SVG Working Group text question.

Hi Behdad,


I'm working on an issue for the SVG working group.(1) Your name came up in
our discussion of the issue. I'm hoping that you can give us some guidance.

As I'm sure you know, SVG allows characters to be shifted/rotated using the
'x', 'y', 'dx', 'dy', and 'rotate' attributes.(2) SVG 1.1 maps numbers in
these attributes via Unicode code points (although it's not stated as
clearly as it could be). So:

<text x="10 20 30">ABC</text>

would position the 'A' at x=10, 'B' at x=20, and 'C' at x=30.

SVG 2 originally changed this mapping to use UTF-16 code units so that a
character outside the Unicode base plane would consume two numbers. I think
this was done to match how the DOM SVGTExtContentElement methods count
characters.(3) Browser tests show that only Firefox uses UTF-16 code units.
The consensus of the working group is not to use UTF-16 code units and to
revert back to SVG 1.1 behavior which is seen to be more user friendly.

Then came the issue of how to treat precomposed and decomposed characters.
Amelia's tests(4) show that browsers treat 'u' + combining character as
follows:

  * Firefox:    consumes two numbers, translates/rotates as unit.
  * Chrome and Safari: consumes two numbers, translates/rotates separately.
  * Edge: consume one number (even in cases where there is not a
precomposed equivalent).

Everyone agrees that Chrome and Safari's behavior is wrong. There is some
disagreement between if Firefox or Edge behavior is better, the first uses
Unicode Code points, the second uses Extended Grapheme Clusters (UAX#29).

My concern with Edge's behavior is that it requires an SVG renderer/editor
to know what are Extended Grapheme Clusters which I imagine could be quite
complex for some scripts. Pango appears to have this built in. Is this
something that is exposed to a user? Is this something that is well defined
for all (most) scripts? The advantage of using Unicode Code Points (besides
being backwards compatible) is that it is well defined and predictable.

We would appreciate any thoughts you would have about this.

Thanks,

Tav

1. https://github.com/w3c/svgwg/issues/537
2. https://www.w3.org/TR/SVG2/text.html#TSpanNotes
3. https://www.w3.org/TR/SVG2/text.html#InterfaceSVGTextContentElement
4. https://github.com/w3c/svgwg/issues/537#issuecomment-417823060