Re: transform property on <tspan> and <textPath>

On Fri, Jan 15, 2016 at 11:13 AM, Tavmjong Bah <[email protected]> wrote:
> On Fri, 2016-01-15 at 14:35 +1100, Cameron McCormack wrote:
>> In the SVG 2 draft, we allow transform on <tspan> and <textPath>.  I
>> realise now however that this is inconsistent with what CSS allows in
>> non-SVG documents, and if we are defining SVG text layout in terms of
>> CSS text layout (plus an additional layer of SVG glyph positioning),
>> then we might want to reconsider.
>>
>> Specifically, the transform property is defined to apply only to
>> “transformable elements”:
>>
>>   https://drafts.csswg.org/css-transforms/#transformable-element
>>
>> Notably this doesn’t include inline elements, which is what we are
>> considering <tspan> and <textPath> to be.
>>
>> As far as I know nobody has implemented this.
>
> Actually, rsvg seems to support transforms on tspan elements!
>
> I would have thought that transform on tspan elements would be treated
> as part of the post CSS layout step. However, I don't have a strong
> opinion on keeping transforms on tspan and textPath elements.

CSS's issues are mostly with inlines broken across a line, as there's
no good answer for what coordinate space the transforms should operate
in (particularly rotates/skews).  Thus our restriction to blocks and
atomic inlines (like display:inline-block elements).

SVG's text model is a little different, but putting the restriction on
<tspan> amounts to the same thing.  A <text> can span multiple lines,
but it in itself is a reasonable "container", like a block in CSS.  A
<tspan> can contain text split across different lines, tho, which runs
into the issues we had.

(There is no "post-CSS layout step". Transforms are part of CSS
layout, and SVG is just using an unspecified CSS layout mode which
resembles forced absolute positioning.)

~TJ