Arabic text: changing colour changes form

[Paul Sladen]

Arabic requires advanced OpenType contextual layout: in Arabic each characters has four forms (standalone, initial, intermediate, final) depending upon its positioning with the surrounding characters.

If one (intermediate) character is selected in Inkscape and its colour is altered, then the composition is broken, and the standalone form being shown.

Ideally changing the colour of one character should not alter the glyph that is shown for that character.

[Paul Sladen (sladen)]

SVG source for above.

[su_v (suv-lp)]

> Ideally changing the colour of one character should
> not alter the glyph that is shown for that character.

AFAIU this is a limitation of the underlying SVG code: if changing the fill color attribute of an individual letter (or a sub-string), the differently styled character needs to be wrapped into a separate (nested) <tspan> to override the inherited style, making the individually colored character "standalone".

The same can be reproduced with any font which uses ligatures (e.g. 'Aller' or 'Lobster'):
<http://imgh.us/ligatures-ttf-inkscape-1.png>

[su_v (suv-lp)]

Setting status to 'Confirmed' for now (reproduced with Inkscape 0.48.2 and 0.48+devel r10672, on Mac OS X 10.5.8 (i386)).

[su_v (suv-lp)]

Related to (or duplicate of)
- Bug #332580 “Applying colour to text breaks Arabic shaping "
  <https://bugs.launchpad.net/inkscape/+bug/332580>

[Tavmjong Bah (tavmjong-free)]

There are two separate issues that need to be addressed:

The first is that it is not clear from the CSS and SVG specs what the correct behavior should be. What should cause stand-alone forms to be shown? Changing font size? Changing font weight? Adding letter spacing? Moving characters using the 'x' or 'dx' attributes? The latter does result according to the specs in breaking non-mandatory ligatures.

The second issue is the amount of code that would need to be rewritten. Currently Inkscape handles text one tspan at a time. As any change in color results in a new tspan, Inkscape would have to be change to handle multiple tspans together. Pango does have a function that is designed to shape text across changes in style (pango_shape_full())[1] but it would still require quite a bit of code rewriting. I am hesitant to try to make any changes until the first issue is settled.

[1] https://developer.gnome.org/pango/stable/pango-Text-Processing.html#pango-shape-full