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Re: [css-transforms] 'transform-box' defaults in svg

From: Amelia Bellamy-Royds <amelia.bellamy.royds@gmail.com>
Date: Mon, 28 Sep 2015 15:44:16 -0400
Message-ID: <CAFDDJ7y4b8MO8319RXwMsK6r=_GEc9g=a+shqZYLRtqjn_XZ-w@mail.gmail.com>
To: Erik Dahlström <erik@xn--dahlstrm-t4a.net>
Cc: "www-style@w3.org" <www-style@w3.org>, "public-fx@w3.org" <public-fx@w3.org>
The default `transform-box` for SVG needs to be `view-box` in order to be
consistent with current (SVG 1.1) behavior.  SVG transformations are by
default relative to 0,0 in the userSpace coordinate system.  So the SVG
default transform-origin is 0,0 and the default transform-box must somehow
work out as equivalent to view-box.

It's true that many designers (and especially animators) coming from CSS
would prefer to work with bounding-box for percentages, but that's why the
`transform-box` property was introduced to allow the defaults to be changed.

That said, implementing this by mapping `border-box` to `view-box` could
introduce unpleasant inconsistencies with other specs, such as filters and
masking, which also have reference boxes but different SVG 1.1 defaults.

Since we're already agreed to use the user agent style sheet to set the
SVG-specific `transform-origin`, it might make sense to use the same
mechanism for setting `transform-box`.  Then come up with a universal
mapping of CSS reference boxes to SVG reference boxes that will be
consistent across all specs.

~Amelia


On 28 September 2015 at 15:27, Erik Dahlström <erik@dahlström.net> wrote:

> Hi,
> I was reviewing the 'transform-box' section in the css transforms spec,
> https://drafts.csswg.org/css-transforms/#transform-box.
>
> The initial value of 'transform-box' is 'border-box', which is fine.
> However, the spec continues to state that:
> "For SVG elements without an associated CSS layout box, the used value for
> border-box is view-box."
>
> It seems to me that a used value of 'fill-box' in this case would lead to
> more easily understood behavior than 'view-box'. So, I'd like to know what
> the rationale behind the current choice is.
>
> An example, http://jsfiddle.net/fs6cLt38/1/.
>
> Cheers
> /ed
>
>
Received on Monday, 28 September 2015 19:44:44 UTC

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