[librsvg: 1/4] Start a roadmap document for text layout

[Marge Bot <marge-bot src gnome org>]

commit 74744ae9acf2985a3254a1369456e991af19a284
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Oct 22 15:40:03 2021 -0500

    Start a roadmap document for text layout
    
    Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/620>

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+# Text layout in librsvg
+
+This document describes the state of text layout in librsvg as of version 2.52.3,
+and how I want to overhaul it completely for SVG2.
+
+## Status as of librsvg 2.52.3
+
+Basic supported features:
+
+* Librsvg supports the elements `text`, `tspan`, `a` inside text, and `tref` (deprecated
+  in SVG2, but kept around for SVG1.1 compatibility).  See below for the `x/y/dx/dy`
+  attributes; librsvg supports single-number values in these.
+
+* `text-anchor`.
+
+* SVG1.1 values for `direction`, `writing-mode`.  Non-LTR or vertical text layout is very
+  much untested.
+
+* SVG1.1 values for `letter-spacing`, `baseline-shift`, `text-decoration`.
+
+* `font` (shorthand), `font-family`, `font-size`, `font-stretch`, `font-style`,
+  `font-variant`, `font-weight`.
+
+* `text-rendering`.
+
+Major missing features:
+
+* `text-orientation` and `glyph-orientation-vertical` fallbacks, SVG2 values for `writing-mode`.
+
+* SVG2 `white-space` handling.  This deprecates `xml:space` from SVG1.1.
+
+* Support for multiple values in each of the attributes `x/y/dx/dy` from the `text` and
+  `tspan` elements.  Librsvg supports a single value for each attribute, whereas SVG
+  allows for multiple values — these then get used to individually position "typographic
+  characters" (Pango clusters).  In effect, librsvg's single values for each of those
+  attributes mean that each text span can be positioned independently, but not each
+  character.
+
+* Relatedly, the `rotate` attribute is not supported.  In SVG it also allows multiple
+  values, one for each character.
+
+* `glyph-orientation-vertical` (note that `glyph-orientation-horizontal` is deprecated in SVG2).
+
+* `textPath` is not supported at all.  This will be made much easier by implementing
+  `x/y/dx/dy/rotation` first, since each character needs to be positioned and oriented
+  individually.
+
+* `@font-face` and WOFF fonts.
+
+Other missing features:
+
+* `display` and `visibility` are not very well tested for the sub-elements of `<text>`.
+
+* SVG2 text with a content area / multi-line / wrapped text: `inline-size`,
+  `shape-inside`, `shape-subtract`, `shape-image-threshold`, `shape-margin`,
+  `shape-padding`.  This is lower priority than the features above.  Also the related
+  properties `text-overflow`,
+
+* `text-align` (shorthand), `text-align-all`, `text-align-last`, `text-indent`, `word-spacing`.
+
+* Baselines: `vertical-align` (shorthand), `dominant-baseline`, `alignment-baseline`,
+  `baseline-source`, and SVG2 values for `baseline-shift`.  Note that Pango doesn't
+  provide baseline information yet.
+
+* `line-height` (parsed, but not processed).
+
+* SVG2 `text-decoration`, which translates to `text-decoration-line`,
+  `text-decoration-style`, `text-decoration-color`.
+
+* `font-feature-settings`, `font-kerning`, `font-size-adjust`.
+
+* CSS Text 3/4 features not mentioned here.
+
+Features that will not be implemented:
+
+* SVG1.1 features like `<font>` and the `glyph-orientation-horizontal` property, that were
+  deprecated for SVG2.
+
+## Issues
+
+https://gitlab.gnome.org/GNOME/librsvg/-/issues/795 - Implement the unicode-bidi property.
+
+https://gitlab.gnome.org/GNOME/librsvg/-/issues/795 - Implement SVG2 white-space behavior.