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Re: [css3-background] bg-position syntax clarification

From: Brad Kemper <brad.kemper@gmail.com>
Date: Sat, 22 Aug 2009 11:41:57 -0700
Cc: www-style list <www-style@w3.org>
Message-Id: <2ED44AB2-D185-4CBC-8B70-A040D2820F2E@gmail.com>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
And this does not strike you as adding a whole extra level of  
complexity that is completely unnecessary, compared to just giving a  
single measurement for each of the first or last stops along the  
gradient path? The total (100%) gradient path length is going to be  
either the distance between two corners, or a distance determined by  
an author-specified angle (the "outside" distance). Any pointed  
plotted in a 2-dimensional space, as with <bg-position>, can also be  
found in a line that intersects the gradient path at a right angle.

I would much rather err on the side of keeping the grammar simple,  
than on adding complexity that would only matter tiny bit, and only  
then for a tiny, tiny, tiny fraction of the times they are used.

On Aug 22, 2009, at 10:16 AM, Tab Atkins Jr. wrote:

> The gradient discussion has revealed that many people (me included)
> simply do not have a good grasp of what the syntax for a <bg-position>
> really means, beyond the most basic examples with two keywords or two
> offsets.  I don't believe this is any true fault of the syntax, but
> rather simply a failure in the explanation.  I think the explanation
> in the Backgrounds and Borders module is geared toward helping
> developers parse the value (thus the constant references to "if x
> values are specified...") rather than helping authors write the value.
> I have tried to write a more clarifying explanation of the syntax
> below, with a holistic focus starting from the full syntax and
> explaining what happens when certain things are omitted:
> <bg-position>:
>  <horizontal-component> || <vertical-component>
> <horizontal-component>:
>  [ [ left | right ]? <offset>? ] | center
> <vertical-component>:
>  [ [ top | bottom ]? <offset>? ] | center
> <offset>:
>  <length> | <percentage>
> A bg-position is defined by two components, a horizontal and a
> vertical.  Each specifies a side and an offset from that side.
> Positive offsets indicate a direction into the box, negative offsets
> point outside of the box.  For example, "right 10px top 20px"
> specifies a point 10px to the left of the right edge, and 20px down
> from the top edge.
> Generally, the horizontal component must be specified first, followed
> by the vertical component.  You are allowed to swap these if and only
> if both components have their side specified.  As well, as specified
> in the grammar above, the side (if present) must be specified before
> the offset (if present).
> If an offset is omitted from a component but the side is specified,
> the offset defaults to '0'.
> If the horizontal component's side is omitted but the offset is
> specified, the side default to 'left'.  If the vertical component's
> side is omitted but the offset is specified, the side defaults to
> 'top'.
> Either component can be replaced by 'center'.  This is exactly
> equivalent to omitting the side and specifying '50%' as the offset,
> and is subject to the normal rules and restrictions that are applied
> when you omit the side from a component.
> An entire component can be omitted only under particular
> circumstances, as this introduces the potential for ambiguity.  The
> horizontal component can be omitted (and defaults to 'center') if and
> only if the vertical component's side is specified (the vertical
> offset may be omitted).  The vertical component can be omitted (and
> defaults to 'center') if and only if the horizontal side *or* the
> horizontal offset is omitted; you cannot omit the vertical component
> if you specify both parts of the horizontal component.
> I've obviously played a little loose with the language; the phrase
> "background positioning area" doesn't appear anywhere in my prose.  It
> can be easily patched up to be more precise.  As well, further
> clarifications such as what exactly percentage values for offsets mean
> are already adequately explained in the B&B module, so I've left them
> out here.
> My specification actually breaks from the one given in B&B in one
> place: if three values are specified, my syntax allows them to be a
> side and two offsets, while B&B requires them to be either two sides
> and an offset or a side, an offset, and 'center'.  That is, "left 10px
> 20px" is allowed in my syntax and expands to "left 10px top 20px", but
> it is not allowed in the current B&B syntax.  Is this acceptable?  I
> think that this case is pretty easy to parse, and it would complicate
> the explanation to disallow it ("left 10px 50%" is at least no harder
> than "left 10px center", which it should be equivalent to.).  I
> suspect that the only reason it's not allowed currently is precisely
> because it's more difficult to explain it in the current style;
> disallowing it actually simplifies the current language.  In my
> language it's very simple - you've just omitted one of the values, and
> it's very clear what effect that has.
> Thoughts?  Improvements?  Does this actually help people understand
> the complex syntax of bg-position?  I expect this syntax to be reused
> many times in the future, as it's very useful, and want it to be as
> accessible as possible.
> ~TJ
Received on Saturday, 22 August 2009 18:42:40 UTC

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