I don't quite see what is the difference between my example above
and this:

<svg>
  <g visibility="inherit"/>
</svg>

The point is that when an implementation is doing the cascade, it
has no choice but to initialize it's style properties structure to the
Initial Values; those values are then cascaded down. So it doesn't
matter if you start at the <metafile> node, the <picture> node, or on
the <grobject> node... as soon as you see 'inherit', it will be
replaced by 'on' (the initial value).

I tried to adapt the CSS wording to WebCGM when I first wrote it, and
it was me who replaced 'element' with 'Application Structure', which
may be introducing the question of "Is the picture node an APS?. I
think that's the only possible source of confusion on the matter. What
is a good replacement for 'element'?

LH> -Lofton.

-- 
 Benoit   mailto:benoit@itedo.com