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[css3-images] cycle detection and element()

From: L. David Baron <dbaron@dbaron.org>
Date: Sun, 5 Feb 2012 19:47:39 +0100
To: www-style@w3.org
Message-ID: <20120205184739.GA2797@pickering.dbaron.org>
http://dev.w3.org/csswg/css3-images/#element-cycles
(Overview.html revision 1.269) says the following:

  # Populate the dependency graph initially by having every element
  # depend on each of its children. Then, whenever a property on an
  # element A uses the ‘element()’ function to refer to an element
  # B, add an edge to the graph by having A depend on B. If a
  # dependency cycle is detected, any ‘element()’ functions that
  # produced a dependency in the cycle must represent a fully
  # transparent image with no intrinsic dimensions.

It's not entirely clear, but this sounds to me like it's defining an
algorithm that happens in a certain order (given its use of words
like "whenever ... add").  In other words, it sounds like this
algorithm might be suggesting that with this markup:
  <div id="a" style="background: element(#b)">hi</div>
  <div id="b" style="background: element(#a), element(#c)"></div>
  <div id="c" style="background: element(#a)"></div>
a cycle would be detected after adding the backgrounds for a and b
to the graph, voiding (i.e., making transparent and dimensionless)
the element(#b) background on id="a" and the element(#a) background
on id="b", and possibly (the spec isn't clear on this point) then
removing the corresponding edges from the graph.  If those edges are
then removed from the graph (which I believe they should not be),
then the element(#a) background on id="c" is *not* voided, which I
believe it should be because of the c -> a -> b -> c cycle.

There's no reason for this algorithm to depend on the order of CSS
style computation.  That's a dependency that could be very confusing
for authors even if that order were defined, which it's not (and
shouldn't be).

Additionally, the "add" wording is confusing because when the
computed value of the property no longer contains the element()
function, the edge should definitely be removed from the graph,
which might cause a previously-eliminated background to render.
Doing otherwise would violate the principle that the sequence of
dynamic changes that led to the current state shouldn't affect how
that state is rendered.


I believe this section needs to be rewritten to describe the state
of the graph in terms of the present state of dependencies rather
than describing it in terms of addition operations that happen at
certain times.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂
Received on Sunday, 5 February 2012 20:59:30 GMT

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